[AMRadio] AMRadio Digest, Vol 132, Issue 12

John Coleman jc at pctechref.com
Fri Jan 16 17:34:13 EST 2015

Since we are on the transmission line stuff.  I will add some finding of my
I have develop a method for measuring very accurately the Peak Rf voltage
across a 50 ohm dummy load, but that is another story.  I was running a rig
that would easily put 300 volts Peak across the dummy load.  I tuned it up
into a length of 100ft RG8 with the meter and dummy load out in the yard.
Making very precise measurement on the plate current dip and the plate
supply voltage and the local SWR meter showed no return voltage at the most
sensitive setting.  I then quickly went out to the dummy load to measure the
RF peak voltage before the dummy load boiled over. Made the note and ran
back to kill the XMTR.  I then connected the dummy load to 60 feet of open
wire line which was suspended in the air by strings from the trees and came
to a balanced Johnson match box mounted under the eve of the house.  Ten
feet of coax went to the matchbox from the same SWR bridge. I then place the
XMTR with a very low plate voltage and tuned the matchbox for no return
voltage. Then flipped the switch on the XMTR to normal HV and checked that
there was still no return voltage on the SWR bridge. And that HV and Plate
current were same as before.  I then quickly went and measured the peak RF
voltage at the 50 ohm dummy load and found that it was a few volts higher
than with the coax straight.  I did this many times at various power levels
and it was always the same. The balanced open wire line with the KW Johnson
MatchBox delivered a little bit more and when I say a little bit I mean very
little more than the straight COAX.  The difference was extremely small but
I was expecting it to be more with the straight coax 100ft long.  So if
someone tells you those tuners and balanced line have a loss, it is true,
but not as much loss as 100 feet of COAX.  BTW this was all at 3.885 MHZ

John, WA5BXO  

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