Dear Stefan, LF Group,
The loop I am using is detailed at
https://sites.google.com/site/uk500khz/members-files/files - see the file
"BPloops2.pdf". These loops use a 1m x 1m single turn made of copper tube.
The unloaded Q of the single turn is quite high, >100, provided a very
low-resistance capacitor (about 400nF for 136k) is used. So the Q is similar
to a multi-turn loop, although the impedance is very much lower. The low
impedance means it is easy to feed a 50ohms input, and it is very
insensitive to de-tuning. The loop is used in a bandpass design that gives a
flat response over about 10kHz bandwidth without re-tuning, combined with
better out-of-band rejection.
A single loop gives a figure-of-8 directional pattern with 2 nulls 180
degrees apart, and it is well known that combining equal-amplitude signals
from a loop and a vertical with the correct phase gives a cardioid pattern
with a single null. If the signal amplitude from the vertical is reduced,
the result is a skewed figure-of-8 pattern, with less than 180 degrees
between the two nulls. With appropriate adjustment of the vertical signal
level and the loop direction, you can have 2 nulls seperated by any angle
from 0 - 180 degrees. I used this type of arrangement when Loran-C was being
transmitted from Rugby a few years ago - then it was not possible to null
both the Rugby and Lessay signals with a simple loop at this QTH, because
the difference in bearing between the transmitters was only 100 degrees or
so. The procedure was to aim the loop on a bearing half way between the
bearings of the two transmitters, then adjust amplitude and phase of the
loop/vertical signals until both signals were nulled. This was quite
effective, and it was possible to effectively eliminate both Loran signals
while getting good coverage of Europe. Of course, the usefulness of this
type of system depends on the direction of the wanted signal being well away
from the interfering transmitters, but looking at the map, it should be
possible to make it work quite well for receiving signals to the west of
Heidelberg while rejecting DCF39 and HGA22
Cheers, Jim Moritz
73 de M0BMU
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