How it technically may have been accomplished





Page started on 9 June 2012

Status: 27 August 2012



Our Nachtfee survey has reached a stage which is showing a slowdown in progress; together with a deadlock (stalemate) partially due to conflicting views how it technically may have worked.

Hans Jucker, as well as in some respect Günter König, are more or less strongly opposing my hypothetical ideas how the Nachtfee system was managed.

Günter is strongly believing that it would have been possible letting both system clocks, in some way or another, operate in full concert; where the virtual clock pointers keeping equal vectors, without any additional means.

This section is to be skipped as new data has come available major objection to this view is: that watching actually the temperature dependency of the Nachtfee quartz controlled time base, where I once monitored from room temperature up to 60° C (oven temperature) that the (about 500 Hz) signal deviates 0.57 Hz, is preventing this. We know that the original quartz frequency is 15000 Hz, and that we have to count with a division by 30. Hence, 0.57 x 30 = 17.1 Hz. I strongly doubt that by such a behaviour a second quartz could indeed be manufactured, having exactly equal (partially non linear) frequency versus temperature curves. Not mentioning that several systems should have had exactly the same time base parameters. The airborne environment is extremely harsh and likely causing more difficulties than is already encountered on the ground. It is found that this data is no longer valid.


It is found still necessary to consider the pro and cons of some of the virtual system concepts.


What do we know so far?


Synchronism between both the ground as well as the airborne system clocks is essential. In my perception, which also is the result of many foregoing experiments, it is likely that in some way quasi synchronism was maintained manually. Maybe even accomplished on both sides of the chain. Thus, on the ground as well as in the aircraft. Where operational procedures may prevailing to one side or the other.

In quite many e-mail discussions a point is often ignored, or at least not emphasised upon, which is the existence of the 'Phase' control on the (shore) Nachtfee control panel. We have already noticed, that coherence exists when the Nachtfee signal output is being feed back to the control-input of the Nachtfee apparatus. Constituting a coherent loop.


The signal feedback principle, where the Nachtfee data output is directly returning towards the Nachtfee main chassis

Please notice the section: (Bladwijzer32)


To my understanding it is essential, that in the above shown situation - the 'Phase' control is not playing a role whatsoever. In the closed system loop the Nachtfee 'order' or command setting of 'Phase' control doesn't change the data-vector at all! Thus no phase shift is shown on the controlling CRT screens! Why is it maintaining coherence?   

Because the Nachtfee signal processing, and the time base of the controlling screens, is both derived from a common signal source just after it has left the 'Phase control goniometer' stage, please notice the red line in the second schematic (at G1 of Rö5 of the next block schematic). Coherence can be maintained for as long as the blue line system concerned. The upper line is part of the Nachtfee 'order' or command chain, whereas the lower blue line is concerned with deflection of both the LB2 control screen and the dual trace 'Rang Adjustment Tube'. When thus the upper signal is being feedback someway towards the two CRTs signals (yellow line) coherence will be maintained; even when it has passed through the FuG25a aircraft transponder. However, when a different kind of signal is delivered onto the CRT screens blips are non coherent. It is, though, possible that both coherent as well as non coherent signals are appearing on the LB2 CRT screen.


Let us first consider the Nachtfee block schematic 


The Nachtfee block diagram

Please double click on it as to open it in PDF

It has though, become necessary to implement an additional principle schematic diagram of the Nachtfee main unit, which is particularly showing the three kinds of signals dealt with. The next figure is showing the three kinds of signals to be dealt with. 


The three coloured routes represent the different kinds of signals being dealt with. Please also notice the previous block schematic and previous descriptions

Please double click on this drawing as to open it in PDF

Please remember: the red line shows the common quartz controlled signal route; the blue signal line is regarded being interacting in coherence; the yellow line is the feedback signal. Which however, will act in a closed feedback loop being coherent, and may also acting in non coherence when the signal originate from an independent source, like the one originating from the simulated aircraft time base. Please notice further down about these aspects.

The so-called 'Range off-set' control is having a scale between 0 and 300 km (Bladwijzer39), which is effecting the controlling CRT time base only. Hence, its setting is not effecting the Nachtfee 'order' or command signal either (as long coherence exists) though, it is, of course, rotating the local time base vector; resulting in a rotation of the 'order' blip (vector) painted on the LB2 CRT screen.


The directly feedback order signal shown on the LB2 Control screen. Photo taken during earlier experiments


The purpose of 'Range offset' is to neutralise the operational signal time delay caused by their mutual (increasing or decreasing) system distance (0 - 300 km). When Range offset is being operated, the shown blip vector will rotate in either direction, as this is the essence of Range offset adjustment.     Please remember that 500 Hz is equal to λ = 600 km, in radar terms 300 km, as the trajectory is to be passed twice. Though, how could they compensate a loop parameter when they have no returning (signal phase) information to adjust upon? A returning signal picked up just about the EGON antenna is still in coherence, and 'Phase control setting' is, in this case, not influencing the control blip on the LB2 screen.

Where is the 'Phase control' entering the system loop? It can only influence the Nachtfee 'order' signal in the 'time domain' (against a second reference). The airborne system clock is a free running device, which's signal phase is acting as a system clock as well. Let us consider that both clocks are operating at equal frequency. Does this imply that the clock pointers are having equal vectors? No, even when atomic clocks being involved, they are (almost likely) not pointing at the equal vector angle. Their mutual phase difference will stay for a long lasting period, keeping a constant phase difference. Quite recently I used our two LF synthesisers and set them both at 499.993 Hz, and changed only the BNC cable from one to the other. The painted order blip on the simulated aircraft display was exactly kept in concert. Thus did not move against the painted time base circle. What only happened is that the painted 'order' blip appeared not at North but moved instantly to about East (about 90° phase difference in a positive sense, it could, however, be any value between + or - 360°), staying though in concert. It is just measured a different value, why I don't know, but please consider also: new data has come available

It is thus understandable, that such phenomenon is also occurring in the Nachtfee system. There has to be a means to control both the actual ground- as well as aircraft time-base-phase. In other words, bringing both clocks in line, so that the clock pointers are having equal vectors (having exactly the same time). Here is where 'Phase control' is entering the arena.      Please bear in mind, that the Nachtfee 'Phase' control is not capable of manipulating the real aircraft time base phase. What it does though: is delaying or advancing the actual Nachtfee 'order' or command phase in such a manner that it gets an offset (+ or -) in phase time, so that its signal blip arrives just at the CRT screen when the (painted) aircraft time base rotation is actually at North (= 0°) (Bladwijzer40). Exact knowledge of the actual 'Range offset' value is therefore a must (crucial)!     When employing Freya-Polwender, the blip is pointing South; although, phase time is still adjusted upon 0°. This may sound confusing, but Freya-Polwender is only interchanging the two data output wires of Nachtfee, thus changing its signal phase 180°and not their mutual phase time difference. Hence, when alignment is incorrect the blip will not pointing exactly due South (in the hypothetical aircraft). Bear also in mind, that in our experimental concept we are not viewing the Nachtfee data signal, but we are watching the returning simulated aircraft time base signal phase; what only count is their mutual time base phase relation. Hence, are both virtual clock vectors pointing exactly equal.

My current trial is having a constant, thus an un-interrupted, simulated airborne time base feedback towards Nachtfee. Being, of course, differentiated pulses when the Nachtfee time base is just passing through 0° in positive direction, where just a new sine wave cycle begins. Indicating only when a new cycle starts. Similarly is done with our simulated Nachtfee data signal. This may well have been periodically interrupted, as it is not essential to monitor 500 times a second its actual status. However, enough pulses should remain as to allow sound 'Phase' control.

A strong point of the British drawing (Fig. 6) is the information: 'Range Adjustment Tube'. The existence of this dual trace CRT device is essential for Nachtfee's proper functioning. It is highly accurately showing what the received feedback signal-phase (the returning aircraft time base signal) is against the Nachtfee internal CRT time base phase (my estimation  < 1° deviation). In other words: it is allowing manually controlled calibration against both the returning- and internal deflection timing, also owing to the manual 'Phase control setting'. Both parameters have to obey to 0° phase-offset (neglecting 180°).


The dual trace HRP2/100/1,5(A) is showing two signals, but both originate from a single signal source

T.von Hauteville's very clever trick was, crossing both deflection plate systems. Thus one channel is painting from right to left, whereas the second channel is writing from left to right. Please remember: both originating from the same signal source! Similarly is done with both vertical deflection systems. By doing this both signals can only coincide when these are exactly in the centre (middle) of the deflection trace. I opted for aligning both vertical signals in a back-to-back manner. This is a very handsome way of doing. It allows us to distinguish between 0° and 180° signals (the sharp edges changed position). The screen shown above is adjusted to 0°. I cannot, however, say whether they did it exactly this way during war days. The CRT time base is directly influenced by the 'Range offset' settings. This is in our experimental setup clearly visible; as adjustment can only be achieved when Range offset is set accordingly.

Why 'Range'? Because its coincidence is only valid when the Nachtfee's 'Range-offset' (0 - 300 km) control is adjusted correctly against the distance between Nachtfee and the controlled aircraft. When this is set right, the 'order' blip on the aircraft display is also pointing correctly at North (using Freya-Polwender it will be South). (Bladwijzer38)

Another, new thought: we have noticed that the Nachtfee 'order' scale is having also two commands: nal 1 and nal 2; the meaning is: Switch on recognition key 1 or 2. (Bladwijzer33) This implies, to my understanding, that the switch isn't in function during some period of the operation (BG25a is having a neutral position between key 1 and 2)(Bladwijzer37). How could the ground controlling Freya-EGON station still knowing what the tactical distance and course (flight path) of the aircraft under guidance is? EGON is relying on the FuG25a transponder signal. Thus, sending a pulse towards the aircraft and measuring the time delay of its response; in this case also controlling the exact flying path towards the target. Were they perhaps relying upon the signal phase of the aircraft time-base-feedback, or was, nonetheless, still one of the two keyed recognition channels in operation??     Nevertheless, proven is that, although the time base reference is constantly transmitted towards the Nachtfee shore system the transponder keyed-signals can be monitored. Thus recognition is still possible (Bladwijzer34) + (Bladwijzer35).     Please bear in mind, that we deal with two kinds of signals; the Freya-EGON/Nachtfee signal and the time base reference both approaching the EGON/Nachtfee console. The first one is received by the FuG25a receiver apparatus and then being re-transmitted, the second one is originating from the aircraft time base (only). Both free to rotate (act) in their own domain.

On Tuesday 12 June David Shelborn visited our museum. Nachtfee was demonstrated and explained for several hours. During the course of explanations I switched on Key 2 of the FuG25a IFF transponder. As expected, a second flickering blip appeared on the LB2 control CRT (it is switched on and off due to its Morse code recognition) (Bladwijzer41). Its blip flickering is more heavy owing to the fact that the returning Nachtfee/EGON signal passed the route twice (distance or range) through a wobbled receiver front-end, knowingly of the FuG25a and the Gemse receiver. What I knew from former experiments, but was incidentally not aware of, is that the transponder blip wasn't inflicted by the setting of the 'Phase' control on the Nachtfee front panel at all!!! Nevertheless, its vector rotated when the 'order' pointer is being moved; maintaining quasi coherence. When the order pointer was moved the simulated aircraft time base reference did not move its vector blip on the LB2 CRT screen.     For simple reason: there does not exist an internal signal connection inside the Nachtfee apparatus, beside its quartz controlled time base; pulse signals have to enter the (separate) video system via the feedback or 'Impulsamplitude' input (interface List-Stecker pin 1)(the yellow lines in the schematic above).

It was also encountered, that after a quite long time the Nachtfee frequency stabilised more or less at about 499.993 - 499.992 Hz (against the modern PM5193). Please bear in mind my previous comment: that the returning Nachtfee time-base-phase did not have to be available at a rate of 500 times per second. When a quasi equilibrium was reached, it may even have been transferred in intervals, albeit, in my hypothetical understanding, transfer should be long- and frequently enough to allow proper alignment. Of course, when the aircraft time base was equally stable, which is still rather doubtful.


The aim of this new discussion page is to discuss the pro and cons of how Nachtfee could have been accomplished. During the writing and thinking about the many open queries, it is found that we must go also into some technical details of the FuG25a keying. Although, I first opted for a schematic free discussion space.


Shown is the detector output and the limiter/keying valve circuit of FuG25a


The problem with modifying the FuG25a transponder, is how should we combine both the the EGON/Nachtfee 'order' signal and the aircraft time base signal phase, with the least change of its original design concept and preventing mutual interference?

The detector signal is fed more or less directly onto the FuG 136 aircraft display where its blip is representing the Nachtfee 'order' or command vector (directly galvanic from the anode of Rö4 onto test socket pin 9). Logically, the Nachtfee signal may not directly be overlapped with the returning aircraft time base phase, as it otherwise will only being available when 'recognition keying' is taking place, and, it will most likely confuse or jam 'order' presentation. Rö5 acts, according the FuG25a manual, as a Morse code switched 'on and off' limiter stage. Currently I inject the aircraft time base directly onto the 'Keying of IFF transmitter' line (after first order differentiation), which is directly connected onto g1 of Rö8. The Morse code recognition switching arrangement (module) is de-blocking (keying) the cathode of Rö5. Technically the circuit is quite simple; is there a better method of combining both the Nachtfee recognition with the aircraft phase (signal) feedback? Likely there is, but this may require at least an additional valve, which has to be placed outside the FuG25a unit, for which is no internal space left.

In my vision, the way we are currently combining both the Nachtfee signal and the time base phase signal returning towards the Nachtfee/EGON ground simulation is the best possible means under the given circumstance.    



We have in the meantime dealt with so many Nachtfee related aspects, we should take notice of our current system concept again



The block diagram of our current experimental and hypothetical Nachtfee setup.  Nal1 and Nal2 constituting the recognition keys (Bladwijzer41

In our experimental Nachtfee concept the data output is, after signal processing - HF pulse modulation, fed at 124 MHz onto a dummy antenna. Its signal is received by the FuG25a RX (bridging ca. 1.5 m). Whereas the FuG25a TX transmission is received at a distance of about 2.5 m by the Gemse RX.

In our hypothetical concept is the Nachtfee signal also constituting the Freya-EGON pulse signal. It may at least occasionally have acted as the Freya-EGON's system clock (considering the huge problems faced by phase deviation versus temperature and time, it is most unlikely that Freya's own time base (being a RC oscillator) was comparable phase stable as is the Nachtfee internal reference). We have also noticed (Bladwijzer42)(please notice both pulse diagrams): that in Hans Jucker's perception two signals were conveyed towards the guided aircraft is not workable. His thought was: that the EGON signal could synchronise the aircraft time base (this is what he suggested to me, when we met in Dresden during breakfast on 29 April 2012). Such two signals, knowingly the EGON and the Nachtfee data, may well causing mutual uncertainties. Because, when both are having equal (overlapping) pulses in the time domain, the system could no longer distinguish between the EGON and the Nachtfee data signal. It may also causing trouble when the two single signal pulses being received with a slightly different pulse-phase. It may, in my understanding, easily have been possible that the Nachtfee pulse ran further being the time base and the EGON pulse (neglecting its less stability compared to the one originating from Nachtfee) was acting like the Nachtfee order signal. Again, like (some) others, his perception is totally neglecting and rejecting the existence of 'Phase' control. Would have 1940 techniques allowed them a faultless data transfer? To my judgement, most unlikely. Considering such concept downsides, we should wait until new relevant information is found. Hence, staying to our current experimental system concept for the time being.   


The current setup of our hypothetical aircraft display, being a substitute for a LB2 like CRT

The Nachtfee signal blip is pointing at say three minutes past the hour. The circular time base frequency is 500 Hz (PM5190/5193). The purpose of Nachtfee 'Phase' control adjustment is to delay or advance the Nachtfee signal phase such that the painted trace and the Nachtfee signal both just coincide. Normally, according British investigation, the reference point is at due North, as this is also the only phase reference alignment available on the Nachtfee 'Range Adjustable Tube' (Bladwijzer40). Being, of course, due South when Freya-Polwender is selected.


On 19 June 2012

Hans Jucker asked me about circuit details of the Nachtfee quartz oscillators

To be skipped, New data has come availableAs have been discussed before, it is found that after say about 100 minutes of operation, the Nachtfee signal is getting its equilibrium about 499.993 Hz (499.992 Hz). What is its stability in modern terms? It is about 1 to 2 ppm or even better, when we consider 499.993 Hz versus 0.001 Hz. I think, for 1940 technique reasonable. Whether this was obtainable in an aircraft environment is questionable. Measuring such accurate frequency domain is not very easy. Our Dana 1999 counter provide 10 digits, but the lower significant digits cannot be trusted. I prefer a Lissajous, which is allowing very accurate comparancies.


I also took in consideration whether they may have had two FuG25a sets on board, but their mutual interference may have prevented them from doing.


I will not say, however, that my current hypothetical system understanding is faultless, but it is the fair result of all my actual knowledge and experience, which progress is extensively documented already seven months on this website.  My invitation is, please come over and be part of the experiments. Nearly all previously undertaken experimental steps can still be reproduced. I can assure you, watching the Nachtfee system interactions and implications is thrilling. Not good? Money back! I would say.



On 8/10 July 2012

As so often, Nachtfee's wartime concept is capturing my mind even during dreams. All hypothetical Nachtfee system options goes through it time and again. I know, that some crucial aspect is lacking, but which one and how? Because it is so different from what one may estimate, or because it is all too simple? The only solution in my working concept is trying out all imaginable options. Deo volente, sooner or later maybe a brainwave engendering the ultimate clue. 


We have so far used Nachtfee channel Q5, which is providing a signal of 500 Hz; equalling the Freya-EGON PRF. My previous system reconstructions were all based on this concept. 


Bearing in mind the fear opposition against my current hypothetical Nachtfee concept, it appeared necessary to set up a new hypothetical system reconstruction. Taking into account the above notice pointed in a early 1944 report file

Transcript: We have listening evidence 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.     May this imply that in the first observed German signal have operated channel Q5 and in the second occasion another Q-channel?  The British wording may imply that they operated a quartz channel above Q5, as they likely would have mentioned 'that the moving pulse walked backwards'. What the implications may have been is discussed is another Nachtfee webpage (Bladwijzer42).    It has to be said though, that this query cannot yet been answered definitely; still too many open queries left.


What would happen when we simulate the case that Nachtfee is running at a different channel, viz. a different channel quartz instead? Directly nothing, as long as the simulated aircraft time base is adjusted accordingly. You may perhaps remember that each channel step differs 2 Hz.   Q5 being quasi in its centre  providing 500 Hz. Is it possible to simulate this system concept with the available equipment and modules? YES


New Hypothetical system concept, relying this time on the transmission of two signal domains. 1. the simulated Freya-EGON signal with a PRF of 500 Hz; as well as the existing Nachtfee 'order' or command signals. → 2. Receiving and displaying both signals on the simulated aircraft display. → 3. (Re)transmitting towards the simulated ground station, the eventually periodically interrupted aircraft-time-base-signal (whether this aspect will be existing in the ultimate concept understanding stays open; as long as there does not yet exist a better solution, we have to maintain direct time-base-phase transmission towards the hypothetical ground station), and on demand, the real EGON signal passing through the required recognition  keys Nal 1 or Nal 2. The red and pink arrows indicate whether the EGON blip rotation is clock- or anti-clockwise. On the 'ground' nothing changes, as far as Nachtfee is concerned, where the EGON signal was already visible on the LB2 control screen.

Please double click on this drawing as to open it in PDF

Symbols and additional explanation:    E = Freya-Egon signal; N = Nachtfee 'order' signal, being either 492 - 494 - 496 - 498 or 502 - 504 - 506 - 508 - 510 Hz. (skipping Q5 at 500 Hz). Please notice also that the channels Q 7 - Q9 and Q10 are defect, for what ever reason; The red box, constitutes the hypothetical existence of  a Freya-EGON system. Its output signal will be derived after first order differentiation from our second digital PM5190 synthesiser, set at 500 Hz.

It is clear, that we will not have to simulate the EGON display; we only should generate its virtual output signal (PRF of 500 Hz). For simplicity, this signal can be mixed up with the TTi Pulse modulator output, or using a separate HF-modulator input. We have already discussed the option of interrupting the retransmission of the aircraft time base phase, as it is nonsense to repeat its existence 500 times a second towards the ground station. After this system simulation is working, it may make sense to investigate how the signal feedback-interruption is practically responding. Particularly in respect to 'Phase alignment' on the dual trace CRT (HRP2/100/1,5 A)(Bladwijzer 44 up this page).

Accomplishing this experimental setup have to wait until the beginning of September 2012.  Plenty of time left considering all the pros and cons of this new hypothetical concept  simulation.

The consequence of doing it this way, is that the Freya-EGON blip will be spinning in either clock- or anti clockwise; depending on whether the EGON PRF is higher or lower than the Nachtfee data signal, in casu, the aircraft time base frequency, which should be kept in quasi-synchronism to the Nachtfee data frequency. Which is, as we have noticed, selected by the channel switch to Q1 - Q10. Each channel step is causing that the EGON blip spins virtually two times per second faster (or a multiple, 2, 4 ..), be it clock- or anti-clockwise. Hence, each successive selected channel number (up or down) is adding two rotations per second.  It is clear, that a spinning EGON blip is easily identifying itself, and may not have confused the wartime operators.

What, however, remains open yet is the still not solved problem of keeping both the Nachtfee ground apparatus in synchronism to the aircraft display time base. We have countered this in current experiments by using the 'Phase' control on the Nachtfee front panel. This does make only sense, as long as both time references are more or less in quasi synchronicity and can directly being compared. For this I lower (or increase) the PM5193 frequency successively (in  Δ 0,001 Hz) until quasi synchronism exists, for the time being; I never have reached this fully, as the experiments were terminated after three hours (or slightly more) maximally.

Some have considered that in the wartime aircraft (e.g. Ju88) a similar quartz system was carried. I highly doubt this. Because: aircraft environment is extreme harsh. Aircraft tend to vibrate heavily and strong shocks like by landings may be encountered, not mentioning when crossing different air pressure areas (Luftsäcke). From a practical point of view, the Nachtfee ground apparatus is fit with a thermostatic controlled oven, which has continuously to be kept connected onto the mains, even when the electrical circuit is being switched off. Aircraft temperature may vary between -30/40 up to + 50 degrees C. German aircraft were compared too Allied aircraft having quite crampy cabin space, consequently, electronics has to be stored in the not temperature controlled section of the fuselage. Have you ever left on burning a small car light? And did you car start the next day without external help? Bearing this in mind, is it practically impossible to fit aircraft with such delicate equipment, fully relying on the ultimate aircraft battery capacity when on the ground? I don't think so. Another dimension is, the Nachtfee ground apparatus is being fit with X-mode quartz resonators, where the quartz bar is quite long. Their mounting inside the metal housing is very delicate. (Bladwijzer51) Mechanical shocks may well destroy or temporarily switching-off quartz crystals. Although, I cannot prove it yet, maybe I will never do so, though, my guess is that they may have used electrically (fine) tuneable tuning-forks modules (for each quartz channel seperately). To my understanding, they may have known which channel number or numbers may have to be operated in advance. Tuning forks may be not as stable as is a quartz controlled devices, but practically these are much easier to maintain. Important, they are highly reliable and likely still stable enough. (Bladwijzer15)and following lines!     I have, nevertheless, to stress that this is my perception, which may well be incorrect, but owing to the lack of an alternative keeping this as a practical solution yet.


We are becoming now more in line with Fritz Trenkle's concept drawing. From previous discussions, we may believe, that Fritz wasn't well informed on aspects of Nachtfee. What he knew partly was what  he could derive from the mysterious Felkin papers of 1945 (he may likely have possessed some, but was not entitled to share his knowledge with somebody else), of which a bunch transcripts is available on our website (Felkin papers). These paper series were kept classified up to at least the second half of the 1970s. Maybe also from some information passed by T. von Hauteville (Bladwijzer4) previously as well. Too late for discussing these matters with living eyewitness after the 1970s. Please be aware that many facts of the Felkin Reports are simply not according reality. The R.V. Jones Reports are more in line with what we can confirm. Jones was a very smart personality. By far smarter than most others.

What is lacking in my concept understanding, is the feedback from the Gemse receiver towards the Nachtfee feedback input. Those saying that this proves that there did not exist such a connection, have to answer why they have implemented CRTs as these do make sense? Not discussing the existence of 'Phase' control and its implications shown on the dual trace CRT (HRP2/100/1,5 A). Trenkle simply was not: Im Bilde! How could he? Nachtfee was still stored somewhere in the US; and those possessing it were not understanding what it is about. It is even doubtful if all its secrets will be lifted. Considering British NA (PRO) practice:   they got rid of most of the original German documents and kept mainly British reports or, eventually, its translation like is Radar News 19. Even here, it is most likely that the last drawing page is not real German, but is drawn from British report(s). Whether we may find these is most doubtful, as my friend Phil Judkins could not trace any technical files. The original German language Radar News 19 issue is in the NA/PRO index, but is not traceable. The only ones are the Felkin Reports and are R.V. Jones' A.D.I. (Sc) papers, like Report 101 (Bladwijzer50). Sometimes interesting, but hardly technical.


On 16 July 2012 The meaning is 



Das FuG136 ist Fernbedient. Das Sichtgerät bekommt der Flugzeugführer

The meaning is: that the aircraft display of system FuG136  is mounted in the vision of the (Chief) pilot and that it is remote controlled. What does mean remote controlled?

What should be remote controlled? Keeping the aircraft-time-base in synchronism to the time base of the Nachtfee ground system? Does this imply the a second display was existing, or was the electronics placed somewhere in the fuselage and was the pilot also watching its synchronism? 



On 26 August 2012


New thoughts


Let us consider the last New Hypothetical system concept again


Would it be possible, to omit the time base feedback? I have considered it, and to my understanding: there hardly is a way around of keeping, in what ever fashion, the feedback of the aircraft time-base-phase.

Let us first setting the main parameter for to day: no changes in the FuG25a apparatus concept


There exist only a single pin at the FuG 25a test-connector, being pin 9. Whatever we would like to do, this line has first to convey the Nachtfee 'order' signal towards the FuG 136 aircraft display. It is, however, also electrically containing the EGON signal, which is, at will, keyed by the Morse-code-key: nal 1 or nal 2. When a signal, of whatever kind, have to be feedback to earth, it has to pass this keying mechanism. Let us regard that we implement a coincidence or strobe circuit, its output signal will interfere with its input, as both come from the same (single)line-point (the premises is: no apparatus changes). This will hardly work. It will also lay the full strain on a crew member of keeping both the Nachtfee signal in exact concert to the actual aircraft time base. My previous hypothetical concepts always focussed on just preventing this!

The dual beam CRT incorporated in the Nachtfee concept had according the Radar News 19 drawing:


The function of "Range Adjustment Tube"; what does mean Range? The only range adjustment available on the Nachtfee front panel is, what I call: Range offset, which is calibrated quite coarsely  0 - 300 km.

It is thinkable, that the returning Morse-keyed Nachtfee signal is adjusted upon. Though, experiments have clearly shown: that the returning Nachtfee signal is still in coherence, whatever their actual phase shift may be (phase difference being constant). Its phase difference originating from  the 'offset of range' as well as being delayed owing to the various electronic circuitries. We may estimate, that the trajectory to be passed towards the controlled aircraft and its return path is equal. We need, however, as has been proved, a second signal phase as to 'align upon'.

What would happen, when the time-base interrupter output would be linked onto the recognition-switch input? The consequence would be, that an interference will be additionally painted at the simulated aircraft display, together with the the 'order' signal + the spinning EGON signal (remember, we have to count on with a single signal line from the FuG25a detector at 'pin 9' of the test connector). The advantage would be, that the aircraft time-base-phase will return towards the ground system. A second advantage would be, that the number of returning (time-base)pulses towards ground is (also) reduced. However, there might exist a technical bottleneck, in the electronic design of the FuG 25a apparatus, and this is the existence of a circuitry preventing that two pulse is to be transmitted following shortly to one another. 

Overlooking all this, it might be worth experimenting whether it works smoothly. However, one of my first experiments (Bladwijzer175) pointed in this way, but its, maybe confusing, time-base-phase wasn't pained onto the 'order' or command screen yet. Please scroll a bit down for the block diagram after you have clicked onto the Bladwijzer175 flag.   

Resuming the current considerations

This block diagram does reflect my new thoughts

Please, double click on this drawing as to open it in PDF

Instead feeding the Nachtfee time-base-phase onto an extra adapted point following the recognition key-selector, the time-base first order differentiated pulse is fed onto the FuG25a receiver output (pin 9 being the anode of the detector valve). Adaption of its injection level has to be determined experimentally. All returning signals from the FuG25a transponder will be keyed in a Morse-code sequence. Whether this will be a nuisance when viewing the simulated aircraft 'order' display remains open; and, the resulting alignment of the dual trace CRT accordingly will remain open as well.



On 27 August 2012


As so often in the very early morning, new hypothetical ideas came up


Let us estimate that eventually the previous concept works, it may then be possible to blank-out the feedback of the time base pulse. Only for a short interval. I might on the other hand conflicting with the Nachtfee 'order' signal; when both just point in the same phase-vector

Please double click on this drawing as to open it in PDF

Whether it works is still to be empirically figured out. It might work as long as the blanking pulse is short enough, compared to the duration of the differentiated Nachtfee 'order' signal. His particular experiment is only to be commenced as to prove that it hypothetically works. Everything undertaken in the Nachtfee project is hypothetical, as long as we lack original documents; at least say 1945/46 papers.




Maybe continued in due course 


My final comments

This page is setup with the idea that too many different web pages have been dealt with. Only those having followed the entire 'coming into being' may be able to grasp the system concept. Its growing complexity is even for myself becoming a problem, as to trace back some of the things done or simply copying some earlier used materials. When I started this page on 9 June 2012, I thought it would only focus on certain Nachtfee aspects, as the rest is easily traceable on our website. Which is after all not so simple.

The concept of this page is showing signs of inconsistency. We have to live with it, but for those reading its content for the first time they will not facing problems.


Please consider also our next Nachtfee Quartz page


 Arthur O. Bauer      

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