[Elecraft] KX1 output power measurements

[Don Wilhelm]

Paul,

Yes, what you observed is true.

The KXAT1 wattmeter will indicate more power with increasing SWR *if^ 
the load impedance is higher than 50 ohms, but if it is lower than 50 
ohms, the power indication will be lower.  In ATU CAL mode, when the 
power output is 4 watts into a 50 ohm load - if the load is 25 ohms pure 
resistive, the power indication will typically be 2.2 to 2.5, and if the 
load is 100 ohms pure resistive, the power indication will be typically 
6.8 watts (the max value that will be displayed).  I cannot say how the 
indication varies with complex impedance loads, but I see the same thing 
here with the precision 25 ohm and 100 ohm loads that I use when 
adjusting for the SWR=2 point.

This phenomenon seems to be due to the absence of total power 
calculation by the firmware.  The difference of the squares between the 
forward power and the reverse power must be calculated to indicate the 
actual power to the antenna (with the phase angle properly included in 
the calculation) - yes, the calculation seems to be incomplete in that 
regard, but the relative indication is still informative.

After a tune cycle which reduces the SWR to somewhere near 1.0, the 
power reading will be correct, but I have not measured just how close to 
SWR=0 that must be.

That same characteristic is also true on the KAT1 wattmeter used in the 
K1. and at the moment, I can't recall what happens with the K2 tuners.

73,
Don W3FPR

Paul Huff wrote:
> I was using my KX1 in the backyard yesterday to make some final adjustments and tests on a portable vertical that I will be using for field day.  I noticed that when the KX1's antenna tuner indicated a high SWR it also indicated a much higher "output power."  I assume that the reflected power throws off the measurement.  Does anyone know what the maximum SWR is that will still provide a fairly accurate power reading?  
> 
> Thanks and 73,
> Paul - N8XMS
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