From: "Michael J Underhill" <m.underhill@eim.surrey.ac.uk>
To: rsgb_lf_group@blacksheep.org
Date: Sat, 1 Feb 2003 16:33:13 -0000
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Subject: LF: RE: RX loop designs
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Yes please, Jim.

By the way, in experiments with my version of the CFL, I have made
double tuned HF transmitting loops with similar bandwidth improvements
to yours but having at least four times the Q values that I guess you
have. I have not yet achieved the claimed CFL bandwidths with the same
efficiencies and sensitivities that I get from my other loops. Ganged
tuning is necessary and it is difficult to keep this accurately enough
in step to cover even a single HF band. At the moment considered 'not
worth the hassle' for the HF bands.

73 de Mike G3LHZ

m.underhill@surrey.ac.uk

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of James Moritz
Sent: 01 February 2003 11:33
To: rsgb_lf_group@blacksheep.org
Subject: LF: RX loop designs

Dear LF Group,

With the recent chatter about LF loops, I finally got round to writing
up 3 
of my receiving loop designs, which I have been using for the last year
or 
so. They seem to be significantly different to the designs other people 
have come up with, and work well, so could be of interest.

The usual problem with a tuned loop is that the bandwidth is very
narrow, 
so remote tuning is needed. Resistively loading the loop increases BW,
but 
reduces the loop output and SNR, so a bigger loop is needed, and also 
out-of-band selectivity is reduced. My idea was to add an additional
tuned 
circuit coupled to the loop to produce a bandpass response, which can be

designed to give a flat-topped or slightly double-peaked response
covering 
the whole 136kHz band without re-tuning. This has turned out to work
well, 
and the article describes one 1m^2 loop with a bandwidth of about 4kHz,
and 
a 2m x 2m loop using a single turn of tubing with a bandwidth of about 
18kHz. With the simple pre-amp also described, the noise floor of both 
these antennas is well below the band noise even on a quiet day.

The article also includes a wideband loop based on a low-pass rather
than 
bandpass design, which covers 10kHz -200kHz, also a single turn 2m x 2m 
loop. This is slightly less sensitive than the bandpass designs, but
more 
than adequate for general use over the LF/VLF range. The relationship 
between field strength and loop output for this antenna is predictable,
so 
it can also be used for field strength measurement.

The article I have written is 5 pages, with several diagrams, and is a
.pdf 
file of about 340kB. It is too big for the reflector, but if you are 
interested, let me know, and I will e-mail it to you as an attachment.

Cheers, Jim Moritz
73 de M0BMU