[Elecraft] powermeter

[Fred Jensen]

Only naive if you've been around a hundred years and still don't know, 
Ross. :-)  It depends on what you want to measure.  If you put it 
between the antenna and the tuner, you'll measure the fwd/ref power from 
the antenna.  If your antenna is resonant, AND the feedpoint impedance 
is equal to the characteristic impedance of the feedline [I'm assuming 
coax here], then you'll measure your forward power and reflected will be 
zero ... or close.

If your antenna is not resonant, and/or its feedpoint impedance is not 
equal to the characteristic impedance of the feedline, you'll measure 
the forward and reflected power ... which could be anything and won't 
mean a whole lot.  Incidentally, many ham power meters actually measure 
voltage not power and as reflected power goes up, so does the forward 
indication.  The "power meter" needs to be able to handle the output 
power of the amplifier.

If you put it between the amp and the tuner, you will measure the SWR 
that the amp is looking into.  Ideally, the tuner will make that 1.0:1 
and the forward power will be your output power.  I wouldn't really 
worry about protecting the amp, today's amplifiers are pretty well 
self-protected, and you adjust the tuner at low power anyway, you'll 
know then if you got a good match or not.  The power meter needs to be 
able to handle the drive power to the amplifier from your transceiver.

Neither position tells you very much after the first check that you 
don't already know.

And "Incidentally #2":  "Antenna Tuners" do not "tune the antenna" and 
never have.  We used to call them antenna couplers, which is closer to 
the truth but still no cigar.  They are really just matching networks 
that will transform the complex impedance at the end of your coax [or 
other feedline] to what the radio/amp wants to see, usually 50+j0 ohms. 
  Most "autotuners" these days are L-networks, but manual tuners are 
sometimes T- or Pi- networks.  Tuners that feed high impedance open wire 
line are sometimes parallel resonant tank circuits [Johnson Matchbox 
series].

Hope this helps.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 50th Running of the Cal QSO Party 3-4 Oct 2015
- www.cqp.org

On 5/12/2015 4:06 PM, Ross Biggar wrote:
> This is probably a very naive question, but where should a powermeter
> that measures Forward and reflected power be positioned? Should it be
> between the Amplifier and the Tuner, which could safeguard the
> amplifier if there was a high SWR, or after the tuner, (ie, between
> the tuner and the antenna)?