[Elecraft] K2: Stability of Output Power Level

[Ken K3IU]

Hi Michael:
In case you don't have an antenna analyzer and the OHR dummy load you refer
to is their "100 Watt RF Load," I just measured the impedance of my OHR
dummy load with my MFJ Analyzer. It shows 50 ohms resistive and no reactive
component at all until the freq gets up to about 16 mHz. So I would say that
your dummy load is probably OK for the purpose.
73,
Ken K3IU
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of W3FPR - Don Wilhelm
Sent: Tuesday, June 28, 2005 12:19 PM
To: mlinden at rcn.com; elecraft at mailman.qth.net
Subject: RE: [Elecraft] K2: Stability of Output Power Level

Michael,

OK, so you are like the man with 2 watches who never knew what time it was
<G>.

Since you don't know which one to really trust, may I suggest that you
connect an RF Probe directly across the dummy load (a TEE adapter at the
dummy load connector makes a convenient way to attach the probe) - measure
the RF Voltage - your DVM connected to the probe will read the voltage in
RMS Volts, then calculate the power P=Vsquared/R, and in your case you know
the R is 50.3 ohms.

In addition, I must also ask how you determined the dummy load was 50.3
ohms - a DC resistance reading will not be sufficient - measure it with an
antenna analyzer to be certain of both the resistance and the reactance,
ideally, the reactance should be zero across the entire frequency range that
you will use the dummy load, but you must know its real value at RF
frequencies.  If it is reactive, that alone may explain the difference
between the WM-2 and the K2 readings.

Repeated presses of the TUNE button will normally produce different power
output, it will be within a couple watts of the requested power.  At the
extreme low power levels, the voltage produced is so small that it gets
'lost' in the digital translation.  Try measuring the power with a keydown
condition rather than using TUNE and you will find the power output more
consistent, and it should be close to the requested power if the load is 50
ohms resistive.

I might add that I have found the base K2 (RF probe type) provides a very
good measurement of power if the dummy load is truly 50 ohms non-reactive.

With the either KAT2, KPA100, or KAT100 installed, the power is measured
through a true wattmeter circuit rather than an RF probe type detector, so
the dependency on having a non-reactive load is greatly reduced, and after
proper calibration will provide accurate power measurements.

73,
Don W3FPR