Dear Jim,
Thank you for the comments, useful as ever.
I know most of the needs you mentiond and also the problematic match.
Well, certainly there is an advantage by using buffers to minimise the
interaction between the antennas. However there is a limitation for
special low noise amplifiers for Martin. So we have to try what we can
achieve with this simple design. At least both his E and H field antenna
has a BW of above 3 kHz and the levels are quite high, i.e. about 30 to
40 dB above the soundcard noise. DCF is audible most of the time at night.
The plan is to permanently toggling the switch while slowly increasing
the pot from 0% to 100% and listening to DCF39. Hopefully a difference
can be heared in a certain position. Then, some fine adjustments could
take place in that region and we could try to slightly change the
resonance of the loop to achieve a phase of 180 deg between E and H, to
maximise the F/B and so the S/N.
At least the loop performs better than the wire, so we won a few dB
(looks like at least 3 dB). The winter season may give us another few
dB, giving others the chance to leave traces in SA.
Thanks and 73, Stefan/DK7FC
Am 25.08.2012 18:19, schrieb James Moritz:
Dear Stefan, LF Group,
In order to achieve one or two nulls using combined E- and H-field
antennas, you need to be able to adjust the relative amplitude of the
unwanted signal from the 2 antennas to be equal, and the phase
difference to be 180degrees. In your circuit, assuming the wire signal
is bigger than the loop signal, you should be able to achieve
amplitude balance by adjusting the pot. You will be able to get some
adjustment of the phase by altering the resonant frequency of loop and
wire antennas, although if you are unlucky, the required phase shift
may be outside the practical adjustment range. Adjusting to obtain a
null might be quite difficult; both amplitude and phase adjustments
are quite critical , and in this circuit, the 3 possible adjustments
all interact - adjusting phase by changing resonant frequencies will
also change amplitude, altering the amplitude pot will change the Q of
the tuned antennas and so the phase shift.
There are some rather simple noise cancelling circuits that work in a
similar way - usually, these try to make the amplitude and phase
adjustments reasonably independent of each other. This mainly relies
on having quite high attenuation of at least one of the antenna
signals, which is OK provided there is a large SNR margin to begin
with, and the receiver has good sensitivity. If this is not the case,
much better results can be achieved by inserting low-noise buffer
amplifiers between the antennas and ampltude- and phase-adjusting
networks. Also, it helps if the antennas are fairly wide-band,
otherwise the null is only achieved over a very narrow frequency range
I have used systems of this type for several years, with small loop
and vertical antennas. For a point source of QRM, they whork very well
- for instance, critically adjusting the amplitde and phase contols at
my QTH can reduce the level of DCF39 or HGA22 to almost inaudible. Of
course, this does not help much if you are trying to receive DL
stations... But they are less effective at reducing QRN, I expect
because QRN impulses are usually arriving from a wide range of angles,
while the antenna null is quite narrow. But if the QRN is mainly from
a single large storm, there can be a useful improvement.
Probably the simplest way of achieving a directional null is using
"terminated loops" such as the K9AY and EWE type antennas - I have
found almost any large loop with a suitable adjustable terminating
resistor will work quite well/ But the loop area must be quite large ,
because this type of antenna is rather inefficient in terms of
received signal power.
Cheers, Jim Moritz
73 de M0BMU
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