K2: Stability of Output Power Level

[Michael J. Linden]

  I just completed construction of an Oak Hills Research WM-2 QRP
wattmeter. So, of course I wanted to compare its readings with that of
the K2.
 
  Here are some test results. Please let me know if what I'm seeing here
is normal or if you think Elecraft's "Optional K2 Power Control
Modification" may make any of this a little better. All of these tests
were performed at 7.145 MHz into the WM-2 wattmeter and then into a good
(50.3 Ohms resistive) OHR dummy load. For each test, I key the radio
several times using the Tune function (no KAT2 installed, yet).
 
#1
Set the K2's power control (request) for 4W and the output varied
between the following:
 
K2 Disp  WM-2
-------  ----
5.2      ~7
3.9      ~5
3.3      ~4
 
#2
Set the K2's power control (request) for 2W and the output varied
between the following:
 
K2 Disp  WM-2
-------  ----
1.5      ~2
2.4      ~3
 
#3
Set the K2's power control (request) for 0.5W and the output varied
between the following:
 
K2 Disp  WM-2
-------  ----
0.4      ~0.7
0.8      ~1.2
 
#4
Set the K2's power control (request) for 0.2W and the output varied
between the following:
 
K2 Disp  WM-2
-------  ----
0.2      ~0.4
0.3      ~0.5
 
#5
Set the K2's power control (request) for 0.1W and the output decreased
with each activation of the Tune function until there was no detectable
output:
 
K2 Disp  WM-2
-------  ----
0.1      ~0.2
0.1      ~0.1
0.2      ~0.05
0.2      no noticeable reading on 100mw scale
 
  I'm primarily concerned with cases 1 and 5. The variance in case 1 is
fairly broad and is among three values. Case number 5 concerns me
because the output level drops with each successive press of the Tune
button until no output can be detected by the WM-2 on its 100mw scale.
Clearly, I would not be able to reliably use my K2 when set for 100mw.
 
  Will this situation (with respect to output power sensing) get any
better after I install the KAT2? I haven't completed construction of the
KAT2, yet. Any input/explanations/ideas would be appreciated.
 
  Thanks! -Michael N9BDF, K2 #4137
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[Don Wilhelm-3]

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

> -----Original Message-----
>
>
>   I just completed construction of an Oak Hills Research WM-2 QRP
> wattmeter. So, of course I wanted to compare its readings with that of
> the K2.
>
>   Here are some test results. Please let me know if what I'm seeing here
> is normal or if you think Elecraft's "Optional K2 Power Control
> Modification" may make any of this a little better. All of these tests
> were performed at 7.145 MHz into the WM-2 wattmeter and then into a good
> (50.3 Ohms resistive) OHR dummy load. For each test, I key the radio
> several times using the Tune function (no KAT2 installed, yet).
>
> #1
> Set the K2's power control (request) for 4W and the output varied
> between the following:
>
> K2 Disp  WM-2
> -------  ----
> 5.2      ~7
> 3.9      ~5
> 3.3      ~4
>
> #2
> Set the K2's power control (request) for 2W and the output varied
> between the following:
>
> K2 Disp  WM-2
> -------  ----
> 1.5      ~2
> 2.4      ~3
>
> #3
> Set the K2's power control (request) for 0.5W and the output varied
> between the following:
>
> K2 Disp  WM-2
> -------  ----
> 0.4      ~0.7
> 0.8      ~1.2
>
> #4
> Set the K2's power control (request) for 0.2W and the output varied
> between the following:
>
> K2 Disp  WM-2
> -------  ----
> 0.2      ~0.4
> 0.3      ~0.5
>
> #5
> Set the K2's power control (request) for 0.1W and the output decreased
> with each activation of the Tune function until there was no detectable
> output:
>
> K2 Disp  WM-2
> -------  ----
> 0.1      ~0.2
> 0.1      ~0.1
> 0.2      ~0.05
> 0.2      no noticeable reading on 100mw scale
>
>   I'm primarily concerned with cases 1 and 5. The variance in case 1 is
> fairly broad and is among three values. Case number 5 concerns me
> because the output level drops with each successive press of the Tune
> button until no output can be detected by the WM-2 on its 100mw scale.
> Clearly, I would not be able to reliably use my K2 when set for 100mw.
>
>   Will this situation (with respect to output power sensing) get any
> better after I install the KAT2? I haven't completed construction of the
> KAT2, yet. Any input/explanations/ideas would be appreciated.
>
>   Thanks! -Michael N9BDF, K2 #4137
>

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[Ken Wagner 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: [hidden email]
[mailto:[hidden email]] On Behalf Of W3FPR - Don Wilhelm
Sent: Tuesday, June 28, 2005 12:19 PM
To: [hidden email]; [hidden email]
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

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