Hi all,
not long back home again after an enjoyable few weeks in Normandy, (not far
from Lessay but the grandsons pinched the semtex for use as playdough) so,
full of enthusiasm and biere rousse I set about fettling the antenna system
for the autumn/winter season. During the course of this exercise I got round
to measuring the length of the bit that does the actual radiating and found
that it is less than 8m. (24ft. 11in. in real units)
Can I therefore claim the record for the best distance to antenna height
ratio? ;-))
More seriously, in the course of preparing a talk on LF for a local club and
knowing that there are one or two members thereof who excel at picking
holes, I went over all my voltage and current measurements and calcs (for
the umpteenth time) and got a few surprises. No shocks on 73KHz where the
actual power fed to the ASMU is 175W of very nice sine wave giving an
antenna current of 2.2A during the last few weeks of operation on the band
(after I had the line of trees under one leg of the top-loading lopped by
four feet).
On 136KHz things are not nearly so satisfactory.
The output waveform from the long-suffering BK starts going from sine to
triangular at a pretty low level of input, and this seemed to start at an
output power level around 25W. I normally monitor the output waveform
'off-air' and so after the Q of the loading coil and antenna have done their
work, and at saturation my measurements had indicated that I was getting
108W so I assumed I had got a 'good' amp and this was the figure I had used
for calculations of system and earth resistance etc. However like the
knights of old I am much addicted to simmering my carcase in lots of hot
water (with bouquet garni of course) and while indulging in this pastime one
evening the other brain cell kicked in to give a dawning realisation that
with the output waveform triangular and not sine I would have to remeasure
and recalculate - with fairly predictable results.
The 136 antenna current (nice and sine) is still 2A, but the triangular
output power from the amp now works out at 68W -much more in line with what
I had heard of these beasts.
Quite a shock to the system, so with no more ado the M0BMU mods to the BK
were carried out - under epidural.
Now the bitter bit. Things were now even worse, with it proving impossible
to get anything like the previous output figures or a decent waveform on
136 or even 73 at any over a couple of watts so I hastily put things back
the way they had been and opened the Pommeau.
So todays puzzler is;
How come? Anyone got any ideas? I've gone into sulk mode.
Ian GI8AYZ/MI0AYZ
PS Since changing over to twin feeder there are no "MIssing Amps".
Accordingly the case is declared closed.
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