Hi Jim
For tuning LF antennas, I have for some time been using an oscilloscope to
monitor the current and voltage waveforms at the TX output. Resonance
occurs when V and I are in phase, and the load resistance is 50ohms when
V/I = 50. This is very easy to use provided a scope is to hand, but not
very useful if it isn't. An SWR bridge can be used, but it only indicates
the degree of mismatch, so tuning is still a matter of trial and error. So
with a mind towards future /P operations, I have built a self-contained
tuning meter.
Although I included a description of your ScopeMatch Tuning Aid in the LF
book (as built by Lech G3KAU) I never did get round to building one myself.
The reason is that I never had a transmitter that had a 50ohm output system.
With the BK amplifier I went straight from 4ohms to the multi-tap toroid
transformer at the base of the antenna system. Resonance is indicated by
maximum antenna current and matching by the current to the amplifier FETs.
The amplifier was coupled to the antenna matching transformer with a length
of heavy duty mains cable.
When I moved on to the Decca transmitter I changed the antenna antenna
loading toroid transformer to an auto transformer arrangement because I
didn't need the isolation. This system is 70ohms and for some time I used
the same mains cable transmission line. I recently changed it to 50ohm coax
(because it seemed not quite right to be using mains cable for such an
application) but it had no effect on the transmitter performance.
The question is would your tuning aid be of any use in my system? I can see
a use when I use the LF transmitter at Amberley Museum, where resonance and
antenna feed impedance are totally different to the home QTH but is your
latest meter for 50ohms only?
I would be grateful if you could send me a copy of the PDF.
Regards,
Peter, G3LDO
e-mail <[email protected]>
Web <http://web.ukonline.co.uk/g3ldo>
|
