Topband: Dedicated topband transceiver

[Steve Ireland]

G'day 

It has been interesting reading the thread between Tom W8JI and Mauri I4JMY
about sensitivity, to pre-amplify or not to pre-amplify.  As Tom says,
there are "horses for courses"

On the drive back from the VK6VZ/6 operation, my friend Phil VK6APH, who
had spent some time (between HF and 6m duties) watching how I operated the
FT1000MP during the Stew Perry, remarked that I was only using a small
percentage of the facilities of the transceiver - and some of them in a way
that was far from optimum from its design.

The AF gain on the rx was turned right up and the RF gain right down.  As
the ambient noise level was only S1 or S2 from the isolated location and
signal levels were high on the inverted vee dipole antenna 180' above the
sea that I was using, there was very little need for much RF amplification
at all.

Phil put forward the idea of making a very simple 1.8MHz-only transceiver,
with a tuned Cohen filter in the front end (see Peter Martinez G3PLX's
famous 'Plagiarise and Hybridise' articles in the RSGB bulletin in the
1970s), virtually no RF amplification, a high level mixer, a single 455 kHz
IF with 500Hz crystal filters at each end, and, finally a 'hi-fi quality'
audio amplifier (a 20W-plus chip run at a low level.  The VFO could be a
direct digital synthesiser, or even an analog VFO (i.e. no phase noise
whatsoever).

The proposed transceiver would be compact and run from a modified PC
switch-mode PSU, making it weigh a fraction of the FT1000MP and ideal for
using on large antennas strung from the top of lighthouses.

For a topband DXpeditioner it would be a great radio, but maybe not a lot
of use to anyone else.  However, we are going to try and put it together
over the winter here, from various junk box parts.

To make it even more of a "horse for a course", we are talking about
putting in some old-fashioned bits that we both used in in our early
topband days that work well.  If anyone has an old HRO dial in their
shacks, even in poor condition, that they wouldn't mind parting with for
some dollars, please drop me an e-mail...

Vy 73,

Steve, VK6VZ

  




    


>The old argument or idea you never need an amplifier is just as silly 
>as saying you always do need an amplifier.
>
>I would guess the "rule" we never need -140dBm sensitivity is 
>based on a suburban location, perhaps with a wide filter, or 
>perhaps a very efficient antenna. It is almost certainly NOT based 
>on a very directional antennas, in particular inefficient antennas, 
>since the more directional the antenna the lower the noise level is.
>
>Suggesting people always do NOT need an amplifier or a very 
>sensitive receiver because we read it someplace is like telling them 
>they always DO need one. Both are very wrong, because the 
>circumstances vary greatly!