Nachtfee - EGON


During the 35 year NVHR anniversary held in Egond aan Zee on 23- 24 March 2012


I discovered that it is seemingly difficult to understand the interaction between EGON and Nachtfee


This webpage started on 26 March 2012

Status: 15 April 2012

In progress


What is EGON?

EGON is an acronym for: Erstling - Gemse - Offensiv - Navigation



I would not wonder when EGON had been the first secondary radar system used for measuring distance operationally in the world (of course, neglecting that it IFF generally is connected to distance as well). The Germans however, used it tactically; in combination with Nachtfee even operational data was being conveyed.     Like in most Western Air Forces, I.F.F. was rather widespread used as a means to identify 'Friend or Foo'. A rather important means as to correlate an anonymous radar target onto a particular friendly object on a radar display. It was similarly used in Germany.

My guess is, that from 1943 onwards EGON gained significance as it neutralised in some respect the effectiveness of Allied radar jamming. An advantage is also that the radar range improves a lot. Freya's range was say 100 km, EGON could be operated up to about range of 250 km.


The Freya-EGON system is shown on the far right-hand side


To my understanding, there existed a mix of systems. Some being purely relying on IFF technology. Like KUH (also known as Q-Sender)  - FuG25a and Gemse; or it was possible selecting between regular Freya or EGON operation.(Bladwijzer12)

Both systems relied on a 500 Hz PRF; also Nachtfee is using this frequency.

We have already discussed this aspect in the previous pages.


Like radar, EGON used (relied upon) signal pulses in the order of several kilo-watts of a few µs wide. Radar signals must rely upon high transmission power because the signal has to be bounced back towards the radar receiver again. Most of the energy will be lost for ever as it will be scattered away from a target randomly. In case of Würzburg only 10-15 to 10-17 is returning to the radar system again. Whereas in case of EGON the returning signal is hampered by the squared power per unit distance only; in contrast a normal radar signal has to pass a trajectory twice, thus it is losing energy to the forth power.

A minor disadvantage is that the signal is not obeying free space physics directly, but it has to pass through many transponder components before it transmission velocity equals 'c' again. Inside the transponder system a signal delay is occurring, which is causing some kind of inaccuracies. Whereas modern radars can measure distance very accurately, secondary radar is counting in a few hundred metre tolerance.

Nowadays, it is used widespread as among it main purpose to identify an aircraft, it also provide data on: flight number - destination - and other means. But also technical data like the state of its engines; so that ground service is automatically informed what kind of action have to be undertaken after landing.




The principle of EGON in combination with Nachtfee (simplified version)


The FuG25a IFF transponder is inside the aircraft. The red dotted line inside it represents the responding IFF signal returning towards the Gemse receiver inside the radar cabin.

Just where I wrote 'Nachtfee - EGON feedback signal' is the point where the returning IFF signal is entering the Freya-EGON measuring system. Freya as well as EGON both relied on 500 Hz PRF, the returning Erstling signal could also been handled by means of the Freya measuring chain (Messkette). Of course, provision has to be taken for the distance off-set caused by the FUG25a circuitry.



The upper antenna as well as the right-hand section of this line drawing represents the EGON system

It is, however, also visible the Gemse signal is being fed onto the ranging screens. These signals being compared with the one derived from the Freya calibrated measuring chain (Meßkette). 


One might get the impression from the above sketch that there does not exist a feed back from the Nachtfee display inside the aircraft. As I have discussed extensively in the previous pages, it is very most unlikely that without a kind of deflection-feedback Nachtfee could not have been operated!    We have nevertheless to consider that Trenkle, although being a most accurate person, that he had no technical information as to how Nachtfee and/or FuG136 had worked in detail. He relied heavily upon P/W statements reflected in the Felkin papers; which is found to be inaccurate or simply faulty! And of no direct technical relevance. Maybe having got some information from Mr. T. von Hauteville, who had designed Nachtfee at Rechlin.(Bladwijzer8)



On 28 March 2012


I had some new thoughts about the possibility and consequently how, the Nachtfee deflection data could have returned to the Freya-EGON ground station.



Part of a British wartime evaluation on German navigational aids (Bladwijzer14)


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.


This information is in our current context interesting. Though, we may ask ourselves, have they really intercepted the signals of the Freya-EGON ground station, or have they intercepted the EGON signal originating from the FuG25a equipment in the German aircraft? Don't expect from me an answer, but we have to consider all possible parameters.



A bit disappointing though, is that it proves being hardly possible to convince some who may think from their previous respected occupation or status that 'the truth' is theirs; and that is the only truth there is.

Our survey however, is proving that most is different than was being thought previously. Only by following the course of our survey is unravelling the curious discoveries. Not because we cannot make misjudgements, but the Nachtfee project itself is keeping most of the clues in its housing or eventually concept. Supported by some British reports.




On 15 April 2012

It is just appears in my mind:



Since August 2012

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


And, the PowerPoint progress page (converted into PDF)




To be continued in due course


By Arthur O. Bauer






By Arthur O. Bauer



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