[Elecraft] K2- Is the Power display really representative of PEP?

[W3FPR - Don Wilhelm]

Robin,

The base K2 power display is a simple diode detector (an RF probe is 
similar), and it will be reasonably accurate if and only if the load 
inpedance is 50 ohms resistive.  Your antenna may be something quite 
different.  You might say that the base K2 power output indication shows the 
power that would be present if the antenna load were actually 50 ohms 
resistive.

If you have the KAT2 or KAT100 or KPA100 installed, they contain a true 
wattmeter, but these must be properly calibrated to provide a good reading.

In general, wattmeters can be trusted to only 10 to 20% of their full scale 
reading - so if you are reading on a 100 watt scale, the actual power may be 
off by as much as 10 to 20 watts!!!
Use the lowest scale that will handle the power being measured.  Actually 
some are better than that, but should not be trusted until they are properly 
calibrated.  I calibrate (or verify) wattmeters by connecting a 50 ohm dummy 
load and measuring the RF voltage across that load with my 'scope (an RF 
Probe could also be used) - then do the math to determine the power.  When 
using the 'scope, the formula I use is Vp-p ^2/400 and for an RF Probe that 
reads RMS voltage the formula would be V^2/50 (these formulas are only for a 
50 ohm load).  Check the dummy load beforehand with an antenna analyzer to 
be certain it is a good 50 ohm resistor at the frequency you are using for 
the calibration.  In other words, trust nothing for accuracy until it is 
verified.

73,
Don W3FPR

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


I've probabaly seen this discussed before, but can't remember ...

I have noticed that the power output on my K2 differs from measures PEP 
going to the antenna. Mostly it displays between 14.8and 15.2.

However, when measuring with a wattmeter and dummy load I notice that on 18m 
I get 18W PEP, and on 15m down to 12W. On 10m it's at 8W.

So my question is, is the power level that you seen on the display really 
just a scale for adjusting power, or is it at true reading. I'm starting to 
wonder if my Watt meter is inaccurate (a Yaesu 60 model) ?