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
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