[Lowfer] RF Ammeter recommendation

[John Andrews]

Mark,

> I hope to be at-it by this Fall, at least that's this years target date
for
> getting my lowfer beacon on the air from R.I. I just received some
> information from a guy who is selling some analog RF ammeters. They come
in
> various F.S. values of 1-4-6-10 Amps. I have wanted one or more of these
> guys for awhile, but I'm wondering which range would be the most helpful
for
> lowfer work?  Thanks.

Fundamental question: What sort of transmitting antenna are you planning? If
you will use a vertical antenna working against ground, you can expect 20
ohms or more of loss resistance. The radiation resistance will be small
compared to this, so the antenna RF resistance is effectively the 20 ohms.
In that case, the current will be 0.224 amps (for 100% xmtr efficiency), and
a 0-1 amp thermocouple meter would be fine. The RF resistance of the meter
will be negligible compared to the 20 ohms, so there's no trade-off.

On the other hand, if you use a transmit loop, you might wind up with an RF
resistance of less than 1 ohm. Now, there's no bargain here, because the
radiation resistance is MUCH MUCH  lower than that, and the efficiency will
probably be worse than the good vertical antenna installation. Anyway, let's
say you wind up with an RF resistance of 0.8 ohms. That translates to an
antenna current of 1.12 amps, again assuming 100% efficiency in the xmtr PA.
You should resist (no pun intended) the temptation to use a thermocouple
meter in this application, as it may have enough internal RF resistance to
cost you some power. Most of us who have played with xmit loops have used
homebrew RF ammeters with toroidal current transformers, 1N34A diodes, and
DC meters. That does give a peak, rather than RMS indication, but the
antenna current is pretty much a sine wave, so that's OK. The trick is to
use a high turns ratio in the transformer, like 1:50 or 1:100, thereby
reflecting a very small RF resistance into the antenna circuit. If you want
more info, just ask.

John Andrews