[Elecraft] Where to put the wattmeter

[Bob McGraw]

While I totally agree with Alan, N1AL, I fear that one who is reading an 
external power meter located in the feedline after the tuner will be 
mislead by the power indications.  At the same time, SWR indications are 
typically valid.

Case and point.   On one of my antennas, on a frequency where the SWR is 
high, the FWD power indicated on the external meter is 200 watts.  No 
way the radio is producing 200 watts.  At the antenna matched frequency 
the external meter indicates 100 watts.

The original post indicated with the OPs K3S the P3 Sensor located after 
the KAT500 antenna tuner was showing a power value of some 300 watts or 
so.  Even more non believable values with his KPA500.  Thus my reason 
for questioning the position of the measuring sensor in the feed line.   
Most power measurement devices are calibrated for a known impedance, 
normally 50 ohms. Therefore placing them in the feedline where an 
unknown impedance exist, one will also have a false power indication.

Exceptions are a calculating power meter that measures both voltage and 
current to determine power.  Most power meters I find in ham usage are 
only voltage sensing devices.

That's all.

73

Bob, K4TAX


> Date: Mon, 10 Apr 2023 10:03:10 -0600
> From: Alan Bloom<n1al at sonic.net>
> To:elecraft at mailman.qth.net
> Subject: Re: [Elecraft] Where to put the wattmeter
> Message-ID:<f792d09d-367a-7ff3-eec0-10d84f20bad2 at sonic.net>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
> I changed the Subject line since this is a new topic.
>
> If the wattmeter (in this case the P3 sensor) is placed after the tuner
> then it will read the SWR of the antenna.? To determine the net forward
> power, just subtract the reflected power from the forward power.
>
> To calculate reflected power, use the equation:
>
> P(ref) = P(fwd) * [(SWR-1)/(SWR+1)]^2
>
> Since the K3 and K4 already have a built-in wattmeter located before the
> tuner, it makes sense to me to put the P3 sensor (or external wattmeter)
> after the tuner so you can see what is going on with the antenna.
>
> Alan N1AL