[Elecraft] K2/100 Power Calibration questiion

[Don Wilhelm]

Jerry,

Yes, use the 'scope and precision dummy load because it will provide more
accuracy than an uncalibrated wattmeter.  A 10x probe must be used for
accuracy - connect it directly across the dummy load (a coax TEE adapter is
handy for that if you cannot get the probe on any other way.  Results will
be great if done at 40, 30 or 20 meters with a 100 MHz scope and a 10x probe
rated for at least 100 MHz.  If your 'scope and probe are rated for only 50
MHz, stay at 40 meters and lower frequencies.

The K2 will attempt to re-level the power to what it thinks is the requested
value during the adjustment, so yes, adjust in small increments - keep
trying until it is right.

When using the 'scope, the easiest way is to read the peak-to-peak voltage
and then use the formula  (Vp-p^2)/8R or (Vp-p^2/400 for a 50 ohm load) to
obtain the power directly without first converting to RMS voltage.  The
derivation of the formula from the basic V^2/R is 'left to the student'
(Hint - plug in the formulas for conversion from P-P to RMS and reduce the
equation).

You may find it easier to do the initial adjustment at 20 watts and adjust
R26 shooting for 89.4 volts p-p and then check at 100 watts looking for 200
volts p-p on the 'scope.

73,
Don W3FPR


> -----Original Message-----
>
> Just finished my 100 watt add on to my K2 (# 4472) and I have a few
> questions on the power calibration page 45.
>
> I am using the power meter in my Palastar tuner as a external power
> meter and I know it is not calibrated, but it reads about 12 watts when
> I drive it with my Icom 703 at 10 watts and when I had a 1000MP(sold it
> to buy the K2) I remember it being about 5  watts differnce at about 50
> watts out.  I know I can' t rely on it being accurate, but its
> repeatable..
>
> Anyway, when I set power on the K2 to 50 watts and hit tune+ display I
> adjust r26  so the reading on the k2 equals the reading on the external
> pwr meter, the reading on the external pwr meter changes also.  Is this
> correct?  I know I am supposed to make small adjustments, which I do.
>
> My reading are.
> Band       K2 pwr set       external pwr meter    k2 pwr display
> 80           10                          15
> 10.2
> 40           10                          13                          11.4
> 20           10                          13                          10.4
> 15           10                          12                          10.7
> 10           10                          11                          9.7
> 80           50                          60                          49
> 40           50                          54                          52
> 20           50                          50                          51
> 15           50                          48                          52
> 10           50                          46                          51
> 80           100                        102                        104
> 40           100                         99                         100
> 20           100                         92                         106
> 15           100                         82                         103
> 10           100                         79                         104
> The warc bands follow a similar pattern.
>
> I measured my current draw on 10 meters at 100w and it was about
> 15.4 amps.
>
> I made a few contacts on 30 meters and had good reports.
>
> The power drops from 102w at 80 meters to 79w on 10meters, is this
> normal?
>
> I also measured with a textronix scope at 50w on 80 meters and read 124v
> p-p using a precison dummy load , giving me 39 watts out.  Should I use
> the scope to do the adjustments?
>
> Jerry, N2UZ
>