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<n...@sonic.net>
To:elecraft@mailman.qth.net
Subject: Re: [Elecraft] Where to put the wattmeter
Message-ID:<f792d09d-367a-7ff3-eec0-10d84f20b...@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
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