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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