Greeting all
My first posting to this list though I've been receiving mesages for some
months, and also listening on 136 KHz since the band came available in the
UK.
On the subject of receivers, for a long time I used a Trio TS430 which was
very deaf indeed on 136, made worse by an indifferent aerial...a base
loaded 100 ft sloper the top being 80ft agl.
With a tx under construction (valve job) it seemed appropriate to look at
getting a better rx, and having as another interest the collection of
vintage radio gear, I turned to what I had. Some might remember the AR88LF,
an old valve rx which tunes down through 136 to 73 KHz, but which has poor
if skirt selectivity (by modern standars). Also the Racal RA17 which
doesn't go much below 500 KHz, but which has a nice 100 Hz bandwidth
crystal filter. By taking an o/p at intermediate frequency from the AR88,
and feeding it into the RA17 tuned to that frequency, what seems like a
very good rx system emerged. In effect the AR88 has become a tunable
converter.
The result is that I can hear very much more than I can work, though
perhaps the 85 watts or so rf o/p from the tx is too modest. Suffice it to
say that on 4 March I was receiving OH1TN calling cq at a nice 559 though
he couldn't hear my puny signal and no one else was heard calling him
whilst I was listening. My rx system can hardly be described as hi tech,
but it does the job (though isn't exactly portable), and anyone with an old
rx covering the 136KHz band might care to consider a similar arrangement,
perhaps using more modern gear as an if amp/detector.
When I heard OH1TN, the aerial was a 240 ft inverted vee with apex at 80 ft
agl inductively loaded at the feed point. Since then it has been changed to
a triangular loop with the loading coil at the far end of the inverted vee
from the feed point, resonated with a series vacuum variable cap to a
return wire trailing on the ground back to the feed point. Initial
reception tests suggest a worthwhile (and anticipated) improvement over the
original base loading arrangement. Maybe further improvements can be made
with it as experience/knowlege of lf techniques is gained.
Incidentally, as an aside, when listening to 136 at night, I sit in near
darkness, since it soon became clear the energy efficient bulbs used in the
shack were producing a nasty rf noise blanking all but the stronger
signals.
73..Howard/G3RXH
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