Evaluation and Conclusion


Page created on 20 January 2012

Status: 23 June 2012


This final report is in progress

and is meant to explain what the Nachtfee technology is about. Though, also discussing pro and cons from newly accessed references. However, its ultimate purpose is also drawing some conclusions



We are now about to evaluate the findings and discoveries though, ultimately also drawing conclusions. The latter doubtless leaving aspects open, as after 67 years have been passed  since, and the Nachtfee project was (only) a preliminary  experiment. That we ever may find the conclusive files is very unlikely. Never mind, putting together all the facts, sometimes fit together from tiny bits and pieces, we have come further than anyone could have predicted some months ago!


It seems, however, the more bits and pieces of information is seeing daylight, the more open questions we have to deal with. And each of us have to judge the provided parameters, and bringing them in line with our personal understanding (judgement) of what likely was possible in this wartime period.




Michael Svejgaard was so kind to cooperate and, although, not much of technical nature, it provides the state of Britsh Intelligenge just before the conclusion of the war in Western Europe. The sources on which A.D.I. relied originated from: captured German documents and equipment, PoW interrogations and from espionage sources; all filtered and judged by R.V. Jones' office.


On page 17 we notice the following information:


To my understanding, this paragraph on page 17 is clearly pointing at Nachtfee

The Germans have used various methods of communications from ground-to-air in connection with Egon control. Ordinary R/T was seen abandoned owing to our jamming and W/T was substituted. From several prisoners' accounts it appears certain that they are now using a method of coded pulses. The signals are displayed on a cathode-ray tube in the aircraft. The display part of the tube has a circular time base divided into 10 sections numbered from 0 to 9. The blip normally shows opposite 0, but when a signal is being sent it moves to a position opposite  one of the  other numbers (like is accomplished by the Freya-Polwender switch, AOB). Number 1 to 3 are usually used to indicate which of the aircraft controlled is to obey the order. The other numbers give course corrections or the bombing signal. (such as 'Rolf' or 'Lisa' which was standing for right or left 10°, AOB) (part of  A.D.I Report 101 page 7) (Bombing command was called: Pauke)

It may give us the clue wherefore the 'Freya-Polwender' switch is implemented in the Nachtfee controlling panel. It rotates the signal phase 180° (π). A marker being at, say, 0° (zero degrees) will at once pointing at 180°. Though, also when the 'order' marker is set at say, East it will rotate to West vv. Please consider our graphic explanations lower this page.

We have already discussed the likely application of the ominous 'Freya-Polwender' switch; of which until now I have omitted calling it a selector. Because this implies, that its setting is causing a wanted phase change. There exists, nevertheless, a particular wiring, and the output connections are numbered at the List-Stecker (edge connector) accordingly. When they maintained this wiring (phase) order up to the stage where the signal is being conveyed (being transmitted towards the aircraft under guidance), then we may go for the idea that the front panel text 'Freya-Polwender' is also pointing at its direct purpose; and that is: rotating its signal phase over 180° temporarily - thus bringing it in its waiting position. Hence, the switch-handle is to be pointing upwards in this case. Of course, this is only my guess, and maybe an omission.



The next paragraph on the same page 17 and top of page 18

It continued: We have listening on two occasions of a Freya on about 125 Mc/s giving a second pulse. In one case the Freya was known to be an Egon Freya; the second pulse was phased at a constant interval from the normal pulse and the two blips were of equal size. The latter point excludes the possibility  of the second blip being the permanent echo. In the second case the phase difference of the two pulses varied rapidly. It appears highly probable that the second pulse in each case was in fact giving a signal of the type described above.

To me it isn't clear how the Nachtfee signal we have determined would fit in this latter system.

My understanding of it is: that the Nachtfee data signal is of sinusoidal nature and its signal phase is to be watched in the time frame (time domain). Because both the actual signal phase in the Nachtfee system as well as the CRT display in the aircraft should always staying in concert. Otherwise it is uncertain at which instant both time base phase is. However, a down side of FuG25a Erstling is, that its transmitter is responding with 300 - 400 W pulses. To my understanding, the technical layout of the FuG25a transmitter power supply was such that it could not cope with continuously running. This implies, that the Nachtfee signal is by whatever means most likely transmitted in a kind of packages. How this might have been accomplished I do not know. Although, another option would have been: when the Nachtfee signal was transmitted continuously and that in some way the Erstling transmitter was prevented from responding continuously. 


Viewing the way our Nachtfee apparatus has been constructed hand made, and particularly the experimental nature of it, my guess is: that the latter paragraph is dealing with another system. It may well have been possible that they have experimented with a different technique. Hypothetically, maybe still relying on a (more or less) similar aircraft display unit.

Another, not earlier appearing in my thought, is that Hans Jucker as well as Helwig Schmied pointed time and again the limitations of the regular FuG25a. Though, are we really dealing with a regular FuG25a Erstling? No, we don't! The to FuG136 adapted (converted) IFF set had a separate receiver output, which provision didn't exist in the regular FuG25a. Isn't it possible that they changed the way the Erstling transponder worked a bit, as to fit to the Nachtfee signal concept? In my perception: regarding the many open queries, it is most likely they actually did!

Nachtfee provides only a single signal. Thus, when Nachtfee was kept in concert with Freya, there exist in this system no means to split Freya from Nachtfee. Or, Freya operated independently from EGON-Nachtfee. In this case, both Freya and Nachtfee signal phase might have 'walked slowly away from each other'. Nevertheless, British reports speak about Freya having an extremely stable prf source which is normally 500 Hz.


On 21 January 2012

Continuing our evaluation



Watching a single cycle of a continuous sinusoidal wave, actually having about 500 Hz. Constituting the Nachtfee data output (please, remember the influence of the selected quartz controlled channels versus the Nachtfee signal)



As to visualise what actually is occurring when a feedback signal is displayed on the control CRT (LB2).

The rectangular field represents the virtual window within the returning feedback- or controlling signal is made visible



The 'order' pointer is rotated and set at East (O).

The dotted line is only showing from which position the 'order' pointer has been rotated. The virtual window has moved 90° π), which equals the vector setting of the 'order' pointer below



Both pointers are set at East (O).

The larger one is not connected onto any kind of command. In my perception, it might have acted as a kind of mechanical memory (the right disk is connected onto the memory pointer, the left one having only a single lock, which actually is its 'North' (N) point setting; and is connected with the 'order' or phase shifting servo (Drehfeldgeber) on the far left hand side)


'Order' pointer set at 'Pauke' (S) equal to: 'Attack', 'Strike' or 'Go'



'Order' pointer still set at 'Pauke' but the Freya-Polwender switch has been operated

According the previously noticed A.D.I reference, where in the waiting for commands stage, the actual marker (signal) painted on the aircraft crt screen, is virtually rotated 180° (π). Only when a new command is (soon?) to come their way, the pointer moves into an opposite position. To my understanding, is North (N) the position from which the 'order' or command pointer was starting its vector rotation. This is the only mechanical lock (Rast) available in the 'order' phase shifting system, whereas the, what I call memory pointer, is having a mechanical lock (Rast) at any designated sector of the 'order' compass. (please notice the codes like: '1R' or '1L' or 'knh' or 'fad' or 'kus' or 'jjj', and so forth.

Just a brainwave: putting all we know together, wouldn't it have been possible that signal towards the aircraft was sent only during the virtual window, which I have introduced in the above drawings?

This would solve quite some of the open questions. The Erstling transponder (FuG25a) is getting say every 2 ms a few µs wide signal pulse. Actually, the only thing that matters is a triggering pulse, which indicated at what vector the 'order' pointer is being set. Maybe, for those days a bit too demanding technology.  


There are still a lot of open questions remaining


Recalling only a few of the many queries bothering us


For better understanding it is at least necessary to get an idea how the Nachtfee circuitry was designed.


Let us first start with the quartz and frequency divider unit


The block diagram of the Nachtfee quartz and divider unit

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

This stage is very important, as it controls (generate) the Nachtfee data output frequency. Which is about 500 Hz and depends on the selected quartz channel (1 out of 10). Quartz channels are each separated successively in 60 Hz steps. Q5 constitutes the centre frequency and is 15000 Hz. Which provides, after division by 30, a 500 Hz output signal. Consequently, each quartz channel step generates 2 Hz frequency shift.

Please notice also (Bladwijzer45), which is showing the principle schematic of the quartz oscillator plug-in module


What the implications of these tiny frequency deviation is we don't know (yet?). It was shown on the Nachtfee control CRT (LB2) that between Q1 and Q8 the vector or 'order' marker rotated about, say, 13 degrees in a negative sense (anti-clockwise).

The quartz oven is thermostatically controlled and kept at 60° C. The temperature is only known, as this data is mentioned at the quartz crystal housing. When viewing this photo, please notice the mercury thermometer on the right hand-side. Which has 2 Pt contacts for switching off when temperature is reached.


On 27 January 2012

We continued


The next evaluation contribution is the block diagram of our Nachtfee apparatus


The preliminary block schematic of the Nachtfee apparatus. On 30 January it is modified

Please double click on this schematic as to open it in pdf



The post war Felkin report is rather vague and may be of minor value in this retrospect. (Bladwijzer113)


102.    Nachtfee was the term used to describe a system for the transmission of control instructions to a pilot in the beam of a controlling Freya through the medium of a C.R.T. indicator similar to the Liechtenstein range tube. The airborne equipment used was known as FuGe 136 (FuG136, AOB) and weighted 12 kgs. 

103.    The original purpose of Nachtfee was to provide a solution to the jamming of R/T control systems, and it had been used operationally by the Pathfinders of K.G.66 for laying mines Scheldt estuary*. It was to be adapted for the use by night fighters to overcome our jamming of the night fighter commentary.   

* This report jumps within this paragraph between February 1944 and to late 1944. The Allies suffered a lot of problems getting Antwerp harbour operational again. As the Germans kept for quite a long time control over the estuary, which is know in Holland as "Westerschelde".

104.    Nachtfee was a pulse system using the FuG25a as receiver.

105.    The presentation screen in the aircraft was inscribed with the various both on the inside and outside of a circular time trace. There was a stationary zero blip in the 12 o'clock position, and when an instruction was radiated the pulse received caused a second blip to emerge from the first and travel round the C.R. tube in a clock-wise direction, presumably by a slight change in p.r.f. This blip came to a stop and a the command corresponding to its position could be read off on the inside of the time trace where a total of about eight different instructions were inscribed. It took from one to two seconds for the blip to travel around the tube.

106.    It was thought that the further eight commands inscribed on the outside of the time trace were denoted by the blip making a complete circle and starting round a second time therefore coming to rest at any one of the eight sectors. In this way a total of sixteen different orders could be given. For night fighters such instruction as "Turn left/right", "dive" etc. appeared on the inner side and figures for transmitting bearings on the outer side of the trace.


107.    This system was used operationally, but it was found that, apart from the susceptibility to jamming, other pulse transmissions could interfere by unlocking the system and thereby cause wrong positioning of the indicator blip. Nonsense, by the way, AOB! Please notice the following link (Bladwijzer17)

108.    Another profound objection to Nachtfee lay in the fact that that it was necessary for a member of the crew to watch the C.R.T. indicator uninterruptedly, for missing one blip indication might give the message a false value. It was not therefore possible to use it in a single-seater aircraft

 So far, A.D.I.(k) Report No.357 of 1945, page 11 Please notice the full transcription of this report

In its foreword, they also give the limitation of this report:

The following information has been obtained from P/W. As the statements made have not yet been verified, no mention of them should be made in intelligence summaries of commands or lower formations, nor should they be accepted as facts until commented on the Air Ministry Intelligence Summaries or special communications.

First, that the many Trenkle publications were heavily relying on this source [101].

Second, that the CRT inside the aircraft cabin was similar to the one in our Nachtfee apparatus, as Liechtenstein b/c used a similar type (LB2). It also proofs, that the writer of this report did not had a good understanding of Lichtenstein radars. The already in late 1943 becoming obsolete Liechtenstein b/c he referred to, was proceeded by Liechtenstein SN2, which did not use a circular range CRT.

However, maybe significant is the suggestion that the Nachtfee used eventually a different prf. Hence, a different quartz channel might have been selected.     When we consider that the aircraft circular time base was, by what ever means, kept rotating at a certain (constant) frequency. What would happen when the ground signal being displayed at their CRT screen was having a different data frequency (rotation frequency)? We have already noticed, that in our coherent system a vector rotation is being caused (the notch was set at North for Q1). What would happen when there does not exist coherence? The Nachtfee signal may well have caused for a next quartz channel two rotations more or less per second (each quartz channel caused 2 Hz frequency difference). Doubtless, the marker or pointer blip will make rotations, or at least will move against each other.    This might give us the clue why quartz channels have been implemented into the Nachtfee system!   

Having Nachtfee at hand, an seeing how the 'order' or commands were likely to be given, I cannot understand how the available gear can give a dual command? May this have been the reason for adopting the extra large pointer, which only provide a means of visual memory? We may assume, that the Nachtfee signal was as is shown in the first five line drawings, how would it have been possible to switch from an inner to an outer marker blip?    To my understanding, the feedback signal is derived from  a first order differentiated pulse, whether negative of positive is not important. Such a pulse should create a kind of window, limiting the size and polarity of a painted marker window. How could a ground signal causing a polarity switch-over, without coherence?

In this respect I trust the in the beginning of this page sited A.D.I. (number not yet known) where they mention that the pointer or blip rotated over 180° (π). This is in full accordance to the existing "Freya-Polwender" switch or selector in our Nachtfee apparatus.

Another interesting aspect placing our Nachtfee apparatus in a time frame: I found on some of the "Jahre" capacitors '4.44', which clearly tells us, following German practice, that this component had produced or have been accepted in the factory in April 1944. How long would it take before all components had been assembled, and all had been eventually made Nachtfee? In this respect, it is also interesting that some of the original Nachtfee valves carry acceptance date between March and November 1944. Our apparatus was a late version, whether equal to the early version, we don't know.

Also wondering: we have found quite some references on Nachtfee, that one may get the impression, that in some way it was a regular operational GAF system. Apparently it was not.

Already discussed previously, the nature of our Nachtfee apparatus is completely hand-made. No military components were used. No wire is having an accompanied information. It was, and maybe is, German practice to give every wire (cable) a number; as well as all points where it is connected onto (Potentialzahlen). In our case this information is totally lacking. Clearly a sign that it was an experimental (GAF) project.         


During an e-mail exchange with Hans Jucker, I realised that it does make sense trying to simulate an aircraft display.     What we need is an oscilloscope by which means we paint a lissajous figure on the CRT screen. When both signals have a phase difference of 90° it is possible, when both signal amplitudes are selected appropriately, painting a circle. Maybe first providing the Nachtfee signal by means of Z-modulation. A whole range of new experiments is to be undertaken.

Dick Zijlmans managed to obtain for us a Philips LF synthesiser, which I would like to employ as an external  frequency source.


On Monday 30 January

We proceeded with the experiments


Before moving our old Tectonics scope I tested it, but it failed showing any trace at the CRT screen. Not wasting time, as a substitute I took an old Grundig oscilloscope. But this one also failed after a few minutes running (we should use them more often). Then I took another Grundig scope which worked fine, but it has no Z-modulation provision. Luckily our HP scope was quickly setup for the new task.


Before it is possible painting a Lissajous figure by means of the X and Y channels, it is necessary to create a 90° phase shifting network.


For this experiment my aim is: keep it simple

It might sound strange using a 120 kohm resistor, but this value was empirically found the optimal value. The reason is that we have to cope in the way the Z-channel is interacting on an external load and/or signal.

I firstly thought of an all-pass network type, but this is not necessary as a simple dual R/C combination is doing what we need. Today we used the less stable Wavetek generator as Dick Zijlmans should bring our recently purchased LF synthesiser next Wednesday.


After adjusting the amplification of the X- and Y channel and some fine tuning of the two pot/trimmers this circle is shown. That some distortion is left, might be due to distortion of the Wavetek output signal. Or is being caused inside the HP scope, then likely within the X-channel


Our next step was finding out what the best way is to supply onto the scope the "impulse feedback signal". This is the same feed back as is used in our Nachtfee system.


This is what we see through the camera when a different quartz channel is selected

 It is found, that the Nachtfee feedback spot rotates anti-clockwise when a lower quartz channel number is selected; and rotates clockwise when the channel number is having a higher number. I took as centre frequency channel 5 (= 500 Hz data output frequency). The rotations are visible, maybe not directly countable due to its rotation speed.


The luminance is set such that only the most bright spot is just made visible. What the marker spot is not very sharp originate from the small frequency drift of the Wavetek signal generator versus the Nachtfee apparatus, because it rotated slightly during exposure time


A different screen setting. The spot looks like broad but this is not the case. Again the relatively long camera exposure time is distorting the observation

We should need a video camera, which we do not have.

The dark section of the circle originate from the remaining, but weaker, positive first order differentiated feedback pulse. In the final setup it should be removed by means of clamping (Klemmung)


It is thus now being proved, that it is possible to convey Nachtfee signals towards an external display unit.

Maybe difficult is keeping both rotating CRT spots in concert (synchronism)

I also tested whether there exists a link between the "grade or number scale" goniometer and the data output phase, which was not the case; as I had already expected. More astonishing for me is that the "Phase" goniometer which is the first one in the signal chain, does have hardly influence on the Nachtfee control screen (LB2) though, it is possible to rotate the data output signal over 360° (2 π) which is shown on the simulated aircraft CRT screen.

Of course, changing the Freya-Polwender selector is having the known effect of 180° (π) pointer or marker rotation.

I have no idea how I can paint a marker or signal pointer on the simulated aircraft display screen yet. The best solution might be, using a circular CR tube in Germany known as: Polarröhre. We do not have one, and first swopping a circular deflection unit including a CRT and then building a working set - I am simply lacking time and, maybe spirit, for it.



The The principle of circular deflection CRT (Polarröhre), like our LB2. Virtual trace with marker or pointer blip in green colour

These CRT types are magnetically deflected. The two tapered (conical) cylinders act as circular (vertical like) deflection system.

It is evident, that deflection in the way is graphically shown is in favour of Z-modulation.


I used also the opportunity finding out from where the power deflection stage Rö14 and Rö15 is being fed from. Not directly expected, it is directly linked onto the grid circuit of Rö5 (acting as a triode). This may be the reason why the number servo is not influencing the data output at all. But does definitely influence the LB2 control, as well as the dual beam horizontal deflections.  The schematic block diagram above is accordingly modified.



In this respect we have to consider some facts which we have encountered currently. The Nachtfee divider stages does not work appropriately before some time have passed (call it warming-up). This aspect should be noticed before we may think of what kind of technology was employed in the FuG136 aircraft system. Some are believing that they have used quite complicated techniques. I doubt, as, already for practical reasons, most aircraft systems used rather conventional technologies. 'High Tec' in a sense - how we would have solved it from today's vision, is most likely out off the question. The total weight of the aircraft system was 12 kg only (such data is often incorporating the quite heavy power generator) . Space to build-in these kind of additional things was very restricted. All my current objections - is still not solving the fundamental open queries. But may limit the possible techniques that might have been used. It should have been kept, more or less, simple.

Aircraft is a hostile environment, where temperatures may be varying between , say, -25° to +40 degrees C or beyond. Rather strong vibrations have to be countered as well. Humidity is also a major factor of concern.

After having simulated the way information was being showed at such an aircraft display, the most important conclusion is, there is no way that such a system could operate without tight synchronism maintained from (by) the ground signal!

A 500 Hz signal represents a period duration of 2 ms. As is shown above, what is needed is only the pulse generated within the, what I call, window. This pulse may be a few hundred micro seconds wide; an existing  technology in those days. How could they have kept synchronism within this context? Nachtfee uses quartz resonators, but challenging the 10 quartz channels they should have operated a hole bank of these still critical components, which hardly may be regarded "shock and environment proof".

Another option, be it hypothetical, however, not yet dealt with is: why not using a tuning-fork? The Rohde & Schwarz portable quartz clock CFQ used one as a tuneable (controllable) signal source at 1000 Hz.  By means of an additional coil it is quite simply possible to tune the fork a bit up or down. Tuning forks were also used after the war for special applications, such as for facsimile transmissions. I know from experience, that for decades photos were nationally as well as internationally exchanged by photo-senders. These signals started with a certain period of sending a 'zero level' signal and also giving just the intervals where the beginning of a photo picture (line) starts. During the transmission over often say 6 or more minutes the tuning fork circuit had to maintain a high degree of (line or drum rotation) stability. Of course, it is my guess, but a realistic assumption. In my perception, a far more realistic option, then trying to tune quartz crystals or other means.

However, it has been pointed in the foregoing discussions that the Germans were capable designing selective filters. What you need is a discriminator circuit, a then very well known technique. Such a circuit provides a controlling signal which polarity is designed as to tend to zero (AFC). Thus, when the frequency or phase difference is passing a level the controlling voltage forces it to become null (simplified), whatever the (direction of) deviation is. Of course, as long as it is within the system capture range.

Some report shown above, points that the pointer blip could be facing out- or inside. This is quite simple to maintain as it only depends on the way the feedback signal is being modulated at the Nachtfee carrier; thus creating a positive or a negative pulse. Which we have done during some tests ourselves. Whether this option is according the facts we don't know. As this particular reports relied heavily upon PoW interrogation. A fact that was found maybe suspect. The way this episode is being described is for me not convincing (please notice 105. and 106. on this page).

As have been pointed above, the FuG25a was modified becoming FuG136 by means of an additional receiver output. This signal output should provide all the necessary Nachtfee signal parameters. It is, in my perception most likely, that the FuG25a had also been modified as to fit optimally for this task. Nevertheless, allowing the FuG25a to operate as a regular IFF transponder as well.

However, we may think of preventing that the Nachtfee signal is activating the IFF transponder. Maybe, maintaining for some time-frame a constant (data) carrier. I was also told, that FuG25a does not respond on these kind of continuous signals though, only upon pulsed (IFF like) signals.

A recent discussion was the implementation of a bank of narrow filters. But Nachtfee provides 2 Hz successive channel steps at about 500 Hz. Although, the Germans were good engineers, designing successive filters with 2 Hz channel separation at about 500 Hz may have been "a bridge too far" from reality; particularly in respect to the hostile avionic environment.


On 1 February 2012

I continued with simulating how eventually a CRT in a German Pathfinder aircraft might have responded onto a Nachtfee 'order' or commanding signal.


As have been many times before, things are different than what anyone before may have thought!


As was promised, Dick Zijlmans turned up at the end of the morning with the Philips LF synthesiser type PM5190X (1 mHz - 2 MHz).


We did first some work on replacing the brightness control of the LB2 CRT. (notice Inbetriebnahme)

After a not too long while, say 5 - 10 minutes, the quartz oven was by far not yet heated up, synchronism was more or less established between Nachtfee and the synthesiser signals. Referring  to channel Q5 the Z-marker was showing minimal phase difference when the synthesiser was set at 500.15 Hz (spot staying for some time at equal vector). After a period this moved to 500.125 Hz, then slowly becoming 500.1 Hz (keeping herewith the marker quasi stable) . This was during the measuring period before the quartz oven began switching on and off, thus when it had reached its working condition.

A very significant fact was noticed: that when the "Phase" control on the front panel was tuned - that a slight phase drift was being introduced. My guess, this may have been owing to slight change of the goniometer electrical loading. Still bearing in mind 500.1 Hz it had moved downwards to 500.09 Hz.


Brightness of the Z-marker adjusted such that only a single spot is visible (originating from the coincidence of the Nachtfee feedback signal)



A new phenomenon was found: when changing Q5 for the lower quartz channel Q1, the screen vector rotated clockwise, whereas changing to Q8 it rotated anti-clockwise. My guess, its direction may be caused by the way the 90° (½ π) phase shifted signal is interacting as to obtain our Lissajous figure.

After a some time vector stability was obtained by 500.08 Hz and then 499.99 Hz stabilising finally at about 499.97 Hz.

However, whatever figure we give, phase synchronism was never fully obtained!


Measuring set up. Synthesiser set at 499,97 Hz. The sine wave is coming from the synthesiser and the pulses derived from the Nachtfee feedback (signal). However, phase synchronism is best noticed by using a Lissajous figure in combination to the Nachtfee feedback pulse


A next step was, after we considered that synchronism is hardly practically possible, whether it does make sense using the Nachtfee signal instead. Beside the nuisance from not having a un-distorted sine wave, preventing a circular projection, it doesn't work! The solution is rather simple: Nachtfee is constituting a phase shifting signal, and using the Nachtfee signal for deflection is never constituting a phase shift at all!     We need thus two time base systems, there is no way around it! Or, Nachtfee should have provided an additional synchronising signal, derived just after its third frequency divider (Rö2 in the block schematic above). But this facilities didn't exist and the Freya-EGON system should have transmitted two independent signals!


It is our intension to show Nachtfee during the 35 anniversary of the NVHR, Dick Zijlmans, forced me to explain how Nachtfee worked; acting as if he is one of the audience.

It may sound silly, but this is a very relevant point. I therefore had to explain it as if he never has heard of Nachtfee.


The upper sine wave is originating from the PM5190 (apparently we have to look why its signal is too much distorted, which clearly is changing a bit during the course of operation), the lower one comes from Nachtfee

Both signal phase were adjusted in line, here, because it is the only means, by setting the "Phase" control on the Nachtfee front panel. This situation is where operational Nachtfee have to starting from (see further down some considerations). We may also ask how they knew what the implication of setting in the aircraft was?


North (N) painted at the Nachtfee control screen (LB2)



'Order' pointer has been set at East (O)



Watching the Nachtfee control screen at East (O) (LB2)


'Order' pointer set at South or "Pauke" (S)


'Order' or Commanding pointer set at South (S) or Pauke


Please don't take their mutual phase relation as being absolute, as the Nachtfee signal versus the synthesiser phase have drifted in the meantime!


It nevertheless is giving a good impression what the nature (implication) of Nachtfee is. It also tells us - that mutual synchronism is a highly delicate matter. So far, no one has ever discussed this nuisance seriously before.

My guess, the fact that the waiting 'order' signal was set at south, was an important system parameter. It gave the operator the possibility to maintain synchronism at least manually. When the pointer was turned to north, this was indicating: that from now on a message has to come their way soon.



Viewing all the parameters we have dealt with, Nachtfee was a difficult (delicate) system to handle. Was this one of the reasons why it never matured? Not unlikely, by the way. As have been discussed previously, a 'tuning-fork' might have been employed as to keep synchronism for a relatively short while. Who in those days was capable adjusting frequency up to a hundredth on a cycle, as we where doing? We may say no one.  Quartz oscillators can be incorporated, but as we have noticed during its operation - at least tiny frequency drift is encountered under all circumstances.

There simply did not exist a feedback system that guaranteed synchronism between both the shore and airborne system. British sources considered the master oscillator of Freya being extremely stable (500 Hz). Whether it was stable within say > 0.01 Hz I doubt. Even this tiny value will causing an unreliable 'order' transfer.

Having employed a tuning-fork would likely have been their only chance!



In my perception yes.

Because: what is necessary keeping tight synchronism between the Nachtfee signal phase and the actual screen vector in the aircraft?

Some might say: why not using tuneable quartz resonators? Because for such low frequency they did not exist at about 500 Hz. Building in a Nachtfee like quartz controlled frequency (divider) system is beyond what is possible. Furthermore, even when this would have been possible, its signal phase had to be variable (controllable), as to maintain strict phase equality; thus both vector blips staying in concert. As we have noticed, tuning a phase shifting goniometer, is also causing an additional phase drift. The only practical means is employing tuneable fork resonators, that is to be kept in concert by manual- or less likely electronic means. Their Q-factor is extremely high, be it not as much as a piezo-electric resonator; but the latter device is not tuneable.


Quartz of channel Q7. It tells us that its fabrication tolerance is +/- 2.10-5 at 60° C

Tc is not directly significant, because it is supposed to be kept in a thermostatic oven. The implication, nevertheless, is - that it lays between 20 Hz per MHz plus or minus. My guess is, that its Tc might be regarded in the same order. Noticing the limits which we have observed referring to Q5 - it moved at least about 0.18 Hz. Hence, the fundamental quartz frequency itself deviated: 0.18 x 30 = 5.4 Hz. This measurement was done under average room temperature of say 20° C, by far not comparable to the aircraft operational environment. It was also found during my experiments - that the adjustable (positive) feedback potentiometer in each quartz module had also a non negligible influence of its initial signal phase. Hence, during our foregoing experiments the frequency of Q5 moved thus about 0.18 Hz;  which was even still clearly measureable after it had been divided by a factor of 30! (@ 500 Hz)

A third option could have been using a R/C controlled oscillator, whatever its configuration, having a Wien Bridge, a ladder network or maybe even a L/C oscillator. I also have strong doubts that its performance could have been better than that of a tuning-fork. Please let us bearing in mind, we encountered frequency deviations in the order of  < 0.01 Hz which proved being "unworkable". (I don't like using the word frequency instead of phase deviation, but its implication is better understandable. Controlling eventually a single beat-note per 100 seconds is unrealistic)

All further expectations is what the Germans call: Wunschdenken! (and beyond reality)



A good watch (horloge) may have had a daily time deviation of, say, 1 to 5 seconds per day. As far as I can judge this, this figure is less promising as we have to operate it at an operational frequency of 500 Hz. Thus, 500 times faster than the seconds steps of a normal clockwork.  ATM j154-5  is in this regard very informative (please notice particularly the section from Bild 6 downwards). The purpose of this paper was to explain new ways in adjusting watches against a standard watch or N-Uhr, within a relative short time span (excellent paper in German language). Although, in this paper is dealt with daily clock deviations of say 1 - 5 seconds, during the day, however, irregularities sometimes causing temporarily deviations of 60 s/d. The time keeping accuracy is only a daily average; and is thus per unit of time not fully reliable.  These kinds of (mechanical) irregularities does not occur when a tuning-fork oscillator is involved. Maybe its decrement is a slight downside, but this is compensated for by the oscillator energy in the circuit. Full synchronism without interference had only to be maintained within a couple of minutes, maybe a bit longer.



Of course, all this is my presumption, and maybe on a wrong track or simply being a flaw.

Is there someone around who is having an idea how the Germans have, or may have, solved this basic problem?  Please contact us at the e-mail address down this page.


On 9 February 2012

New experiments were undertaken. (Bladwijzer116)

After having given the Nachtfee time to warm up, quasi synchronism was achieved against the synthesiser frequency set at : 499,978 Hz. In 8 minutes, thus 480 seconds, the Nachtfee signal phase changed or drifted 360°. When we consider that this process is advancing linearly, in each second signal phase drifts thus: 0.75°. 





On 10 February 2012

After some modification work on the Nachtfee, I also made some phase drift measurements.


It is practically found, that the warming-up time is quite long. It continues quite longer than the time span to which the thermostatic oven has reached its 60° C (thermostat switching on and off regularly).


Comparing the two signal phase. The signal reference derived from the signal synthesiser PM5190X is connected onto channel A and Nachtfee is connected onto channel B. Measured at 499.978 Hz (channel A).

Before preparing my camera I had adjusted such that the delayed window were both edges are just touching. In the meantime, the phase has drifted slightly; which is visible as the edges of the high-lighted are no longer just touching. One may not derive an instant conclusion of this fact, as I am not sure whether the LF synthesiser (reference oscillator) had been set optimally against the Nachtfee data signal. A down side of our set-up is, that every setting necessitates trial and error steps. Although, the PM5190X is facilitated with an IEEE-bus, we do not have the appropriate interface for it. Nor have we experience in programming it with a virtual tuning knob. 


Using, however, the delayed time base of our scope, it is quite easy to discover the actual phase drift 'in degrees' of the Nachtfee signal. (see previous screen shot)


A few days ago I measured what the actual Nachtfee data output impedance is. This was accomplished by a principle practiced by "Berreclouw" in the 1930s (he even got a Dutch patent on his X-Q meter). He pointed, that when you have an unknown impedance and you measure its signal level - that it is possible to determine its actual impedance, by loading the impedance by means of a controllable resistance (purely ohmic) until its signal level reaches half its value. In this case, both the known and unknown impedance are equal. Of course, the parallel circuit should not adding a 'complex' component (the circuit under test should be, in my understanding, being in resonance, thus constituting a real ohmic value). But adding a trimmer-resistor at 500 Hz is fully obeying to this. It was found, that the Nachtfee data output impedance is about 300 Ω. But, the implication is, that its signal output is reduced by a factor of 2. The external pulse forming circuit which is providing the Nachtfee feedback signal could not cope with it. However, it was discovered that visually the Nachtfee sinus wave was having far less signal distortion. As an improved external signal module is being planned, I removed the 300 ohm load for the time being.






On 13 February 2012,

After working on the conversion of the main Nachtfee unit and bringing it into a proper display fashion, my attention focused on the signal phase interaction between both, the PM5190 synthesiser signal and the Nachtfee data output again.


Frequency reference set at 499,980 Hz

 It was also discovered, that even the feedback control in quartz channel module is having a direct influence onto the Nachtfee data output phase. Phase change up to about 90°!



Time-base delay marker adjusted just where both signals were crossing at the CRT screen




Screen shot take after 120 seconds have been passed





420 seconds have been passed since

Please bear in mind, that scope channel A is triggered upon and originates from the synthesiser generator PM5190X.




Photo taken on 17.37.45. The measurement series has started on 16.57.45


From the first three screen shots it is apparent that synchronism is not existing, although, seemingly there only exists a very tiny phase drift when watching what happens at the CRT display.




On 20 February 2012


During my reading of the various Felkin reports which Phil Judkins has traced in Kew, I realised that time has come taking notice of Fritz Trenkle's early publication of 1966. In those days his work was published by: "Ausschuß für Funkortung Sonderbücherei" Ausschuß für Funkortung, Düsseldorf, Am Wehrhahn 94 (but now in Bonn, and known as DEGON). Best.Nr. 1031. Is sometimes to be found on the internet.


Sometimes very unpleasant discussions had taken place, where some were relying on what they have found on the internet. In particularly the recently been made accessible TICOM file of 1945/46. In which Dr. Oskar Vierling in his laboratory at Feuerstein should have designed Nachtfee. (DOCID: 3560798 p.43) My counter comment was: first, that Trenkle is stating differently. Secondly, that when he published his papers most of the eyewitnesses were still alive. In those days impossible was stating facts that was not agreed upon by those who had played their wartime role on it.


After an introduction, he states: Diese Zusammenstellung stützt sich ausser auf eigenen Unterlagen auch auf die Schriftensammlung des Ausschuss für Funkortung. Nachstehend ernannten Herren und Stellen, welche durch Auskünfte und Bereitstellung von Unterlagen wesentlich zur Vervollständigung dieser Schrift beigetragen haben, wird besonders gedankt:

Herrn Dr. Arenz, Feiburg/Breisgau - der Fernseh-G.m.b.H., Darmstadt - Herrn Oberst Greffrath, Hamburg, Herrn Dr. T. von Hauteville, Stuttgart (who apparently had been engaged in Rechlin, AOB) - dem Luftfahrtarchiv G.W. Heuemann, Solingen-Wald - Herrn Dipl.-Phys. A. Mattes, München - dem Max-Planck-Institut für Aeronomie, Lindau/Harz - Herrn Reg.Baurat Dipl.-Ing. F. Meißner, Koblenz - Herrn Prof. Dr. H.G. Müller, München - Herrn Dr. W. Pohlmann, München  - Herrn Dr. Rautenberg, München, Herrn Dipl.-Ing. H. Schuchmann, München - Herrn Prof. Dr. H.J. Zetzmann, Oberpfaffenhofen.  


For our Nachtfee evaluation it does make sense taking notice of what Fritz Trenkle referred onto T.v. Hauteville.

Although, this is not entirely proving it, we may consider that it was T. v. Hauteville who (might have) taught him on the Nachtfee design history (as well as on other things like his FuG16ZY system developed at Rechlin). This is far more in line then what is mentioned in the TICOM document.


Fritz Trenkle told me once, that his 1966 publication was a first attempt to publish what he had so far collected on German Electronic Warfare. He stated, that some details were not in all respect correct. But knowing Fritz Trenkle personally, his later publications were always reflecting the state of his fair knowledge.



On page 125/126: Im Jahre 1943 entwickelde die Erprobungsstelle Rechlin jedoch als ergänzung bzw. Ablösung des "Y-Verfahrens (Kampf)" das EGON-Verfahren (aus Erstling - Gemse - Offensive - Navigation ) zur Führung von Beleuchterflugzeugen (Pfadfindern). Der Standort des Flugzeuges wurde zum Boden wieder mittels eines "Freya"-Gerätes mit Kennungsempfänger "Gemse" aus Richtung und Entfernung bestimmt. .... Die E-Stelle Rechlin hatte auch ein besonderes Kommandoübertragungsgerät "Nachtfee"FuG 136 als Zusatz zum FuG 25a entwickelt und in 5 Exemplaren gefertigt, das bei Teil der Einsätze auch verwendet wurde. 


Let us consider Trenkle's EGON-)Kampf-)Verfahren again


FuMG represents the Freya-EGON-B (eventually EGON-A) site from which an "Pathfinder" aircraft is under guidance



We have dealt with Freya-Egon many times, but how did it work?


The basic principle of Freya-EGON sometimes called EGON-Kampf


VW is the EGON signal channel. It transmits towards the IFF transponder in the aircraft at about 125 MHz and receives its reply on about 156 MHz

PU (Peilumschalter) is facilitating regular- or split-beam operation. Split-beam is actually not shown in this drawing, but it allows very exact D/Fing, where a clear 'left' or 'right' from the antenna axis is being indicated. Just at its centre there is no signal shown as left and right signal are 180° out off phase (nulling)

Weiche VW is a splitter-filter for the receiver and transmitter signals

Q or "Kuh" both being pronounced equally in German language, is the interrogation transmitter

PE "Gemse" is the IFF signal receiver

All other abbreviations are equal to regular Gema designations

Very significant for us is that apparently the Nachtfee data is being fed onto the QN stage, which is the power supply as well as the pulse-driver-stage. My estimation is, that the Nachtfee feedback signal had been derived somewhere from the output of the Q stage. It is even possible to detect it near to the antenna array. Whether by means of a simple diode or other like may stay an open question.



On 22 February 2012



New discoveries.


On our proceeding page Nachtfee-FuG25a, I have started with the implication of the Nachtfee data processing up to what is presented on the aircraft 'order' or command display.


In the course of discussion (Bladwijzer2) I also dealt with the fact that the 500 Hz of the Nachtfee data signal represents a λ of 600 km. As this trajectory has to be bridged twice the operational distance is maximally 300 km.   Like a flash it appeared in my brain that this might be indicated on what I so far have designated 'number' scale. The scale actually is constituting a correction for the distance between the Nachtfee ground- and aircraft systems.


A second consequence is, that also the nuisance of synchronism between the Nachtfee ground signal and the aircraft CRT deflection frequency is possibly solved when the aircraft deflection phase or frequency is being feedback to the Nachtfee ground console. All controls on the Nachtfee front panel are having a sense since. The distance scale (formerly number scale) and the 'Phase' control is allowing correction of the 'order' displayed in the aircraft from the ground station. It is also possible to manually control the 'order' presentation on the aircraft display.

Secondly, Fritz Trenkle's statement that T.v. Hauteville (Bladwijzer4) has designed Nachtfee is making sense since. He is also quoted for having developed FuG16ZY or Ypsilon system. Known by the Allies as Bonito. Both systems are relying on the implications of the wave length of a super-imposed sonic wave for measuring distance. It is not rather strange that both may have been designed by T. van Hauteville of the E-Stelle Rechlin. Albeit, that their application is differing.



Copied from our Nachtfee-FuG25a (Bladwijzer3) page:

We have discovered now an explanation for the until now rather mysterious controls.


However, still some unsolved operational and technical aspects may be left. But this is making our Nachtfee project often so thrilling! Reconstructing operational and technical queries, by judging given facts a thorough thought. Implementing the experiments as to how an aircraft CRT presentation should have been operated and drawing conclusions of the impossibilities of keeping system synchronism practically, lead me finally to consider a way countering the faced downsides. The many experiments done also pointed that some of the Nachtfee controls did not make sense, as, for instance, the 'Phase' control without feedback. Also, why is there a control having a scale covering 0 - 280 (0 = then equal to 300). It was the query does the aircraft speed conflict with the phase of the data signal? After I found that this did not, because during every Nachtfee cycle of 2 ms the aircraft is roughly displacing 16 cm versus λ of the 500 Hz data being 600 km. It is also realised, that the operational range of Nachtfee is consequently 300 km. In a flash I realised that this may be what is given on the 'number scale' control. I rushed to our museum as to check whether this is true. And, it is! Since, most remaining system queries are falling in their holes. Of course, there are still some being left unsolved. I hope, nevertheless, that later experiments can bring most answers. A problem might be encountered as to simulate distance. Here we may have to implement modern digital techniques as, for example, to variably delaying the Nachtfee data signal (simulating changing distance).


On 1 March 2012


I received a kind message by Jørgen Fastner from Norway, in which he quotes from a just received source of information (Bladwijzer30)




To be continued in due course




On 6 February 2012


We received a response onto our call for Nachtfee support; which is worth being added to this discussion. It may not solve everything, but it is an attempt to give it a serious thoughts. As to bear in mind first what this comment is about:


Photo taken just after the scale paper had been cleaned. Though, before it has been reassembled. It was decided to use, for historical reasons, the original scale again



Skala - Nachtfee

"Please notice the scale text symbols '1 L' and '1 R' as well as '2 L' and '2 R'. Likewise is even to day the usual NATO spelling: A = Alfa, B = Bravo etc.; the Germans used A = Anton and B = Berta. Rolf and Lisa may have been used in the communication between the pilot and the Nachtfee operator in the rear seat of the aircraft, he being also R/T- and radar operator (as well as rear gunner). He should have been the person to monitor the CRT screen where the Nachtfee commands where displayed upon. The meaning of the other abbreviations is for us a mystery, as their meaning might be camouflaged. Who knows what, for example, 'jjj' may have stood for?"

meine Annahmen: (den folgenden Text bitte selbst mit "möglicherweise, vielleicht, ?, usw ..... ergänzen :-))


1. Die Skala zeigt eine Art Maschinentelegraf.
 2. Bodenoperator und Bordfunker haben vor sich die gleiche Skalenbeschriftung.
 3.Der Bordfunker sieht die Skala jedoch nur als Darstellung in der Oszilloskopröhre.
 4. Der Bodenoperator sieht in der Oszilloskopröhre über der Skala die gleiche Oszilloskopdarstellung wie der Bordfunker.
  5.Die Skala besteht aus einer äußeren Skala (Seerose) und einer inneren Skala (Bombenabwurfmanöver).
 6.Die Unterscheidung, welche Skala gültig ist, erfolgt mit dem Polwendeschalter.
(This aspect I doubt, as the Freya-Polwender switch is, according other wartime information, to be used when a command is due to be transmitted. It is in no means informing that another kind of command is being in force, AOB)
 7.Die äußere Skala gibt dem Piloten den Kurs an, den er fliegen muß, um das Zielgebiet zu erreichen.
(my comments: Freya-Egon was a system supposed to guide a Pathfinder aircraft towards a designated target. The aircraft had to rely fully onto these 'orders' or commands. The pilot had his own cockpit compass, AOB)
 8.Die innere Skala zeigt dem Piloten den (uhrwerkmäßigen) Ablauf der eigentlichen Bombardierung im Zielgebiet.
 9.Die Abkürzungen knh, kus, fad sind angelehnt an die internationalen Q-Gruppen. (q ersetzt durch k) siehe dazu auch Wikipedia deutsch : "Egon Radar". Bezeichnung Q-Gerät als Kuh-Gerät)
 10. knh : Nach Anreise ins Zielgebiet Flugzeug auf vorgesehene Bombenabwurfhöhe bringen. entspricht qnh: der Flugplatz gibt an den Piloten , der landen möchte, den Luftdruck in Landebahnhöhe durch. Der Pilot stellt darauf am relativen Höhenmesser die 0-meter Höhenmarke auf den angegebenen Luftdruck in Landebahnhöhe.
 11. kus, pauke auto, jjj (am inneren Skalaring durch eine Verdickung zusammenhängend gekennzeichnet)  kus: Das Bodenziel (optisch) erfassen.
 entspricht qus: ich sehe am Boden Zerstörungen, Wrackteile .....
 12. Pauke: Feuererlaubnis AUTO: Umstellung auf automatisches Abwurfgerät?
(My guess, employing a X-Uhr for timing or setting for actual bomb or flare release?,  AOB)
 13. jjj : Darstellung von Bomben, die aus dem Schacht fallen.
 14. fad: Zielgebiet verlassen entspricht qad: starten, wegfliegen
 15. Bezeichnungen 2L, 2R  Das Ziel wird zunächst nicht direkt angeflogen, sondern in Flugrichtung rechts vom Ziel. Nach einer Linkskurve um 20 Grad (2L) fliegt man direkt auf das Ziel zu.Nach dem Überflug und Bombenabwurf fliegt man eine Rechtskurve um 20 Grad und entfernt sich auf dem ehemaligen Anflugkurs.
 Grund: 1.Wie bei einer Landevolte bekommt man bei diesem Manöver einen besseren räumlichen Eindruck vom Ziel.
 2.Fliegt man bei einem direkten Zielanflug am Ziel vorbei, (Ungenauigkeit von Freya), so weiß man nicht, ob man das Ziel rechts oder links suchen muß. Bei einem Zielanflug eindeutig rechts vom Ziel muß das Ziel links auftauchen, wenn nicht, muß man eine Linkskurve fliegen und suchen. (vgl. Peilung nach  Auffanglinie,= Brücke über den Fluß = Peilung nicht Brücke, sondern z.B. 200m rechts von Brücke. Steht man am Fluß, gibt es keine Unklarheit, sondern man geht nach links am Fluß entlang zur Brücke.) Zielsuche links ist körperlich günstiger (vgl. Sportstadion, Läufer laufen Linkskurven lieber als Rechtskurven).
 16. Das Bombenabwurfmanöver startet am roten rechteckigen Symbol. Es ist beendet am schwarzen runden Symbol. (Bodenoperator betätigt Polwendeschalter)
(This is theoretically possible doing this by means of the Freya-Polwender selector, AOB)
Quelle Q-Gruppen:

The contributor, however, preferred to omit his name.


As a consequence of lacking adequate post- or wartime documents I have decided to investigate what the implications might have been after the Nachtfee data (signal) left the Nachtfee 'order' console and its appearance on the CRT screen in de aircraft cabin.

Please continue with:

Nachtfee - FuG 25a


To be continued in due course



Significant for us in the Nachtfee survey project is to get more detailed technical information on Freya and/or EGON. We have not yet been able to trace such kind of technical documentation. Never mind, Phil Judkins helped us with finding some PRO/NA files, but these are so brief, and hardly technical, that for this survey these are meaningless. However, Michael Svejgaard sent us a few days ago useful document copies, which are really helpful. 


Is there someone around, who would like to participate in this Nachtfee Survey Project? Please contact us at:

And type in what you read

Those participating will be recognised and fully referred on to.

By Arthur O. Bauer


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



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


And, the PowerPoint progress page (converted into PDF)





Please go back to, or proceed with: FuG136-Nachtfee introduction page

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: 4 March 2012)

Please consider or go back to:   Nachtfee FuG25a project - investigating how the Nachtfee data was conveyed towards the aircraft display (status: 4 March 2012)

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