From: Jack Smith
Subject: Re: Alan Blumlein site
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Date: Mon, 16 Sep 2002 19:39:19 GMT
NNTP-Posting-Date: Mon, 16 Sep 2002 15:39:19 EDT
Organization: Cox Communications
On Mon, 16 Sep 2002 16:26:02 +0100, John Woodgate
>I read in sci.electronics.design that Jack Smith
>wrote (in ) about 'Alan
>Blumlein site', on Mon, 16 Sep 2002:
>>Knickebein around 30 MHz, X-Gerat 67/75 MHz.
>>Chain Home radars were also around 30 MHz as I recall.
>You are quite right. So why the big mystery about the beam width?
I had a chance to dig out my copy of "The Wizard War" (R. V. Jones),
and refreshed my recollection of his discussions.
With respect to the Lorenz beam, used in Knickebein, Jones provides a
clear description and diagram of the technique. It uses two "fat" beam
antennas, fanned apart. One sends morse dots . . . the other morse
dashes, interleaved with the dots - - - . When you are at the point
where the two antennas provide equal signal levels, the dots and
dashes merge into a solid steady signal. It should be possible for
the human ear to distinguish a dB or perhaps even less in signal level
This technique was well known, I believe, before WW II, in the form of
the LF A/N beacons. One antenna modulated with morse A (dot-dash) and
the other antenna with morse N (dash-dot); when flying down the
equal-signal locus, steady tone. To one side, hear the A or the N.
These were in the 190-535 KHz range, formed from two "figure 8"
antennas. See http://www.navfltsm.addr.com/ndb-nav-history.htm for a
nice illustration. The A/N blend point provided a sharp 3.5 degree
"beamwidth," something that would require an antenna size measured in
miles if done with conventional techniques.
Jones says that the Knickebein signals were between 28 and 35 MHz,
typically 30.0, 31.5 and 33.3 MHz, which were pre-set frequencies in
the German Lorenz receivers used by the Luftwaffe. The RF carriers
were modulated at 1150 Hz. (Apparently, the Germans used MCW
(modulated continuous wave) so that the receivers didn't require a
BFO, and so that frequency stability in the receiver wouldn't be an
Incidently, Jones relates that the Air Ministry had the leading
British propagation experts of the time look at the possibility of
using 30 MHz frequencies over the distance between the transmitter
sites in Germany and France and the UK and they concluded that it was
totally impossible for the Germans to use a beam system over the
British Isles. These experts believed the radiated signals were a
decoy to cover some other method of nighttime navigation. Jones had
to fight the Air Ministry to demonstrate his analysis (and his
proposed counter-measures) of Knickebein and X-Gerat was correct.
Jones has an equally interesting chapter on X-Gerat. It operated on
various channels between 66.5 and 75 MHz, with 500 KHz channel
spacing. The modulating tone was 2 KHz. Otherwise, it seems to have
been similar to Knickebein in the sense that it used the dot/dash
technique, although the user could either listen to the signal or look
at a meter deflection.
It seems to me, therefore, that there wasn't any great secret in the
"narrow beamwidth" techniques used in both Knickebein and X-Gerat.
Rather, it was an application of techniques that were known before WW
Anyone even vaguely interested in the radio and countermeasures part
of WW II should have a copy of The Wizard War. It's fascinating