[Elecraft] Measuring power with a scope...

[John Clifford]

Hello Don,

Thanks for your reply... I think it was the probes.  I was using my probe in
the 'x1' position... it is rated for 200 Mhz and I will try it at the x10
position and see if that makes a difference.  I never realized that the
setting would make a difference.

My voltages were low... about 30% low (measuring about 14.6v peak when
trying to measure 5w).

What I ended up doing is setting the K2 to 5w into a 1:1 load, then
calibrating the DWM-4 so that its forward and reverse voltages exactly
matched the K2's.  I tried this at a 1:1.5 mismatch to get some reverse
voltage so I could adjust the reverse pot in the meter.  I'm hoping that
this means my DWM-4 is as calibrated as my K2 is... :-).  Actually the meter
now seems to agree very closely with the ones built into my K2, my Omni VI,
and my AT11 tuner, so it is 'close enough' for the next day or two until I
pull everything out and try this again.

I will have to order a dummy load, or go down to the local electronics store
and pick up enough breadboard, connectors, resistors, and enclosures to make
one.

As you can probably tell, truly understanding antennas is something I'm
still working towards.  Hmmm... if the K2 tells me that is is seeing 0.0
reverse voltage, then even though that indicates a 1:1 SWR I might not have
a 50 ohm load?

Here is another question... do all HF transmitters transmit at the same
current level, just changing voltage (signal amplitude) to change power?

 - jgc

John Clifford KD7KGX

Heathkit HW-9 WARC/HFT-9/HM-9
Elecraft K2 #1678 /KSB2/KIO2/KBT2/KAT2/KNB2/KAF2/KPA100
Ten-Tec Omni VI/Opt1

email: kd7kgx@arrl.net