[Fwd: LF: Simple receivers for 136kHz]

Andre' Kesteloot akestelo@bellatlantic.net
Mon, 15 Nov 1999 09:59:11 -0500

Kevin Ravenhill wrote:

> 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.