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Message-ID: <D5DC09A7D5814AECA261296B24436E9B@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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In-Reply-To: <003001caf45f$d675b830$0401a8c0@xphd97xgq27nyf>
Date: Sat, 15 May 2010 21:44:28 +0100
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Subject: LF: Re: 9 antennas
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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" <g3kevmal@talktalk.net>
To: "rsgb" <rsgb_lf_group@blacksheep.org>
Sent: Saturday, May 15, 2010 7:52 PM
Subject: LF: 9 antennas


LF/MF
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.
de mal/g3kev