Knickebein lives again!

[Funksammler]

Winter is too cold to work on the Horch, so I am doing smaller projects inside the house. One of the things I wanted to do for a while was build a test transmitter for the FuBl Blind landing installation. So after a few trials and errors I managed to get a first version working on my breadboard:

<a href="http://s672.photobucket.com/user/Funksammler/media/DSC09051_zpshqslg2if.jpg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/DSC09051_zpshqslg2if.jpg" border="0" alt=" photo DSC09051_zpshqslg2if.jpg"/></a>

The setup contains two transmitters, one operating on 30 Mhz replicating the Lorenz beam signal, the second operates on 38 Mhz and replicates the VEZ and HEZ approach beacons.

Here is the reminder of what the FuBl2 setup looks like:

<a href="http://s672.photobucket.com/user/Funksammler/media/DSC09054_zpsrfqrb0zk.jpg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/DSC09054_zpsrfqrb0zk.jpg" border="0" alt=" photo DSC09054_zpsrfqrb0zk.jpg"/></a>

In this case I fitted the Ebl 3F to the top, this receiver can be remotely tuned from 30-33 Mhz and receives the dots and dashes from the beam transmitter. It drives the field strenth and left-right needle in the AFN instrument. The Under that is the Ebl 2 approach beacon receiver. This receiver receives on 38 Mhz only and drives the lamp in the AFN instrument.

So for the first time in many years the Lorenz blind landing signals are audible on the airwaves (only in my attic though....); Knickebein lives again!

I uploaded a number of short clips to YouTube showing the AFN/1 instrument react to the various signals. The video also contains the sound of the signal.

The first clip simulated an aircraft flying into the beam. It enters to the left of the beam, into the dot-zone. You can see that the instrument is pulsing left with the dots. Also note that a strengthening of signal makes the instrument point left. It was important for the pilot to distinguish between the pulses and the more gradual swings of the needle due to changing signal strength:

https://youtu.be/vfSkcZVFe3o

The second clip simulates the aircraft starting on the left of the beam in the dot-zone and flying starboard into the dash-zone.

These first two clips do not only simulate a blind landing approach, but do also simulate the "Knickebein" beam guidance system used in early 1940 at the start of the Battle of Britain. At that time the earlier Ebl.1 receiver would still have been fitted to the German bomber aircraft:

https://youtu.be/guUifTlZcoE

Clip three simulates the "Vor-Einflugzeichen" (VEZ) or first approach beacon. The VEZ was located 3000 meters before the runway. This is a slow pulse, low tone signal. The pilot would approach on a constant height and on hearing the VEZ would start a given descent. Keeping one eye on the variometer and the other on the AFN he would be near the ideal glide path onto the runway

https://youtu.be/YnYwdweLMOM

Clip four simulates the "Haupt-Einflugzeichen (HEZ) or main approach beacon. The HEZ was located 300 meters before the runway. This was a fast pulse, high tone signal; the HEZ is an urgent warning that the pilot does not have a lot of time to make adjustments. The pilot would check his height and had to decide to continue the landing or abort. By now the runway lights should be visible for the final approach:

https://youtu.be/8UsPYerEwiA

The fifth clip shows the aircraft flying along the beam. It starts in the centre, moves to port into the dash zone, corrects but overshoots to the starboard side into the dash zone. The aircraft ends back onto the beam:

https://youtu.be/H8xpMC0SJ9I

The final clip shows all signals in a single clip, the aircraft is flying in the dot-zone and the VEZ and HEZ are triggered in quick succession. Not a very realistic scenario, but it allows to different signals to be compared:

https://youtu.be/sb9Llv5B6xc

So after all those years the FuBl2 is fully functional again. There is still a bit of design work to do on the HF antenna circuit of the test transmitter before I will commit to soldering it onto a circuit board and into a handy box. I hope you agree that the results so far a quite interesting....

regards,

Funksammler

[Dufleuve]

Hello FS,

You're the beacon of this forum.

Great work

Cheers

[LFT]

Great job, Funksammler!
We discuss your post on Il-2 game forum here (russian)

[Funksammler]

Great job, Funksammler!
We discuss your post on Il-2 game forum here (russian)

Perhaps I need to clarify the specifics of "Knickebein" beam system a bit more. The "Knickebein" only used the "beam" signals consisting of the dots and dashes. The VEZ and HEZ on 38 Mhz played to role. So for the "Knickebein", two dot-dash beams on different frequencies were used; the radio operator had to swith the Ebl.1 receiver between the two channels to determine the cross beam. Not ideal, that is why on the later X and Y systems, separate receivers were used for the main and cross beam signals.

regards,

Funksammler

[Funksammler]

A bit more work done on the FuBl2. First of all I transferred the test transmitter design from my breadboard onto a printed circuit. This has now been tested and is working. Now all I have to do is build it into a box and finish it off.

<a href="http://s672.photobucket.com/user/Funksammler/media/DSC09079_zpsszdxqp4q.jpg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/DSC09079_zpsszdxqp4q.jpg" border="0" alt=" photo DSC09079_zpsszdxqp4q.jpg"/></a>

I also build a dipole antenna for the Ebl 2 receiver. Whereas the Ebl1 or 3 receiver used a simple rod antenna for all round reception, the Ebl 2 receiver for the VEZ and HEZ used a dipole antenna arranged lengthwise under the fuselage of the airplane.

To start with the antenna was made of two rods fitted to insulators:

<a href="http://s672.photobucket.com/user/Funksammler/media/FuBl2%20dipole_zps5b6bvznq.png.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/FuBl2%20dipole_zps5b6bvznq.png" border="0" alt=" photo FuBl2 dipole_zps5b6bvznq.png"/></a>

Due to their exposed position, these antennas often got damaged during takeoff and landing, so later in the war, they placed the antenna inside the fuselage under a plexiglass cover, less susceptible to damage and also more aerodynamic.

Rather than rods, I use two lenths of wire, stretched on a 2 meter long wooden plank:

<a href="http://s672.photobucket.com/user/Funksammler/media/DSC09081_zpssodhuavu.jpg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/DSC09081_zpssodhuavu.jpg" border="0" alt=" photo DSC09081_zpssodhuavu.jpg"/></a>

The two halves of the dipole are connected to the DAG1 box, fitted to the other side of the plank:

<a href="http://s672.photobucket.com/user/Funksammler/media/DSC09080_zpswoukcifj.jpg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/DSC09080_zpswoukcifj.jpg" border="0" alt=" photo DSC09080_zpswoukcifj.jpg"/></a>

The DAG1 or "Dipol AnschlußGerät" contains a tuned circuit to adjust the antenna for maximum signal. This was fitted inside the fuselage of the aircraft:

<a href="http://s672.photobucket.com/user/Funksammler/media/DSC09082_zpsctids0oq.jpg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/DSC09082_zpsctids0oq.jpg" border="0" alt=" photo DSC09082_zpsctids0oq.jpg"/></a>

The reason for the use of this complicated antenna arrangement becomes clear if you look at the antenna diagram of a dipole antenna:

<a href="http://s672.photobucket.com/user/Funksammler/media/dipole_polar_diag_zpspizrullp.gif.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/dipole_polar_diag_zpspizrullp.gif" border="0" alt=" photo dipole_polar_diag_zpspizrullp.gif"/></a>

With the dipole fitted lengthwise to the aircraft, the signal is strongest at a right angle to the aircraft. With the transmitter dipole fitted in the same direction, this means that the VEZ and HEZ signals were localised sharply on the glide path, they would have been audible for only a few seconds giving the pilot the most accurate indication of his distance to the runway.

regards,

Funksammler

[Funksammler]

A few interesting documents from my archive: these are pre-war pilot's maps showing the approach to some (civil) airstrips as they were in 1938, I have scanned examples of Rotterdam in the Netherlands and Zürich in Switzerland. The approach beam for both these civilian airports would have worked on 33.33 Mhz.

They show the approach direction (QDM) and height (QFM) and the positions of the VEZ and HEZ beacons. For Rotterdam, the VEZ was placed 5 km before the airfield. They also show and describe the main visual landmarks that the pilot can use. The QFM heights are given at the VEZ and HEZ position, so the pilot should approach the VEZ at 200 meters and descent to 50 meters at the HEZ.

<a href="http://s672.photobucket.com/user/Funksammler/media/FuBl%20Rotterdam_zpshfl2cpvg.jpeg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/FuBl%20Rotterdam_zpshfl2cpvg.jpeg" border="0" alt=" photo FuBl Rotterdam_zpshfl2cpvg.jpeg"/></a>

Because of the hilly terrain, the approach to Zürich was a bit more complex, in the notes, different heights for different parts of the approach are given, here both the VEZ and HEZ locations are drawn on the map. An added complication for the Zürich approach was that the approach route flew over an artillery range, so it was essential to clear this with sufficient height!

The notes mention that morse code üü ( . . _ _ . . _ _ ) was send on 340 Khz as well as on the 33.3 Mhz blind landing beacon frequency when the aircraft overflew an observation post placed on the artillery range about 4 km before the VEZ.

Before reception of the üü signal, the aircraft should not fly under 400 meters, before the VEZ they should stay over 250 meters and cross the HEZ at 100 meters, so a much steeper descent on final approach than in Rotterdam.

<a href="http://s672.photobucket.com/user/Funksammler/media/FuBl%20Zuumlrich_zpsywuyj6d9.jpeg.html" target="_blank"><img src="http://i672.photobucket.com/albums/vv86/Funksammler/FuBl%20Zuumlrich_zpsywuyj6d9.jpeg" border="0" alt=" photo FuBl Zuumlrich_zpsywuyj6d9.jpeg"/></a>

regards,

Funksammler

[Funksammler]

On re-reading the history of the "Knickebein" some interesting observations....

The Knickebein was used operationally from about July 1940, the British managed to jam the beams sufficiently so that it's use was abondonned by about September.... Things were moving very fast during the "Battle of the Beams"...

It was mainly the inexperienced crews that were having problems with the jamming of the "Knickebein" signal, experienced crews could still use it, how come?

First thing to consider is how the "Aspirin" jammers actually worked. The jammers emitted a "dash" signal on either 30 or 31.5 Mhz (the two frequencies mainly used for the "Knickebein" signal during this period). The theory was the the "dash" signal, mixed with the real "knickebein" signal would make crews believe they were flying on the right side of the beam and would force them to deviate left to compensate. However, the Aspirin dashes were not synchronised with the original signal and being relatively weak, the Aspirins had a range of about 30 miles, so only within this radius would the "Knickebein" be jammed.

So Luftwaffe crews could pick up the "Knickebein" signal over the North sea without any interference. Experienced crews would set up their autopilot to fly along the beam. (They would already know the direction of the beam, so the beam allowed them to compensate for wind drift).

Experienced crews would actually not fly exactly in the equilibrium (constant tone) zone of the beam, they would fly slightly to the left, so that the dashes would just be audible. During blind landing this was found to be far more accurate than trying to stay in the equilibrium zone, experienced blind flying crews adopted this method for Knickebein also.

Because the Aspirine Jammers were not synchronised, the crew flying slightly in the "dot" zone could immediately distinguish the fake "dash" signal, as no continuous tone would be formed. While flying on the autopilot, the pilot would know that it would only take minor adjustments to the autopilot to keep flying along the beam.

Since the range of the Aspirin jammers was limited, the signal would be jammed for relatively short periods, and the pilot could clearly hear the "Knickebein" beam again after passing the jammer.

Obviously the British placed their limited Aspirin Jammers near their most strategic targets to make it as difficult as possible to hear the signals during the bombing run.

Altough the experienced crews would still get through, unfortunately for the Luftwaffe experienced blind flying crews were in a minority. The inexperienced crews had far more difficulties following the Knickebein beam. They woud try to stay in the equilibrium zone (as they were instructed) and were far more susceptible to be fooled by the fake jamming signal. Upon losing the beam the inexperienced crews often panicked. There are stories of crews abandonning their aircaft when they lost the signal, so overall the Aspirin jammers were having the desired effect.

It is however inaccurate that the beams were "bend" by the jamming signal. This would require the jammers to be synchronised and this was never achieved against the Knickebein signals (but was with the later "Y" beams).

regards,

Funksammler