Paul W9AC wrote:
> In reading through some materials from R.H.G. Matthews, owner of 9ZN in
> Chicago, early spark stations generally employed one device to determine
> antenna radiation efficiency : An R.F. ammeter placed in series with the
> vertical portion of the antenna offered the clue. The multiwire "T"
> evolved
> as a result of the ammeter. As more wires were added to the flat-top, the
> ammeter displayed more current.
When I was looking for a RF Ammeter with a thermocouple in the late 50's to
get my Marconi T working well (and they were more available then at a
reasonable price) an OT admonished me and said there is a better way. He
describe what amounted to a simple tuning device. Remember the loop of wire
couple near the tank coil and a #47 bulb to get your metal 6L6 MOPA working?
He recommended wrapping a few turns of insulated wire around the junction of
the vertical portion and the horizontal portion of the Marconi "T". then
run enough power to illuminate the bulb. Lower the halyards on each end and
foldback a foot of wire at a time and raise them up, apply the same power
and see if the lamp get brighter. If so, keep shortening the ends until
you achieve maximum illumination available at the set power level you
started with. Of course this needs to be done at night but the final results
insure that the current maximum is at the top of the "T" which is supposed
to be the case for optimal low angle radiation.
After finding the "sweet spot" be sure to remove the bulb loop detector or
crank up the power to burn it out so neighbors don't call 911 while your
operating and report a "UFO" hovering over your house because of your
strange antennas!
Regards,
Herb Schoenbohm, KV4FZ
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