[Amps] Measuring RF Power

[John Coleman]

	A number of years ago I came up with a very accurate method of
measuring the power on a heath kit "cantenna" dummy load.  They had a piece
of junk circuit in a box on the top for some relative metering indication.
I removed the little diode and resistor etc. and mounted a 7 ping mini tube
socket in the top for a 6AL5.  I connected the plates of the 6AL5 to the
dummy load resistor and the cathodes to the little RCA jack that was in the
side of the box. I placed a .01 uf ceramic cap from cathodes to ground.
Knowing that the filament to cathode breakdown of the 6AL5 may not be very
high I decided to use a 6V lantern battery for the filaments and I taped it
to the side of the "cantenna".  Theory of operation is that the .01uf/1000V
cap will charge to the peak of the RF wave form. Example of calculations is
as follows.

One has to make sure that the dummy resistance is really 50 ohms. Some have
been found to be 60 ohms probably due to over heating.

Using a RCA VTVM
Measured DC on the RCA Jack = +300 Volts

Using the formula for Peak to RMS conversion
 
.707 * Epeak = Erms

.707 * 300 = 212.1 Volts RMS


Average Power = (Erms * Erms)/ resistance
(212.1 * 212.1)/ 50 ohms = 899.7 watts (should be 900 watts, error due to
rounding)

Since the resistance is always 50 ohms on the dummy load, and (.707) is
really a rounded off (SQR of 2) then the formulas can be reduced to 
(Epeak * Epeak)/100 = Average Power

I had several people check it with there so called calibrated meters and
some show a little high and some lower. 

I always felt that the 6AL5, having a very high Xc at HF would not alter the
tuning and it has very little forward voltage drop and of course extremely
low reverse conduction (if measurable) when reverse biased, would be as good
a measuring device as a ham would ever need.

Using a very high Z meter such as the VTVM proved to be a plus.  The time
constant was long enough to measure PEP if the RF was SSB or AM.

John WA5BXO