Actual power out and the WM-2

[Vin Cortina]

Hi everyone,

I have been enjoying my K1-4 immensely for many months now, and am getting ready to build the KAT-1.  But I decided to build my OHR WM-2 first, so as to get a good idea of how the antenna tuner is actually performing.  Well, the meter went together on a Sunday afternoon/evening very nicely, but there is one thing I noticed.

My K1 goes from 100 mW to about 7.1 W according to the front panel display (of the K1).  Now, when I look at the power out on the meter, I see good agreement with that from around say 2 watts to about 5 watts.  But at the low and high end, the meter reads high.  Approximately 300 mW when the radio says 100 mW, and about 6 watts when the radio says 5 watts.  At the top end, the difference is even a little worse, approx 8.5 watts on the WM-2 when the K1 says 7 watts.

Yes, I definitely checked and re-checked the calibration of the meter according to the manufacturer's instructions.  It occurred to me that maybe there is nothing really wrong, and that the K1 is just delivering more than it indicates at those levels.  My years of training and experience in electronics tells me that that is probably not the case.  The news is NEVER good.

I am certain many of you use the WM-2 with your K1's, so certainly someone must have some info on this.  And the bad news is...?


Vin    KR2F

K1 s/n: 1977
Vin

Mount Tremper, NY
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[Dan Barker]

You don't mention the quality of the dummy load on the end of the K1 and/or
WM-2. It really matters.

Dan / WG4S / K2 #2456

<snip>
But at the low and high end, the meter reads high.
</snip>

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[Don Wilhelm-3]

Vin,

What you are observing at the low end is differences in the diode
characteristics.  The WM-2 has a diode compensation circuit which attempt to
correct the readings at low power levels while the KAT1 does not - so the
WM-2 is likely more correct at the low end

The difference at the high end does not have such a straightforward and easy
answer.  You might look into your dummy load, the length of coax between the
KAT1 and the WM-2 and various other factors for the answer.  Try another
band, you could find the difference is frequency dependent too.

If you have a good 50 ohm non-reactive dummy load (check it with an antenna
analyzer to be certain), you can read the RF voltage across the dummy load
with a calibrated 'scope or an RF probe and calculate the power.  This
method is quite accurate (better than most wattmeters), but is highly
dependent on the accuracy of the dummy load.

BTW - I do not perceive anything really bad here - just inaccuracies.
Equate that to the man with two watches - he never knew what time it was
<G>.

73,
Don W3FPR

> -----Original Message-----
>
> I have been enjoying my K1-4 immensely for many months now, and
> am getting ready to build the KAT-1.  But I decided to build my
> OHR WM-2 first, so as to get a good idea of how the antenna tuner
> is actually performing.  Well, the meter went together on a
> Sunday afternoon/evening very nicely, but there is one thing I noticed.
>
> My K1 goes from 100 mW to about 7.1 W according to the front
> panel display (of the K1).  Now, when I look at the power out on
> the meter, I see good agreement with that from around say 2 watts
> to about 5 watts.  But at the low and high end, the meter reads
> high.  Approximately 300 mW when the radio says 100 mW, and about
> 6 watts when the radio says 5 watts.  At the top end, the
> difference is even a little worse, approx 8.5 watts on the WM-2
> when the K1 says 7 watts.
>
>

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