FuG25a test set-up

Reconstructing the German I.F.F. system (Freund-Feind-Kennung)


Started on 9 March 2012

Status: 23 June 2012


Up until now we have dealt with purely Nachtfee maters. Nachtfee, however, was a system in which the data transfer relied fully upon the temporarily signal phase shift; this signal had to be conveyed towards the German pathfinder aircraft and was displayed at a special CRT screen. Which, most likely was about similar to the one of the real Nachtfee apparatus (Erstling)

Please take, before continuing with this webpage, notice of the previous:  

FuG25a considerations




Nachtfee testing rig, known as: FuG 25a Prüftafel



Replica of the FuG25a testing set-up. Antenna mounting not yet prepared


Our aim is not reproducing an exact copy of wartime "Prüftafel", but having a set-up which is facilitating our transponder simulations. The Erstling receiver (of FuG25a) will be fully operational, whereas the transmitter is straight linked onto a dummy load. We may estimate, however, that the very nearby receiver Gemse is nevertheless sensitive enough to detect the remaining very weak HF signal. (may be at 1 m distance).


FuG25a mounted, without its cover hood as well as main screening plate



The main mounting frame has been taking of, as to facilitate the wiring of the FuG25a system



Our own version of a Gemse testing rig

Looking at this photo, I notice that I have to cut some of the wooden central mounting plate. As it otherwise is impossible to access the right hand side of the Gemse mounting set-up


On 12 March 2012

The test rack has been wired and is also partially operated



FuG25a test set-up is been wired and already under test. Please notice the tiny single winding turn just near the antenna-rod base, which is injecting capacitive HF from the SMS signal generator. The receiver loading coil of AAG25a has not yet been tuned for optimal signal pick up. It is left as it once was accomplished during the war

Signal being picked up from the anode of the detector or 'Audion' stage Rö4. This signal is also available at the test socket pin 9 against ground at pin 1. For practical reason a separating capacitor of 4700 pF has been integrated inside the test plug.





The signal at pin 9

Please be aware that what is measured is also the HF signal component of the IF frequency as well as the HF signal at 126 MHz. Though, this signal is also wobbled over a wide band of about 6 MHz. Please notice, that the camera exposure time is long versus the screen changes which is causing a quite fuzzy screen picture


As to understand the implications of the FuG25a IFF system, please consider Hans Jucker's paper on it.


R&S SMS generator settings: frequency adjusted at 126.000 MHz which gave the max. signal output at the test point (pin 9 at the test socket = Prüfstecker). Generator output 0.5 V through a separating capacitor as to prevent uncontrolled dc at the SMS signal output. Signal injected by means of a single turn near to the antenna-rod base. Modulation 400 Hz at 90%. 1000 Hz modulation was not good visible, as the FuG25a was designed to prevent responding upon any other signal than 500 Hz pulses. Hence, 400 Hz is also not yet the optimal test signal. We have to repeat this experiment as soon as we can switching on and off the generator output. This has to be accomplished by means of an external circuit. I believe that a resistor has to be put in series with the generator output; at the open end I would like to adapt a power fet (transistor) which is kept conducting against ground. And thus short-circuiting the HF signal. These types represent often a Ron of < 0,1 ohm (the gate often supplied with 10 V against its source). This is the condition for most of the pulse duration. Only when a pulse has to be transmitted the fet will be switched off. This external circuit can be used in combination with all kinds of generators as it acts frequency independently.



SMS signal being switched off (the signal residue is caused by the receiver and IF noise)



The next step is to pick up the signal from output of Rö5, just as has been proposed in the modified FuG25a schematic (detected signal being amplified)




Please consider the wire where I have connected onto the pointer Nachtfee

Please double click at the schematic as to open it in pdf



The detected 400 Hz signal

Please notice the two signal contents being the 400 Hz demodulation of the SMS signal as well as the wobbled receiver signal. During this wobbling process the signal sweeps over a band of about 6 MHz. The consequence is that we also will see the noise spectrum of that band.



SMS signal output being switched off. Please notice the absence of IF noise



The recognition-key (Schlüssel) being switched on. For practical reason this is causing a quite high dc level chance and consequently the scope input had to be adjusted accordingly




BG25a (Bediengerät 25a) is set for IFF recognition key 2

This type designation indicate that is was adapted for FuG25a (Erstling Rot)




On 16 March 2012

I continued with investigating the FuG25a, as to get a feeling how signals and its circuitry are interacting.


First, it was decided to leave my concept of a controlled attenuator by means of power a fet (BUZ11) which was pulsed such that only HF could be passed during the existing pulse. It proved however, that this doesn't work as expected. As to keep the test setup simple, I took a BD237 and used it in grounded emitter circuit, whereas the basis is directly fed from our pulse generator as well as from the SMS signal generator set at 124 MHz for optimal signal transfer; of course having a current limiting resistor in series. This concept works quite well.

It was also discovered, that I had made a mistake in wiring the test connector (Bu3), where I thought I was picking up the detector signal from the anode of Rö4, which wasn't the case as we watched the signal at pin7! It was also found that additional smoothing of the about 190 V HT was necessary (bridging C7 and C8).


 As to investigate the functioning of our FuG25a I removed it from its mounting frame and used an extension cable instead. The consequence however, was that the keying circuit was interrupted. After some considerations, a bridging wire was short circuiting it temporarily. Luckily this allowed watching that the FuG25a transmitter transmits as a response onto the received signal pulses; though did not so when a continuous signal wave was received. My attention was drawn to something that flashed which was the neon transmission control lamp inside BG25a.


There doubtless RF existing at the output connector (Bu1) on pin 9

The HF is picked up via C55 and being fed onto Rö7 which is acting as a detector; followed by a relay which is feeding the neon indicator lamp. I believe that this was done as to extend the duration of the visual neon flashes. I also tested whether it is possible to see a small signal lamp glowing. This didn't work. My guess now is, that the rather short signal pulses are simply too short as to heat up the filaments. HF must be available at pin 9 of Bu1, there is no doubt. It was also possible feeding a dc of say 12 volt onto the female connector of the mounting frame. Resulting in a glowing lamp. Maybe another problem is causing the absence of sufficient HF at the end of the coaxial cable. As to prevent unwanted signal radiation, I have not wired the transmitter output onto the AAG25a antenna-splitter. I would like to activate the Gemse receiver as to watch if it is receiving the HF signal originating from the FuG25a transmitter.



After having done some test it was found that when a non modulated carrier was existing having enough signal strength this signal appears at the anode of the detector valve (pin 9 of the test connector)


Without signal there exists only a single time base line. What the origin is of the double pulses I don't know yet.



Detector output with a pulsed signal content

It has been tested with interrupting the pulse system and the result always was that there was then no signal detectable.



Signal measured at C35 just where it enters the compartment of Rö6

Taking this photo wasn't easy because the keying mechanism was more or less irregularly changing the dc level causing time base moments vertically.


Please take also notice of my personal  Notebook considerations



On 18 March 2012

I continued with adapting the Gemse as to receive the FuG25a (Erstling) transmitter signal.


First it is necessary to review my thoughts that a not too small lamp could function as a dummy-load.

Let us briefly estimate what the actual output in W/s may be. Let us consider the signals used in Freya and EGON; and guessing that each pulse is having a duration of 2 µs and its PRF is 500 Hz. This will provide a total signal duration of 1000 µs or 1 ms. According information the transmitter is providing 300 - 400 W pulses. Hence, 1/1000 x 400 = 400 mW/s. A value far too limited for getting the lamps I have considered to glow.

First I considered using thermo-couples as to detect the existence of transmitter HF. These are, however, very delicate devices for which one have to take special care. But why not using the Gemse receiver? After some modification of the wooden Gemse mounting frame, with the assistance of Alfred Breur, it was positioned just next to the FuG25a test setup.   


In the forefront we look at the right-hand side of the Gemse receiver

The short green cable is just a single dipole leg of the provisional Gemse antenna. In the background next to the wooden FuG25a test-mounting, we see on top the (cubic) 500 Hz sine wave generator and below it the triggered pulse generator, providing short pulses of a few micro seconds. Please notice also the test connector (at pin 9) about the centre of the FuG25a which is, for this occasion, watching the FuG25a detector (Audion) output.



The Gemse receiver just next to the FuG25a (please notice the red wire which is the other experimental dipole leg for the Gemse receiver. The scope probe it connected onto the video output of the Gemse receiver

The four connections just below the metal head of the LD1 oscillator valve belong to the Ferraris wobbling motor, which is wobbling the Gemse receiving frequency spectrum. As to pick up also signals being off frequency centre (156 MHz); similarly is occurring within the front section of the FuG25a receiver.



After having played with the Gemse receiver controls at its front panel FuG25a signals could be received. I had to take a series of photo shots as to get this picture. My guess is, that owing to the receiver wobbling, pulse appearance is getting quasi an erratic nature   

It is quite simple proving that the signals originate from the IFF system; by interrupting the HF generator signal. Or, disconnecting the pulse triggering, both resulting in an absence of video output. Also the neon indicator inside BG25a is telling us when HF is being supplied onto the transmitter antenna connector. 


Our next move should be transporting both wooden frames to the Klooster premise and starting up implementation of the virtual I.F.F. system into the Nachtfee feedback chain. We have to bring along also the SMS signal generator, the pulse generating test gear and an additional power supply for the HF pulse modulating circuit. This latter device has, however, first to be build in a proper high frequency manner, as the experimental one laying on the lab.-table is too fragile to be operated. Also taking provisions for preventing damaging of the modern test gear by means of absorbing (limiting) devices. The Nachtfee is working with relative high voltages whereas the modern pulse delaying unit, though also the digital signal generator SMS, may be handling pulses of 5 to 6 V maximally! Marc Simons provided to us special SMD devices. According his information it consists of a pair of high speed zener-diodes cutting off at about 6.5 V.




On 21 March 2012

I built the HF pulse modulator in a proper housing



The HF pulse modulator build-in a Al housing

On the right-hand side is the HF generator cable, on the left-hand side we see the HF output cable


The arrow is showing that this is the signal input originating from the SMS signal generator. The coaxial cable just at the other end of the housing is giving the pulsed signal bursts. The keying (pulse) signal is fed onto the T-connector



The HF signal bursts are clearly visible. The shape of the base-line as well as the HF bursts on 126 MHz is originating from the dc changes every time the keyed (PRF is 500 Hz) transistor is being switched on and off (in our case experimentally, say, 50 µs). Whereas Freya and EGON used about 2 µs pulse duration. In our case, signal is only passing through when a keying pulse is existing. Otherwise the dc level is at a high level (20 V at the collector when no current is flowing). The HF signal burst itself will not be effected in the HF sense, as its HF amplitude is more or less constant during the keying period


On 27 March 2012


Time has come to move the FuG25a testing mounting (Prüftafel) to the Klooster premise.


Our Nachtfee had been in working order on display during the 35 Year Anniversary held in Egmond aan Zee on 23 - 23 March 2012. It was a great success.


After the Nachtfee apparatus had be reinstalled in the Klooster, it stroked after it had been switched on. Why?

Starting with some nervous feelings, it proved that during transport a solder tag was touched and it brittle brass was broken. Tracing the cause, it proved that 6.3 V filament voltage was lacking. After repair everything ran as usually.

It was now time to get the FuG25a system running, which didn't cause any problems.


Experimental setup of FuG25a now in conjunction with Nachtfee

For convenience the HF pulse-modulator is being fixed temporarily on a lag of the table on the right-hand side. Its housing is too small and the three coaxial cables are time and again pulling it towards the ground. The centre cable is the one that carries the keying pulses for the HF modulator.

I encountered some starting up problems. The pulse-generator (TTi Marconi) refused responding onto triggering signals. I tried all possibilities, but it refused to provide the keying signals. To be sure that nothing had become defect, I tried our previous configuration where the triggering pulse was derived from a sine wave 'tone generator' set at 500 Hz. It worked. Consequently, the triggering input needs to 'see' a sine wave. Why not taking the Nachtfee data output directly? After having checked that the 'peak-to-peak' amplitude is not exceeding 10 V, the Nachtfee data output was fed onto the triggering input. It works!


We are looking at the keying pulses as well as a 500 Hz sine wave signal originating from the PM5190 synthesiser


Changing the Nachtfee 'orders' is directly having an effect on the sine wave. On may ask why? Because the scope is triggering on channel A, the one which provides the keying or timing signal originating from the Nachtfee 'order' data. We are watching however, that the sine wave is changing its signal phase.

Please think this over! This is exactly what should happen in the aircraft. The aircraft display is having its own time base reference and the ground signal is having to interact in the time domain. This must be regarded under 1940s technology being impossible or at least extremely un reliable!





On 28 and 29 March 2012 in the afternoon


I continued with making the FuG25a receiver delivering the feedback signal towards the Nachtfee CRTs.


After some provisions it worked!


Our first successful setup, which shows that a Nachtfee data-signal could be transmitted (distance of about 1 m) towards our FuG25a and being processed again in the Nachtfee feedback channel. The given orders could be compensated for system- as well as distance or range delays

Please double click on this drawing as to view it in pdf


Maybe not yet the optimal presentation, but the screen shot below shows that the loop-feedback is functioning as was expected.


The a bit fuzzy signal due north originates from the FuG25a receiver output, which is being feedback towards the CRTs inside the Nachtfee apparatus. My estimation is, that noise though, also frequency wobbling in the front-end of the FuG25a receiver, may be debit to it as well



Please notice the virtual line between the two horizontal lines (one above and one below the CRT centre)

This is the a bit clipped FuG25a receiver signal, after the virtual line has been adjusted by means of the range- or distance control (previously called 'number scale').

I regard this being quite an achievement!


I also encountered problems inside the Gemse receiver, where the wobbling motor stroke. Luckily we have one in spare and it was successfully replaced. It was also found that the receiver is not responding appropriately. My first thought was to repair it as soon as possible.    However, after due consideration I have decided to reconstruct an independently running scope display. In the same manner as we have done before, simply paining a Lissajous circle and the target blip is inserted via the Z-channel. Thus not feeding the signal to the control CRT (LB2) inside the Nachtfee apparatus. First I would like to check what the implications are of viewing the time base signal phase, originating from the PM5190 synthesiser.



On 31 March 2012


I approached the artificial aircraft display for the second time. This time, however, with the aim testing whether it is possible to feedback the time-base sine wave signal. This time painting a Lissajous circle (X-channel being fed directly from 500 Hz and the A-channel being fed from a 90° phase-shift-signal). After some adjustments of signal phase and levels it painted a circle. The Z-channel was directly fed from the FuG25a receiver output. It proved however soon, that shortening the pulse by means of a relatively small capacitor was making the painted dot smaller.



The painted dot is well distinguishable

You may ask yourself why my ballpoint pen is so prominent in this photo? The reason is simple. When you make a series of photos it is often impossible to match (linking) onto a particular experiment. The spot has drifted a bit, which is also the reason of this experimental setup.


It proves that it is possible by simply using the time-base feedback as a calibrating reference. Comparing the two last photos might giving the impression that quite some adjustment error occurs


This is being cause by the following reason.


Course alignment have to be accomplished by means of the "Phase" control on the frontpanel


What counts is, adjusting the edge or trail (on the left-hand side of the pulse) of the upper pulse in line with the right-hand edge of the lower pulse. Even the slightest maladjustment is becoming directly visible, by means of a mutual off-set.

It was also discovered, that the distance scale is an important factor. This information has to be incorporated in an operating Nachtfee. This might be not a too great deal, as EGON is also used for ranging control.

It is also found, no wonder, that the range or distance scale does not have an influence on the 'order' pointer being transmitted, as it only has an influence upon the displayed feedback signal phase (please notice the Nachtfee block schematic). It is however, having a great impact on bringing in synchronism both the time base frequency of Nachtfee-Frey-EGON and the aircraft time base. This can only be countered by knowing their mutual distance. For me it was quite simple because I only had to look around a corner. Though, tactically this is a far more delicate system parameter. It is, however, found, that after getting some feeling of the involved parameters that the distance- or range scale setting came time and again at a range of 100 km.



Maybe for those acquainted with laboratory settings a not too strange situation

On the left the FuG25a testing table, with underneath a power supply and next our TTi pulse generator, which is being triggered by the Nachtfee data signal. The receiver signal is being fed onto the Z-channel of the scope. Please notice the block schematic of this setup. Although, the latter is not yet prepared for handling the artificial time base signal as we just have noticed.

I can assure you, working in this environment is thrilling, as what we experimentally are  reinventing hasn't been done for about >68 years. Reversed engineering is an innervating occupation!



On 2 April 2012

It is time to explain more in details what actually is done


This block diagram is telling us a bit more how the chain of information, hypothetically, have been handled. Of course, translated onto our context!

The Nachtfee apparatus is now effectively the nucleus of the Nachtfee 'order' or command system. Schematically on the left-hand side the phase controlled sine wave signal is in our concept fed onto a pulse shaping system, as should more or less equally have been done in the wartime EGON transmitter system. As to get pulsed HF, I had to modulate it in an additional module (pulse-gated HF modulator); as the digital SMS generator cannot cope with pulses of < 50 µs. This wireless signal is then transmitted towards the FuG25a IFF setup (bridging < 1 m). The FuG25a detector signal is directly available from the test connector at pin 9. This signal is being fed directly onto the Z-channel of our HP scope, which is wired such that a Lissajous circle is painted and the Nachtfee data signal being represented by means of a softly highlighted spot. So far the two time base systems are running independently and no synchronism is existing. Thus there is no vector equality between both the 'order' pointer setting and what is shown on the simulated aircraft display. For this it is essential, that the aircraft time-base-signal is being feedback towards the Nachtfee consol. This sine wave signal is now directly fed onto the Nachtfee input (Impulsamplitude) at pin 1.  It has to be noticed, that for this occasion I have amplified the signal a bit by means of the first stage of the newly build amplifier module; using however, only the first EF14 stage and also first order differentiating the signal a bit at its output before it arrives at the List-Stecker pin 1.

Since the 'Phase' control lacking a setting-scale is making sense. The signals shown on the dual trace CRT (HRP2/100/1,5A) is exactly indicating  the correct adjustment of the Phase control setting. Both sharp edges have to be brought in line and 0° is assured. One important parameter, though, has yet to be incorporated, and this is the range or distance off-set. When this is adjusted appropriately, it is ascertained that both the 'order' pointer set at North and the blip on the aircraft display are pointing North. During the war this information was directly available inside the Freya-EGON cabin. 'North' is thus the referring vector, which is to be kept in concert!

We are now only watching the mutual phase-relation between the Nachtfee pointer and what is actually shown on the aircraft display. The  Nachtfee 'Phase' control is off-setting the Nachtfee data signal such that the pained order blip is at its correct vector position on the aircraft display (thus due North).

The operator on the ground has, however, a very responsible job, as he is controlling (compensating) their mutual phase difference. This can only be successfully accomplished as long as the operator knows their mutual distance- or range off-set.

The advantage is, that the aircraft crew is not being bothered with this kind of nuisance. What I have considered previously using an electrically controlled tuning-fork-oscillator still stands, because the Nachtfee phase control is only capable handling a phase difference between 0 and 360°. (Bladwijzer15) A quartz and divider chain as is used in the Nachtfee consol is out of the question, as it is too bulky, but is also too delicate to handle in respect to aircraft environment. The Nachtfee quartz-oven should run 24 hours. It possesses a separate 220 V ac connection; and even when the Nachtfee mains switch is set in its off position, the thermostatic oven is continuously running. Not in our case, as I regard this being too risky, when nobody is there!

(Bladwijzer17) Someone reported in 1945 that this system was vulnerable to interfering signals; as this would cause that the 'displayed order blip' would changing its information, thus coming out of synchronism, is absolute nonsense! Please consider: (Bladwijzer16)

The consequence of this principle is, that it is not necessary to transmit all the time a Nachtfee data signal. As what only is counting is that the actual time base signal phase in the aircraft is always, or very often, returning to the Freya-EGON ground station, where it can be kept in synchronism all the time. However, from another point of view, the air crew should be kept informed what the state of the system is. As we may estimate that the screen blip is kept at S (due south). Nevertheless, the Nachtfee 'order' pointer is all the time being kept at N (due North). For this purpose the Freya-Polwender selector is to be activated. On the Nachtfee control screens its action is is not visible (remember, we are watching the time base signal not the returning 'order' signal), and is to be trusted upon. I have tested it and it works!     

However, what I cannot judge yet, is what the implication is of the fact that the returning time base signal phase is also having to pass the same distance- or range off-set.

mmmm ... It may be worthwhile, to consider whether a kind of coincidence between both the time base phase and the North blip is making sense.  As long as there are two pulses returning the system is out of synchronism. Although, how to get it practically working is an open question.



On 3 April 2012


My new approach is, experimenting whether it is possible and watching that it would make sense, to mix both the FuG25a video signal with the time base signal. Being of different nature, this would only imply that we we have to cope with different signal slopes. However, both having a 500 Hz prf.


My aim in this project is keeping additional circuits simple and being of basic design


It worked instantly, the only thing demanding some matching was the time constant of the two input capacitors. Also the output capacitor was finally 1.2 nF (1200 pf). After some experiments the optimum solution was found to feed the circuit output directly at pin 1 of the Nachtfee List-Stecker (Impulsamplitude).


It was found that two signals were visible at the LB2 control screen

The returning time base feedback pulse is shown at 'quarter to' and the returning FuG25a detector output is shown at 'eight minutes to the hour' (vaguely).

Interesting is, though, that the signal output of the FuG25a is behaving like the feedback we have created some time ago, which is hardly responding upon setting of the 'Phase' control on the front panel. Both blips are of course responding onto the tuning or setting of the distance or range offset.


This is the block diagram of the new experimental setup


It has some in common as I have discussed previously (Bladwijzer18) the main difference is, that for obvious reasons I have omitted a coincidence circuit, as this could never having worked appropriately. For simple reasons, a signal is only returning when coincidence is taking place and one never know why. That is why I have considered a mixing* system, where both signals are returning to the Nachtfee feedback input. Although, we may not say returning in respect to the time base signal generated in the aircraft.

* After giving it some thoughts, I should naming it combing stage or module. Maybe 'combining stage' is more appropriate.

You may want to know whether it worked, yes it does very well!



When the FuG25a receiver blip is adjusted at North (N) and by means of playing between the two major controls 'Phase' and 'distance setoff' it is finally possible to tune the two steep edges of the dual trace scope in line, everything is then working correctly in the aircraft display simulation as well. This photo wonderfully shows how this should be accomplished. My feeling is that it works within an accuracy of less than 1°! It is even possible to monitor phase drift whereas nothing is visible on the simulated aircraft display.



The two pulse blips being both tuned North. The biggest one virtually in the background is the FuG25a receiver pulse blip and the smaller one in the forefront is the time base pulse. The a bit fuzzy receiver signal might be victim of the receiver wobbling of the FuG25a receiver, as to be sure that also frequencies being a bit off channel could be handled



Shown is the output of the new mixer stage when both signals are out phase

Next the receiver signal, as well as the simulated aircraft time base are being kept within synchronism; which is maintained by the controls on the Nachtfee front panel. Please bear in mind, that the mixing stage should be placed in the aircraft simulation, because its output should later be fed onto the FuG25a transmitter and being received by the ground receiver Gemse, which's signal output (video) should then be fed onto the Nachtfee feedback input.



This time both signals are controlled from the Nachtfee consol being 'in-phase'



Isn't it amazing that we have come so far, that by this rather simple means full control of the Nachtfee data transfer is being fully handled by means of the feedback system? We encountered, however, a steady about + 80° phase error

Why is a guess, but it may well be caused by the fact that the received Nachtfee signal at the output of the FuG25a has passed through several stages and the simulated aircraft time base is quite directly fed onto the feedback system of Nachtfee. I have already discussed that this kind of phase error should be neutralised by the fact that the returning time base signal from the aircraft has to bridge the same distance (in opposite directions) as did the Nachtfee data signal which was sent from the Freya-EGON station towards the aircraft under guidance; which is in our experimental setup not the case.


Finally a photo showing the experimental setup, I first would like doing it in a flying-wiring concept before putting it into an appropriate metal box eventually

This is in my perception the quickest way designing a well functioning circuit. It took very minor steps before the mixing stage worked appropriately. Of course, having a professional background is very helpful, as judging the layout and dimensioning it as well as expecting its interactions, is relying for a great deal upon more than fifty years experience.

The next move should be building it into an appropriate housing.



On Good Friday 6 April 2012

My first attention was drawn towards the TTi pulse generator, which sometimes refused functioning during the Nachtfee demonstration for Phil Judkins on 4 April.

My first thought was, that this may have been caused by the Nachtfee data signal due to some kind of overloading of the first triggering amplifier. I am still not convinced that this isn't today the fact. However, when being switched on again, and is fed from a different signal source it worked as may be expected. When the TTi pulse generator will causing future problems, I would like to implement a simple pulse forming circuit inside the HF pulse modulator box, so that the Nachtfee data signal is directly connected onto this module and the additional pulse generator is made redundant.



For this occasion I took some additional screen shots. The two returning blips (let us call them blips as was done equally in the Felkin reports, although what is shown doesn't fit to it)

The bigger, though a bit weaker, 'blip' in the background is the returning FuG25a receiver feedback signal.



For demonstrating how the LB2 CRT screen look like when the simulated aircraft time base is out off phase


The returning receiver signal is still holding due North because there still exist a kind of coherence between the Nachtfee time base and the returning (FuG25a) receiver signal. Be it, that by means of the distance or range off setting the blip is adjusted due north. However, its vector phase is still being in full synchronism with Nachtfee. The virtual time base in the aircraft is lacking any means of synchronism and has drifted away from their mutual aligned position (at due North).

It is found, that it takes over an hour continuous running before a certain kind of quasi synchronism is occurring. Often about 499,992/3 Hz, whereas the starting frequency is about, say, 500.15 Hz.



11 April 2012

On 7 and 9 as well as 10 April 2012


work continued with the aim getting the Gemse RX responding appropriately and then making the FuG25a transmitter sending its signal towards the Gemse receiver. Covering a distance of about 2 m. Nevertheless, simulating the full signal as well as the time base vector feedback towards the Nachtfee control screens LB2 and HRP2/100/1,5A 



The next move was building the experimental combiner or mixer stage in an Al. box


First I encountered problems with the Gemse receiver, as it had virtually no sensitivity. It proved to be necessary to replace the HF amplifier Rö 1 (RV12P2000)


After some considerations it started responding on HF signals also on the transmitted signals originating from the FuG25a on about 156 MHz



The simulated aircraft display is displaying Nachtfee 'orders' rather well



In the meantime I also found space for setup the Philips synthesiser type PM5193, set at: 500.167 Hz. Below the R&S SMS generator which is set at 124.0000 MHz

Which is providing signals from 0.0001 Hz up to 50 MHz. Significant to me is, in contrast to the simpler PM5190, that it has an up/down setting which allows going up or down in 0.001 Hz steps. Previously I had to enter time and again the full frequency parameters. Enormous clumsy, have you entered a 6 digit figure - it seemingly is too high or low. When you have done this during warming up maybe >50 times, one is most pleased with its stepping features! Thank you very much Paul and Marc for lending this versatile machine to us!!


The final system simulation is clearly visible, albeit, that now the combined dual signals are being transmitted by means of the FuG25a transmitter and is received by the Gemse receiver. Its video output is then fed back (feedback) towards the Nachtfee consol and displayed on both CRTs



The FuG25a transmitter is actually transmitting which is indicated by the neon indicator inside BG25a (control box on the right-hand side). The keying signal is coming directly from the combiner output stage and is fed onto the grid of Rö8 (keying stage, please refer to the schematic). Please remember, that the plug with the small screened cable is providing the FuG25a receiver signal onto the combiner module or directly onto the Z-input of our oscilloscope



The two returning signals are a bit fuzzy, because they have been passing through receiver wobbling. The most fuzzy signal is the returning Nachtfee order signal, which passed twice through a wobbling procedure. This is considerably distorting the signal content. However, the Nachtfee data painted on the simulated aircraft display is rather good visible


Interesting is, that the 'Phase control knob' on the front panel is only influencing the returning time base signal phase. It has very little influence on the returning Nachtfee data signal, as it still is being kept in a kind of system coherence.


I believe, that by means of fine tuning the circuit design as well as optimising the way the feedback signal is injected in the FuG25a transmitter may also improving the overall feedback blips


However, this simulating setup proved that feedback is an option. It may nevertheless proving that the optimal system is returning the time base signal phase towards the ground station. The today's experiments are clearly proving again that the aircraft display has to be synchronised from the Nachtfee system on the ground. There clearly is no way around it!



As to facilitate optimal experiments it was necessary to place the Gemse receiver next to the Nachtfee front panel, as this is the only way controlling fine tuning of the Gemse receiver and watching the results on the LB2 control screen. The returning signals are clearly shown on the LB2 CRT screen

A down side of this method is, that correct phase adjustment is becoming difficult as the signal wobbling and system noise is preventing from clear phase tuning at the HRP2/100/1,5A. It was also found, that the Gemse signal output had to be amplified before fed onto the Nachtfee feedback input terminals.

Experiments have to be done as to find out whether feeding back the time base phase only is an better option. However, we have already found that this is only possible as long as the operator knew exactly what their mutual distance set-off was. Without this parameter, Nachtfee never could have been operated!




On Friday 13 April 2012

I continued with improving the performance of the current experimental Nachtfee setup


After some experiments, as to improve the dual keying of the feedback signal, I first decided to wire an additional connection onto pin 8 of connector Bu1 of the FuG25a transponder. These are empty and the wire from grid 1 of Rö8 was managed just between the chassis and the screening cover plate. It was also successfully tested whether feeding back a single signal derived from the time base phase is the best solution.

This improves the reception of the time base pulse shown on the Nachtfee control screen. Let us first consider what the experimental setup is about now.


We have established an entire closed loop, where only a single pulse is returning towards the Nachtfee consol (the time base phase of the simulated aircraft display)

I know that the description of the mixer module should be changed, as we no longer have to deal with two returning signals. The presentation of the returning signal has improved enormously, not only because we have skipped one signal component, but also due to fine tuning of some of the stages, as, for instance, the Gemse receiver. Also significant is, that there existed a kind of pulse feedback inside the FuG25a circuit. Removing the Nachtfee data signal from the mixing or combining circuit is a great step forward. FuG25a screening cover being replaced and the keying circuit interconnected onto the pins 8 and its ground onto pin 6 of Bu1. 

I gave it a due consideration and a solution may be the fact that the transmitter driver stage of the FuG25a transponder is designed as to prevent for double pulsed transmission, of course, within about 2 ms. Whether this estimation is valid I cannot tell. 


The screened wire just visible near to the wooden base of the FuG25a testing rack is the newly introduced keying input line

We might have to experimenting as to find the optimal way of in injecting the keying signal towards the FuG25a transmitter. I would like to implement a potentiometer and watching what signal level provide the optimal results. 


We are viewing the full electronically conveyed time base signal phase retransmitted by means of the FuG25a transmitter, bridging a distance of about 2 m, nevertheless equal to the most likely means the time base phase was returning to the Freya-EGON-Nachtfee system during wartime. Using a first order differentiated pulse derived from the rising sine wave after it had passed through 0°. All data signals are being handled in the same manor, after it left the data-output of the Nachtfee console

For this occasion the range or distance set-off adjusted about 80 km and phase control aligned accordingly at the phase adjustment CRT HRP2/100/1,5A . Both the feedback blip is adjusted at North by means of the 'Phase control' on the Nachtfee front panel, which major setting has the fundamental influence of the correct 'North' of the simulated aircraft display blip. As to compensate for distance off-set, the according scale number (km) has to be adjusted as well. This information should have been provided by the Freya- EGON system. Under this condition both the returning feedback blip as well as the blip at the simulated aircraft display are pointing due North correctly.

It is also found, that even after several hours of operation an equilibrium was seemingly found. Though, it also is discovered that even with the unimaginable value of less than steps of 0.001 Hz full synchronism can not be achieved. For example, PM5193 set at 499.986 Hz proved over a bit longer interval (a bit more than a minute) being too slow - however - entering 499.987 proved to be too high. It is clear to me, that with 1940s techniques this hardly could have been coped with. The sensitivity of the adjustment dual trace CRT (HRP2/100/1,5A) is such that even within a few seconds phase synchronism is no longer in line, although nothing was seemingly happening with the blip of the simulated aircraft display.

Please bear in mind, that the Nachtfee Phase control is only capable coping with phase differences less than 360 degrees. Is the phase difference too high (due to changing stability conditions) an equilibrium is hardly found, as one is tuning the 'Phase' control constantly, thus hardly getting a stable alignment of the aircraft display at due North. Hence, the aircraft time base must have also a rather stable signal source; I still regard a 'tuning fork' oscillator being the most likely option.



For the first time it is possible to align the signal phase exactly at 0°. Please notice also the small curve which's slope is adjusted accordingly. We are looking at the phase alignment of the Nachtfee versus the returning time base signal originating from the simulated aircraft display



However we still face some kind of feedback from the transmitter keying

The only thing I understand, is that the 'quarter passed' blip is caused by keying the FuG25a transmitter. It is, nevertheless, not shown on the ultimate feedback blip on the LB2 control screen of the Nachtfee console. Maybe adjusting the optimal keying (level) parameter is solving this spurious signal.



Please bear also in mind my actual:  note book accounts(Survey report)


On 15 April 2012


It is just appears in my mind:



On 16 April 2012

I continued with firstly testing whether it is possible to use the transponder keying system parallel to the especially injected time base signal phase.

This worked rather well. What is shown is the received Nachtfee 'order' or command signal, like was created by means of the mixing or combiner module. Maybe even a bit clearer. It is always interesting to watch this aspect. The returning Nachtfee (feedback) signal is still coherent to the Nachtfee system. Whereas the arriving simulated aircraft time base (feedback) signal is behaving non coherent, and is liable to the tuning of the Nachtfee "Phase" control.


Because the closed system loop works well, I decided that time has come to replace the repaired HT transformer, which should provide the about 2 kV for both CRTs. As you may remember, we encountered some problems because the LB2 brightness control have been burned out twice. After having checked the modified wiring owing to the fact that the transformer connections have been changed, I couldn't measure HT. Next, I disconnected the RFG 5 anode connection, still no HT; though even some signs of overloading of the HT transformer occurred. The only option left was to disconnect the 6.3 V wires of the special filament transformer. Now full HT existed, touching a single disconnected filament wire onto the transformer caused a heavy sparking. No other possibility than that the transformer winding(s) is having a short circuit onto earth (ground). Although, I couldn't measure a fault by means of an ohmmeter. This transformer is providing three different voltages: 1 x 6.3 V for the RFG 5 rectifier; 1 x 4.4 V (4 V) for the dual beam CRT (HRP2/100/1,5A) and 1 x 12.6 V for the LB2 filament. I suspected this transformer already in the past, as I regarded this the only source of short circuiting the HT fed onto the LB2 g1 and k (cathode). However, we have to wait patiently until also this transformer has been rewound. Special care should be taken as to cope with the rather high voltages against their neighbouring sections (also versus ground). For instance: between the RFG 5 section we have to deal with say 4 kV pp and the LB2 is constantly kept at about 2 kV dc. This is the reason why they have adopted a special separate transformer. Bearing in mind the history, this short circuit may have been the reason why the HT transformer broke down some months ago without it was loaded at all.

I do not yet understand what may cause breakdown against ground of the 6.3 V windings, which is feeding the HT rectifier RFG5.


My first thought was, that the paper like insulation between the solder tags and the transformer core had broke down. To investigate whether my assessment was correct I have put some additional plastic sheets between the contact tags and the transformer body, again with negative results

During the course of eliminating all means that could cause overloading of the HT transformer I disconnected the top contact first. However, without result as well.


Finally we close today with a photo of the empty space transformer space

The series of modern capacitors on the right-hand side is a trial as to substitute the lacking original oil capacitor. Earlier experiments pointed in to the direction that it must have had a substantial value. Don't forget, that we have to deal with about 2 kV dc and several microfarads.



On 12 May 2012 


I continued with bringing our Nachtfee system to live again. Dick Zijlmans managed in the meantime that the defect special filament transformer is rewound. According to a hidden paper-strip: Isolierwandler T 3. Where T 3 means transformer number 3 (remember the 1.4 kV HT transformer carried T 2). Isolierwandler is expressing that its main function is to insulate the three filament sections (6.3 + 12.6 + 4.0 V) against ground, because they all carry rather high voltages against it. Where the 6.3 V windings is having in worst case 4 kV pp against ground and its neighbouring windings.

The next step was also to replace the brightness control of the LB2 control screen. In the passed this had been done twice, as these were, for whatever reason, blowing themselves up. After due consideration it was thought that this was due to a failure in this special filament transformer as well, where most likely shortage against ground occurred above about 1 kV loading against ground. Still keeping my fingers crossed.

Replacing the potentiometer was not quite simple, as the thread-length of the central potentiometer mounting was, like most types, too short. The only solution is to widen the centre hole in a tapered manner. Finally I got it right and the brightness control functions again appropriately.

In the meantime the HT has been increased up to about 1.2 kV under average loading.


Isolierwandler T3 being reinstalled (the left one, on the right the HT transformer)

The piled capacitors on the right are substituting the lacking smoothing C for the CRT HT system. I may have to divide them and wiring them in series of two, as to double the working voltage range. Because we finally would like to operate the system at about 2 kV as was done originally.


A bit on the side, I obtained from Rudolf Staritz in Bamberg two equal servo motors also known as Drehfeldgeber which are used in our Nachtfee system as being a three phase goniometer.


This servo type is about equal to the type used in our Nachtfee apparatus

Interesting is, that according to the label its original design was on behalf of Askania and that it actually had been manufactured by Zeiss-Ikon (likely in Dresden)



Looking now from the front section, where the (tiny) rotor shaft is visible This miniature device, may differ electrically a bit (although, I don't know), but when one of our servos (goniometers) may fail, this one is then most likely replacing the defect device



Alfred Breur had been so kind to purchase a second movable table on which now the Gemse receiver setup is installed (the metallic grey table)


On 14 May 2012

I continued with implementing some modifications


Please notice the two small added resistors

Its function is only to protect the HT transformer for overloading. I don't know the actual HT current, but guess that it will be about 5 à 6 mA. For this low value there doe'sn't exist fuses and a resistor will blow itself up when the voltage, thus its current, is exceeding a certain limit. The second free resistor is prematurely added as to allow a higher tap (80% of the full tension of 1450 V ac) on the HT transformer. 


The FuG25a is now fully operational on the replica of a test frame (Prüftafel)

Please notice the neon indicator in the control box BG25a on the far right. Which is indicating that actually the transmitter is generating HF energy. Although, I have not supplied HF onto the combined antenna, as to prevent for unwanted HF radiation.


Please notice also my Nachtfee project Notebook


In the final days of April 2012 I joined a meeting in Dresden, where I presented the state of our current affairs on the Nachtfee project. Hans Jucker opposed me strongly, and accused me from having a explicitly faulty understanding of the Nachtfee concept. Although, I have closed the system loop successfully, it was, according him, accomplished differently during wartime days. Knowing him for quite a while, this is forcing me to prove that my perception is the only likely way the Nachtfee system may have been operated. Of course, until better arguments is proving that my current understanding is a failure. Which I desperately would like being the case; as we then are coming nearer to the ultimate truth. I still feel some doubts about my concept perception, but I cannot yet find a better operational solution.

Nachtfee versus Hans Jucker


On 16/17 May 2012


I received a very kind e-mail from Günter Koenig

His contribution is worth to be noticed! 


Lieber Arthur,

wie ich sehe funktioniert die Apparatur wieder, nachdem der Heiztransformator repariert ist.

Bei all den Untersuchungen und Überlegungen denke ich, muss unbedingt in diese Überlegungen das "AIRBORN PRESENTATION UNIT" mit seinen wahrscheinlichen Bedienmöglichkeiten mit ein bezogen werden.

Die Darstellung in dem Übersichtsbild aus dem Bericht von 1945 zeigt ja eine sehr präzise Darstellung der an der Funkmessanlage vorhandenen Einrichtung, die Du ja nun eingehend untersuchst. Genau so präzise wird auch das FuG136 Gerät, die Flugzeuganlage, dargestellt sein.

Dort ist ein Schalter sehr deutlich hervor gehoben: "SWITCH FOR RECEIPTING MESSAGES" (Weitere dargestellte Bedienknöpfe sind nicht hervor gehoben).

Dieser Schalter ermöglichte somit die Quittungsabgabe zurück zur Feya-Anlage, dass das gesendete Kommando korrekt empfangen wurde, um genau zu sein, dass der Flugzeugoperator es ausdrücklich als empfangen quittiert!

Wie hat er das technisch voraussichtlich gemacht? Natürlich durch eine Phasenänderung des zurück gesendeten Signals!!!

Möglicherweise ist dieser Schalter gleichfalls ein "Polwender" ???  (Hypothese, eine mögliche Lösung).

ODER:  Möglicherweise wurde über diesen Schalter für eine gewisse Zeit die eigene "Bezugsphase" zurück gesendet, die am Bodenbedienpult dann die Nord-Position anzeigte und somit als Quittung verstanden werden konnte. 

Insofern wäre in dem Bockschaltbild deines Berichtes "On Friday 13 April 2012" diese Bedienmöglichkeit des Quittungs-Schalters der Flugzeuganlage noch mit ein zu bringen.

Das Quittungssignal könnte auch eine Erklärung dafür sein, weshalb der eine Zeiger am Kommandogeber offensichtlich ein "Merkzeiger" ist, den der Bediener jedes Mal nach einer erfolgreich quittierten Kommandoübertragung auf die rückgemeldete Phase stellt
oder , was wahrscheinlicher ist, auf die quittierte Kommandophase stellt, um auch hier den Abschluss einer Kommandoübertragung optisch dar zu stellen.

Sollte der Schalter im FuG136-Bordgerät eventuell auf die eigene Zeitbasis (Bezugsphase), also "Nord"  (möglicherweise kurzzeitig) gestellt worden sein, wäre es darüber hinaus denkbar, dass die Bodenbedienung dies durch Polwechslung auf "Süd" quittiert hat, also wechselseitige Quittierung des Übertragungsvorganges und Rückgang in die ursprüngliche Ausgangsposition "Nord" vor der nächsten Kommandoabgabe.

Wesentlich ist, und das ist unumgänglich bei diesem System, welches  mittels Phasenänderungen Informationen überträgt, dass immer wieder eindeutig die Bezugsphase erkennbar dargestellt wird, so dass gegebenenfalls "vom Boden aus" eine Phasenkorrektur vorgenommen werden konnte
und für alle beteiligten Bediener klare Darstellungs- und Bedienzustände gegeben waren . (Aus der Darstellung der Bord-Bedienanlage FuG136 geht ja ziemlich deutlich hervor, dass dort wohl abgesehen von der Kommando-Quittierung und der üblichen Darstellungsdrehknöpfen für die Oszillografenanzeige keine weiteren Bedienungen gemacht wurden.

Soweit Gedanken von meiner Seite.

Herzliche Grüße!

Günter, DJ8CY


On 20 May 2012

I received today the following answer on to my respons":


Lieber Arthur,

schönen Dank für Deine sehr ausführliche eMail, auf die ich im Detail bezüglich der vielen Unterthemen mit getrennten eMails antworten werde.

Ich möchte mit meinen Stellungnahmen lediglich den Blickwinkel, unter dem man diese vielen Fragen, die sich bei deinem interessanten Projekt ergeben, aus meiner Sicht zu ergänzen. Ich bin mir im klaren, dass das was ich dazu beitrage, nicht stimmen muss! Es sollte also grundsätzlich kritisch betrachtet werden und als
eine wahrscheinliche Möglichkeit gesehen werden, die ich natürlich mit Argumenten hinterlege, was man aber abwägen und insbesondere gegen Quelldokumente aus der Kriegs- und Nachkriegzeit gegen prüfen sollte.

Ein wesentlicher Punkt für mich ist die hohe Stabilitätsanforderung, die an die verwendeten Sytemuhren an Bord sowie am Boden zu stellen war. 

Mit diesem Thema war ich beruflich lange Jahre gleichfalls beschäftigt, als ich ein aus 156 einzelnen und räumlich verteilten Prozessrechnereinrichtungen das ZDF-Sendezentrum in Mainz mit moderner Prozessautomatisierungstechnik geplant und realisiert habe. Über dieses System konnte man nicht nur beliebige Bedienplätze im Fernseh-Studiobetrieb mit beliebigen fernsehtechnischen Maschineneinrichtungen, wie MAZ-Anlagen, Filmabtaster, etc. steuern, sondern genau so von zwei Zuspielmaschinen elektronische Schnitte auf einer aufzeichnenden MAZ-Anlage steuern. Die Anforderung an die einzelnen und in jedem Rechner separat laufenden "Uhren" und die Herstellung deren Synchronität auf Abweichung kleiner 50 ns war eine Herausforderung und wurde gelöst.

Das nur um meinen Hintergrund zu erläutern.

Unter  http://www.qsl.net/dk7nt/cro/croartd1.htm  findest Du auch einen Artikel einer Arbeit, die ich mit zwei weiteren Autoren 2001 im deutschen CQ-DL publiziert hatte, wo es um ein GPS-gestütztes Frequenznormal geht.

Kurz zu den Quarzen bei 15 kHz:

- es ist richtig, dass dies "Biegeschwinger", also longitudinal Schwinger sind. Es gibt diese Quarz-Schwinger im X-Schnitt, NT-Schnitt und XY-Schnitt. Der X-Schnitt zeigt allerdings einen ungünstigen Themperaturgang, weshalb deshalb bei hohen Stabilitätsanforderungen in der Regel der NT-Schnitt verwendet wird.

- der XY-Schnitt zeichnet sich durch einen besonders geringen Themperaturgang aus. Der XY-Schnitt ist allerdings ein "hochohmiger" Schnitt im Gegensatz zum X- oder NT-Schnitt.   

Es wäre interessant heraus zu finden, welcher Schnitt hier vorliegt. Die verwendete Oszillatorschaltung könnte hierüber Aufschluss geben.

Der Aufdruck auf dem Quarzgehäuse benennt es leider nicht. Die dort angegebene Toleranz ist übrigens die Fertigungstolleranz, mehr nicht. Ich bin mir sicher, dass die Quarze letztlich genau über Bauteile in der Oszillatorschaltung auf die korrekte Frequenz mit einer Genauigkeit in der Größenordnung von 10 ^- 7 eingestellt wurden. Da werde ich allerdings noch mal in einer getrennten eMail im Detail darauf eingehen, um den Hintergrund zu erläutern.

Was mich bei der Gelegenheit interessiert: was kann man mit dem Drehknopf, der an jedem Quarzoszillator angebracht ist, einstellen?

Soweit erst mal ein paar wenige Infos von meiner Seite.


Günter, DJ8CY


Everything where we are dealing with is having a perception which may prove finally being a mistake or an error. That I have had incorrect estimations is evident and is well documented in the course of this (survey) report. It is inevitably the consequence of being engaged in a project where there is no technical information around and where we, especially in the beginning, were often unaware what the technical implications of Nachtfee is about.   



I would like to pick up one of Günter's suggestions that I should add a system drawing where the acknowledging pulses or eventually EGON signals can be switched on and off.


Recent findings (New Findings) tells us that the 'orders' nal1 and nal2 were meant for acknowledging by means of switching on either FuG25a channel 1 or 2 on the FuG25a control unit of the IFF set FuG25a. When his is being activated the Nachtfee signal is returning to the ground station, where it both is being fed onto the EGON measuring system as well as onto the Nachtfee control screen (LB2)



Please consider for details the New findings page



On 4 June 2012


I decided to clean up all the rubbish made during the course of all the passed experiments. It is also time to remount the Nachtfee cover plates and to rearrange the total display setting.

After the cover plates have been brought back at their original place (leaving, however, about half of the plate screws out as to ease the fact that now and then a cover plate have to be removed. After the plates were fitted again, I switched on the Nachtfee system and would first check whether it works. Now various units were being mounted elsewhere watching all the time if the system still functions.


On the left the Nachtfee system with on the lower shelf the R&S SMS signal generator, which is set at 124 MHz. Next to it the TTi pulse generator which is being fed directly from the Nachtfee data output. The pulse output is then keying the HF modulator signal (mounted at the upper left-side table leg). Which is then being received by the FuG25a IFF set


On the left the FuG25a testing rack and on the lower shelf its power supply. Next to it the aircraft display simulation. On the right of it the PM5193 digital time base synthesiser. On the lower shelf the Variac acting also a mains isolator (Trenntrafo)


The received Nachtfee signal pointing about North. The bright pulse at 'East' (90°) is caused by keying the FuG25a transmitter


At the same time the returning time base signal originating from the simulated aircraft time base. This is typically showing that synchronism is just right!


Shown the two control screens adjusted correctly together with the illuminated (small) scale of the 'range off-set' adjustment. Which is a 'key control' as it compensates for the range distance between Nachtfee and the aircraft 'en route'. Please notice the pulse shown on the LB2 control screen, which shows the differentiated signal received by the Gemse receiver. That is transmitted by the FuG25a and which originates from the simulated aircraft-display-time-base



On 20 June 2012

Some photos taken during new experiments, where I monitored the Nachtfee recent frequency stability versus time Please notice also:  Nachtfee quarz



Straight from from the beginning in November last year the dual beam CRT type HRP2/100/1,5(A) or HR2/100/1.5 was for safety reasons leaning upon an 'india rubber' device; as otherwise we feared that the heavy socket could break


During the 35 Years Anniversary of the NVHR in Egond aan Zee, I met up with Jaap Keijzer and we discussed whether there is a solution making something which could carry or support the rather clumsy HRP CRTs? It must be a bit universal, as there exist various types; electrically equal, but not mechanically. He came instantly up with a proposal which materialised finally last week. It is made of 'ebonite', which is giving it a grey dark vision. As we have no idea how things looked like in the wartime days, its functioning as well as not being too much modern is making it a good substitute. I believe, that it is highly unlikely that we ever will find the originally used cover-window. Thank you Jaap! He also promised me to look after an adapter for our Würzburg Power Meter. Please notice the page on: Würzburg Repair


Screen shot taken during the quite long lasting quartz frequency measurements. These were maintained against the PM5193 synthesiser. We paint a Lissajous on the simulated circular aircraft display, and received the Nachtfee signal which is displayed by means of Z-modulation. The movement of the light spot is indicating the direction and speed of movement; showing whether we deal with retarding or advancing signal phase. When all was warmed up changes (movements) appeared very slowly. We count then in minutes. A kind of equilibrium was more or less reached, but never absolute stability was encountered

All is adjusted upon the Nachtfee internal reference against 'phase 0°. The small about 45 degree line is indicating that we have exactly adjusted upon 0°!


Viewing the Nachtfee front panel during frequency versus time measurements. The left-hand side CRT is showing 0° adjustment, which is in accordance to the LB2 Control screen. The simulated aircraft blip pointing due North is clearly visible. In my hypothetical reconstruction, this guarantees that the Nachtfee 'order' is pointing due North on the simulated aircraft display as well. This is only true as long as 'Range offset' is adjusted properly 


To be continued in due course

Please also consider my new Nachtfee operational hypothesis

 As well as the new

Nachtfee Quartz page


By Arthur O. Bauer

On 2 June I have started a new experiment, in which I would like to investigate whether our 1000 Hz tunigfork is substituting a tuneable 500 Hz tuningfork.

Nachtfee Tuningfork experiments


Please consider also our addional page on the implication of Freya-EGON and Nachtfee


Since August 2012


Please don't forget to use the handsome: Nachtfee Chronology page


And, the PowerPoint progress page (converted into PDF)






Please notice also our recent new discoveries: Nachtfee new findings (Status: 12 May 2012)

Please go back to, or proceed with: FuG136-Nachtfee starting page! (Status: 5 March 2012)

Please go back to, or proceed with: Nachtfee survey page 2 (status: 8 December 2011)

Please return to, or proceed with: Nachtfee survey page 3 (status: 21 December 2011)

Please return to, or proceed with for the survey pre-phase to: Nachtfee 3a (status 8/1/2012)

Please go back to, or proceed with: Nachtfee MLK Lab. Survey (status: 13 December 2011)

Please go back to, or proceed with: Nachtfee-Inbetriebnahme (status: 5 March 2012)

Please go back to, or proceed with: Nachtfee evaluation and conclusion page (status: 1 March 2012)

Please continue or proceed with: Nachtfee-FuG25a concideration page (status: 10 March 2012)

Please go back to, or proceed with: Handbooks papers and product information