Message-ID: <000501bf2f5c$84687c40$6d8fbc3e@default>
From: "Kevin Ravenhill" <k_ravenhill@radiometrix.co.uk>
To: "RSGB LF Group" <rsgb_lf_group@blacksheep.org>
Subject: LF: Simple receivers for 136kHz
Date: Mon, 15 Nov 1999 11:27:40 -0000
Organization: Radiometrix Ltd.
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Hi all, hope this isn't too "off topic" and will be of interest to
somebody....

I've noticed that, whilst there is naturally a lot of discussion on the
reflector about transmitters and antennas, there is relatively little about
receivers. Since I have a strong preference for uncomplicated homebrew
equipment and an aversion to buying expensive commercial gear, I've recently
been experimenting to find out how simple a receiving setup can be made
whilst still being capable of useful results on 136kHz.

I'm currently using, of all things, a simple regenerative design ("blooper")
loosely based on GI3XZM's original in RadCom Tech Topics Oct 1987. My
version has 7 transistors and 2 ICs in total and includes preamplification
and very selective bandpass filtering before the detector, together with
2-stage active filtering in the AF giving a 6dB bandwidth of about 150Hz at
700Hz. The whole thing is built into a tobacco tin (!) with PCB material
added for mechanical rigidity. Power is provided by a 9V PP3 battery which
is also squeezed into the tin.

Despite the lack of any voltage regulation this has proved surprisingly
stable (to the extent that there is no noticeable frequency drift over long
periods even when using Spectrogram on the narrower settings), and strong
signal handling appears better (at least subjectively) than my old R210
military valved Rx. MDS is considerably better than 0.1uV. The second
sideband ("audio image") can be a problem, of course, but less so than might
be expected because of the narrow audio filtering.

Using a VERY inefficient antenna consisting of 20m of wire draped round the
loft and tuned against a questionable earth, this setup has received (using
normal CW) most of the UK stations and quite a few from elsewhere in Europe
at surprisingly good signal-to-noise ratios.

The antenna now appears to be the limiting factor. Apart from mains-borne
QRM/N, Loran chatter is quite a problem here in East Devon and a frame
(loop) antenna suggests itself as the next step.

I would be the first to admit that this arrangement cannot stand comparison
with a good bomb-proof superhet receiver with narrow crystal filtering,
however it does seem to offer possibilities for those who like an additional
challenge and prefer homebrew equipment (or, one could say, those who like
to make life difficult for themselves!).

Other possible avenues for experimentation are "super-gainer" designs using
narrrow crystal ladder filters, and phasing-type direct conversion, both of
which should eliminate the audio image problem. I would be interested to
know whether any LF'ers have tried anything along similar lines.

73

Kevin, G1HDQ.