Hi Chris,
My advice:
Am 11.03.2011 14:05, schrieb Chris Osborn:
The PA
is capable of giving several hundreds of watts but it has a low output
impedance and the aerial system has a high impedance (several kilohms).
To achieve reasonable matching I use a large laminated-core
mains transformer with a step up ratio of 1:24
Instead of using a mains transformer i would recommend to use a primary
coil that is wound on the cold end of the loading coil. It should be
out of bigger wire, maybe 8x0,4mm dia enameled copper wire (in
parallel). The optimal primary turns depend on the ration of turns, the
losses and the output impedance of the PA and the coupling between the
turns. But is would estimate starting with 10 turns is a good idea.
Then you may measure the current (e.g. above a 0.1 Ohm resistor) and
voltage and calculate the input impedance. You may optimise it to
somewhat between 4...8 Ohm. BTW, when watching current and voltage
waveforms you can securely check if the system is resonated.
Link
coupling to the loading coil may be an option but my early attempts at
this merely shorted out the loading coil and damaged the PA, so I have
avoided it.
If you start with the link coupling, start with low power (=volume). If
you are not sure , just switch a 4 Ohm resistor in series. Just apply
some watts. Then you can measure the current by measuring the voltage
across this 4 Ohm resistor. Try to achieve a 1:1 voltage divider out of
that resistor and the antenna. Then your antenna has 4 Ohm as well.
After that throw out the resistor and slightly increase the power and
measure again. Measure the PA input current and voltage and calculate
the efficiency. Measure the voltage waveform when doing QRO to check
for clipping or so.
For my next experiment i have to wait some time it seems. No wind on
this weekend. Today i found that we have a generator at work for /p
tests. It has 4.5 kVA, single phase :-) Maybe i can borrow it! ;-) :-)
73, Stefan/DK7FC
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