[Lowfer] RF Ammeter recommendation

[Ed Phillips]

John Andrews wrote:
> 
> 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

	There's a pretty cheap way to go as well.  Use a (relatively) large
capacitor as a shunt, with a smaller capacitor in series with a
full-wave rectifier in parallel.  (Meter in the rectifier circuit, of
course.)  Analogous to a DC shunt and meter.  Simple and works OK. 
Convenient for tuning and not bad for accuracy if you go to the trouble
of analyzing the calibration.  Compared to a thermal ammeter has the
advantage of a more or less linear scale.

Ed