GB3BUX 70.000
mhz
I consider myself fortunate to live within
range of GB3BUX on
70.000 mhz. It provides my station with a constant signal
reference and I would like to thank those who provide the
service from up there in them there Peaks. It is an old school
beacon using CW, but very useful nonetheless.
When using digital modes on VHF it is useful to be able to check
the calibration of your radio from time to time. There are many
modern radios which are capable of being 'disciplined' (always
thought that an odd phrase for radio) by GPS or Rubidiun source
or whatever. I have used GPS with my various U2 and U3 kits
(thanks Hans) and a Rubidium source for my Flex 1500. But what
if you want to go on the bands and you do not have this
facility? That's where your calibrated beacon can help.
Using digital modes it is easy to check your radio. For this
(just out of habit) I use Spectran
even though you can just use WSJT-X (see below). I prefer a
separate program not just because it gives an actual audio
frequency on screen (which is nice though not essential), but
also because I have several radios connected to my computer with
several 'installations' of WSJT-X. It can all get a bit busy at
times. Spectran is separate from all of that.
If you tune your radio to 70.000mhz (the transmission frequency
of GB3BUX in this case) and listen chances are you will hear
nothing. Having the radio tuned to zero beat, or close to zero
beat give or take some error, is not so helpful. Instead I tune
to 69.999 mhz on USB. This should provide a 1000 hz tone. You
could tune to 70.001 mhz and switch to LSB to achieve the same
result, especially if your radio does not work below the band -
or if filters change when dropping below 70mhz which could
affect sensitivity. I use USB only because I might forget to
switch it back to USB later and then all would have been in
vain, also there is a possibility of discrepancy between LSB/USB
and it is USB I want to use after all.
So what does this look like. Well here is my trusty old FT847
upon switch on. It has a crystal heater fitted and has been
calibrated for a room at 20C. Even so it takes a while to
settle.
What does this show? It looks like the radio is set 44.5hz too
high. In fact it is, but in a couple of minutes this might not
be the case so calibrate after an hour of warm up, or calibrate
and check back from time to time until you get to know your
radio. My IC-7300 appears to be about 20hz LF on 70mhz and stays
there. I don't tend to worry too much about that. My FT-847 was
over 300hz off frequency when I first tried it. That can be an
issue.
Here is the same set-up at the same time
but using WSJT-X and you can see the same idea. The broken
line is actually the beacon's morse ident.
Below is the radio after warm up and a required tweak of about
11hz - that is about the limits for my FT847 and well useable
even on Wspr where the 'band' is only 200hz. Once adjusted it
tends to remain +/- 5hz all day.
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