[Elecraft] K2 RF Probe accuracy?

[Don Wilhelm]

Kieth,

The RF Probe that comes with the K2 will read the RMS value of the RF
voltage.
The RF detector in the DL1 will indicate the zero to peak RF voltage and the
display on an oscilliscope will be the peak to peak voltage.

It is normal for the power to increase a bit over time due to heating of the
transistor junction.  During a TUNE operation, the power control action is
reduced - think about it, if you are using a manual tuner with the K2 and
the K2 corrected for power fluxuations quickly, it would be difficult to
find a match on the manual tuner.  During keying and normal operation the K2
power control circuits operate in a tighter loop and should control the
actual power output more quickly.

So - those 'creeping' RF voltages MAY be an indicator of what is really
happening.  The capacitor should charge up in a single cycle of the RF
Voltage, and that is only microseconds - a buildup of charge over a few
seconds would only occur if you used a very large capacitor (several uF)
instead of the .001 uF capacitor.  If your dummy load increases value with
heat, that would explain a drift, but 15 volts RMS into a 50 ohm load is
only 4.5 watts, and if your dummy load is large, the heating should be
minimal - you did not indicate the power rating of the dummy load.  Consider
too that your DMM may not indicate the correct value initially - it depends
on how fast it responds DMMs measure in fixed cycles of time, so if you
apply the voltage in the middle of one of these periods, the first
indication will be low and you have to wait for the second period before the
reading is correct.

But if you have a bare K2 (no KAT2, KPA100 or KAT100), the power output
indicated on the K2 display should follow what you are reading on the RF
Probe - if it does not, one of them is incorrect, and it is difficult to say
which one without more information.  All I can say in that regard is that I
have found the 1N5711 diodes (used in the K2 for RF voltage sensing) are
quite consistent.

> -----Original Message-----
> Here I need to confess my ignorance.  I used the RF probe that came with
> the K2 to measure K2 output into a dummy load.  I got odd results.
>
> The voltage would continually increase.  It would start out at say 13
> VDC.  I'd measure for 20 or 30 seconds and it's count up to maybe 16
> VDC.  I'd back off and then put the probe back on the test point and it
> would start at maybe 15.5 VDC and count up, going past 16 and
> continuing.
>
> I figured it might go through some rise time as the cap is charging but
> I never saw it settle out at a value.  What's up with that?
>
> My dummy load is a TenTec.  It's basically a big 50 ohm resistor.  I've
> measured it with the MFJ analyzer and it is flat SWR (50 + 0j ohms) to
> 60 MHz or so.  I've NOT measured it after it heats up though and
> probably should.
>
> Does an RF probe measure RMS, peak, or peak to peak?  So confusing ...
> :-)
>
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