K2/100 Power Calibration questiion

[Jerry Deibel]

Hi all,

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
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[Don Wilhelm-3]

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
>

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[Ron D'Eau Claire-2]

Don is assuming your 'scope is calibrated! No scope is good enough to work
properly if it's out of calibration, Hi!

Usually the most dependably calibrated piece of test equipment in a Ham
shack is the DMM. They are robust and stable unless really abused. A simple
RF rectifier circuit such as that included for power measurements in the DL1
can be used with your DMM to check the calibration of the 'scope.

If using the DL1, keep in mind that the voltmeter circuit in it measures
*half* the applied RF voltage to provide a greater power range without
burning out the diode. Since your 'scope measures peak-to-peak and the
metering circuit in the DL1 1/2 the voltage, the peak-to-peak reading on
your scope should indicate 4 times the DC voltage shown on the DMM attached
to TP1 of the DL1.

Of course, you can rig up your own RF rectifier, using the circuit shown in
the DL1 schematic (it's on the Elecraft under Manuals and Downloads web
site). For best accuracy check the forward voltage drop in the diode you are
using and  add that to your readings as described in the DL1 manual. A diode
like the 1N5711 used in the DL1 will have a quite low drop - in the range of
0.15 to 0.2 volts. A common silicon switching diode will also work but will
have a forward drop in the range of 0.6 volts. For best accuracy, you want
to apply enough RF to be well above the forward drop voltage, but not so
much you destroy the diode.

If your scope is okay at a low voltage, compared to the DMM, it's probably
fine. It's rare for a 'scope to be off on other ranges. Their calibration is
done by setting the gain of the amplifier circuits. The step attenuator at
the input is usually very stable and, unless it's been abused with too much
voltage applied, seldom causes significant error.

Don warned you to use a 10X probe with your 'scope. That's one very
important reason. Most 'scopes that I've seen can only handle a few volts
applied directly to the vertical input without damaging them.

Ron AC7AC

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[Jerry Deibel]

Thanks to Don and Ron or their responses and everyone else also.

I used my scope with the 10x probes and measured the peak to peak
reading from a tee connection going to the dummy load. Squared that and  
divided by 400 to get the power out, can't get too more accurate than
that. Now the K2 putts out 100 on all bands and only drops abt 3 watts
on 10 meters,  not bad.  Of course,  my external wattmeter on the tuner
was 20 higher at 100 watts,  15 watts higher at 50 watts and about 9
watts higher at 10 watts out, so much for that  power  meter.

To make sure my scope is calibrated I measure a brand new  (in a pack)
AA battery with my Fluke DVM and use that to make sure the scope is
right on,  so I am fairly confident about the scope calibration.

Everything is all fine and finished and I even  made a few contacts last
night at 40 watts.
Love my K2.

Jerry, n2uz

snip..

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?


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