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Dear LF Group,
At M0BMU, the original motivation for using
seperate RX antennas was the increasing 50Hz-related noise level on the TX
vertical. This seems to be due to pick-up from the house mains wiring, but
moving the vertical further from the house would result in reduced possible
length, and it is short enough already!
Remote vertical antennas worked fine as far as
eliminating mains noise was concerned, but did nothing to reduce distant
sources of QRM such as Loran. Here I have to use quite selective antennas (the
broadband active-whip type of antenna is not really workable here due to the
20V/m MF signal levels from Brookmans Park). For some time I used single loops
oriented to null out the Lessay Loran signal, and I estimated this reduced
the noise floor for most European signals by 6dB or so, which made quite a few
more stations audible. I found that much experimentation was needed to find
suitable positions for the loops where they would null out Loran, but not pick
up 50Hz noise at the same time - loops seem to be more sensitive to this type of
QRM.
Then the Rugby Loran started up, and the band noise
level in aural reception bandwidths (e.g. 300Hz) increased by 15dB, wiping
out many of the weaker signals at this QTH. I countered this
using both loop and vertical antennas phased together to provide two
adjustable nulls aimed at Rugby and Lessay. This brought the situation for aural
reception of European sigs to about the same place as it was before - but most
signals coming from the west are attenuated by the overall directional
properties of the system.
So, for aural
signal reception at this location in SE England, currently neither loops
nor vertical antennas by themselves work very well, allthough quite good results
are achieved by using a phasing system with both types of antenna to null out
the Loran.
For narrow-band QRSS, etc. signals, the situation
is different, since provided the received signal frequency does not coincide
with a Loran line, Loran does not matter too much. There does not seem to be
much to choose between the loop and the vertical most of the time, however under
quiet band conditions with little QRN I have found that if I use the phased loop
and vertical antennas, and reverse the relative phase of the antennas to favour
signals from the west (i.e. for transatlantic reception), the band noise is
reduced by several dB on the spectrogram compared to either antenna by
itself - I suspect this may be due to a reduction in "Luxembourg effect" noise
originating from Europe to the east.
So here I find it neccessary to use both types
of antenna simultaneously, if I want to achieve good results. This is mostly to
overcome the effects of man-made QRM and to enable aural reception of signals,
which are perhaps the main differences between my situation and the others who
have commented.
Cheers, Jim Moritz
73 de M0BMU
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