powermeter

[Ross Biggar]

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)?

Thanks
Ross ZL1WN
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[Fred Townsend-2]

Ross, not to put too fine a point on it, but a power meter and a SWL meter are not the same. Without a directional coupler a power meter will read both forward and reflected power making readings appear overly high with high SWR. A power meter, i.e. watt meter will only be accurate when terminated into a 50 ohm resistive load.

Now assuming you are using a directional coupler with two signals, the tuning process lowers return power so it should located between the amp and the tuner, usually at the amp end of the coax.

73
Fred, AE6QL

-----Original Message-----

>From: Ross Biggar <[hidden email]>
>Sent: May 12, 2015 4:06 PM
>To: [hidden email]
>Subject: [Elecraft] powermeter
>
>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)?
>
>Thanks
>Ross ZL1WN
>______________________________________________________________
>Elecraft mailing list
>Home: http://mailman.qth.net/mailman/listinfo/elecraft
>Help: http://mailman.qth.net/mmfaq.htm
>Post: mailto:[hidden email]
>
>This list hosted by: http://www.qsl.net
>Please help support this email list: http://www.qsl.net/donate.html
>Message delivered to [hidden email]

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[Joe Subich, W4TV-4]

On 2015-05-12 7:06 PM, Ross Biggar wrote:
> This is probably a very naive question, but where should a powermeter
> that measures Forward and reflected power be positioned?

Sine Wattmeters are only accurate at (or near) their design impedance,
any wattmeter should be installed between the amplifier and antenna
tuner or used in lines to antennas with low SWR

73,

   ... Joe, W4TV


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[k6dgw]

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)?
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[Bob Naumann W5OV]

You definitely want to measure the tuned portion of the path - that being
between the tuner's input and the output of your transmitter/amplifier.  

The SWR on the antenna side will not change regardless of how the tuner is
set, so what an SWR meter might measure at that point is meaningless to you
- you want to be measuring what your transmitter will be "seeing".

So, put your SWR meter between the amplifier and the tuner.

73,

Bob W5OV

-----Original Message-----
From: Elecraft [mailto:[hidden email]] On Behalf Of Ross
Biggar
Sent: Tuesday, May 12, 2015 7:07 PM
To: [hidden email]
Subject: [Elecraft] powermeter

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)?

Thanks
Ross ZL1WN
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