Dear Mal, LF Group,
You probably need to read the e-mails to the reflector on this subject sent
during DK7FC's previous set of tests, where this topic was quite thoroughly
discussed. But, in case there are other readers who don't have access to
these posts, briefly...
Several types of antennas have been used satisfactorily; large and small,
loops and verticals, tuned and untuned. It is rather easy to produce an
antenna/receiver combination where the receiver noise floor is well below
the band QRM/QRN, even with small antennas. Examples include the loop
antenna/preamp design that forms part of the portable RX schematic I posted
yesterday, and PA0RDT's "Mini Whip" with minor mods to extend the lower
frequency range. Many other approaches are possible.
The antenna location is probably more critical than the actual design - any
location near electrical power lines or domestic mains wiring can be
expected to have high levels of 50Hz harmonics, and assorted other mains
related noise. Hence the advantage of a portable antenna and receiver, which
can be taken to a lower-noise location. A loop can be advantageous to null
out at least some noise sources.
Once you have minimised man-made noise, natural QRN is very strong at 9kHz -
several uV/m per sqrt(Hz). This can only be reduced using directional
antennas (loops, basically), if directions are favourable. For reception of
QRSS type modes, the extremely narrow bandwidths are defined by the FFT
process in the waterfall display as usual. A very narrow bandwidth antenna
or RX filter is not particularly useful here. There are no strong adjacent
frequency signals really - the closest are the Alpha beacons at 11.9kHz and
above. I found there was a substantial advantage to having a much wider
bandwidth input circuit feeding a PC sound card (about 3kHz in the loop
antenna / preamp design posted yesterday), followed by clipping to reduce
the energy in the QRN impulses and the high resolution FFT (both performed
in software by Spectrum Lab). This produced around 10dB advantage compared
to just reducing the bandwidth to a minimum.
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: "mal hamilton" <[email protected]>
To: "rsgb" <[email protected]>
Sent: Saturday, May 15, 2010 7:52 PM
Subject: LF: 9 antennas
Not much response regarding antenna development on 9 kHz apart from active
probes and small loops.
My approach is a proper resonant antenna, verticals, long wires etc and
resonant on the freq of interest, giving more sensitivity and a sharp
response, keeping out qrm from adjacent frequencies.
I am referring to RX antennas at this stage.
If qrm is as bad as some say around 9 kHz then small loops and active probes
will only amplify the noise as well as signal, whereas a natural resonant
antenna will eliminate a lot of noise from adjacent frequencies and peak the
signal at the frequency of interest as stated above.