[ARC5] RF Ammeter Puzzle

[J Forster]

Both the ARC-5 and ART-13 [Fixed Antenna terminal]  (and the WS-19 for that
matter) use essentially the same antenna circuit...  a series resonant RLC.

The C is the capacitance of the electrically short antenna.
The R is the radiation resistance (usual a few to a few tens of ohms)
The L is a variable inductor. For the named sets:

     ARC-5  the Roll-A-Ductor
     ART-13 a combination of the variometer and a tapped coil
     WS-19  the variometer

The RF Ammeter is in the series circuit, usually between the variable L and the
antenna, so the meter must be isolated by a transformer or the meter will be RF
hot.

A series circuit has it's lowest impedance (hence highest current) at
resonance, so tuning for maximum RF Amps maximizes the current through the
radiation resistance, hence transmitted signal.

Best,
-John

==========================


Bob Macklin wrote:

> The ARC-5 transmitters had the built in loading coils and the antenna
> current meter was between the output terminal of the Tx and the antenna.
>
> The large aircraft with the ART-13s had trailing wire antennas. But I never
> worked on an ART-13 aircraft installation.
>
> Bob Macklin
> K5MYJ
> Seattle, Wa,
> "Real Radios Glow in the Dark"
> ----- Original Message -----
> From: <sbjohnston at aol.com>
> To: <jfor at quik.com>; <ARC5 at mailman.qth.net>
> Sent: Wednesday, February 27, 2008 7:44 PM
> Subject: Re: [ARC5] RF Ammeter Puzzle
>
> >
> > The ARC-5 was made for short wire antennas on aircraft, wasn't it?
> > Those antennas would have quite low radiation resistance (which is the
> > load that pulls the current detected by an RF ammeter).  The ART-13
> > might have been designed for longer antennas with higher radiation
> > resistances.  The lower the resistance, the greater the current, for
> > the same power.
> >
> > For example, suppose the ARC-5 was used with an antenna with R=10 ohms.
> >   50 watts of power would be about 2.24 amps.  The same 50 watts on a 50
> > ohm antenna would be 1 amp.
> >
> > The formulas are P = I x I x R   or   I = SqRt(P/R)   etc
> >
> > Steve WD8DAS
> >
> >
> > _______________________________________________
> > ARC5 mailing list
> > ARC5 at mailman.qth.net
> > http://mailman.qth.net/mailman/listinfo/arc5
> >