[AMRadio] Re: MFJ-815B Watt-Meter with AM

Jim Candela jcandela at prodigy.net
Sat Feb 10 09:35:36 EST 2007


   I have a good dummy load, and multiple watt meters, and RF ammeter (0-7 amps), and a decent oscilloscope with a good 10x probe. Using the scope and formula:

Watts = [E^2/2R] where E is the peak AC value I agree with both watt meters within +/- 5%. The RF ammeter is mounted in a metal box with SO-239's on each side, and I use the formula Watts = I^2 X R. Of the two watt meters, scope, and RF ammeter, the RF ammeter is the only one way off (~ abt 20%) from the other measurements.I attribute this to the poor accuracy of vintage RF ammeters.

  Since the RF ammeter is thermal responding, I can check it's accuracy with DC current, and maybe make a calibration table, or shift the zero adjust setting (for mid scale use and above). Who knows, maybe the RF ammeter is the only one telling the truth!!

As an interesting side note, my QRO modified Central electronics 20A has very good power supplies within, and can put out 60 watts carrier on 80m, and on SSB the peak power is 64 watts PEP. When I run my Dentron MLA-2500 linear amp (230 volt power) with 8877 conversion, I can boost that 60 watts to 900, but on SSB I peak 2100 watts PEP RF out (for 1 ms on voice peaks). On AM I can run 250 watts carrier and peak at 1000 PEP. The voltage doubler HV supply in the Dentron has terrible voltage regulation, and therefore is a poor performer. I measured the PEP with my scope. The scope and watt meters agree for carrier only. I want to replace that doubler HV supply with something better....another project!

  I am still hoping to make that MFJ-815B more accurate with PEP measurements..


----- Original Message ----
From: D. Chester <k4kyv at charter.net>
To: amradio at mailman.qth.net
Sent: Saturday, February 10, 2007 2:02:07 AM
Subject: [AMRadio] Re: MFJ-815B Watt-Meter with AM 

Why not just use a plain old thermocouple rf ammeter?  I have never seen two 
"ham radio quality" rf output meters that read anywhere near the same.  I 
have an old Mirage, but I use it mostly as a SWR indicator.  It makes a good 
relative power output meter, but on some bands it indicates over 90% 
efficiency in the final, into a 50-ohm dummy load.  I know the transmitter 
couldn't be that efficient, so I take its readings with a grain of salt.  I 
trust the reading I get with my rf ammeter.

Don k4kyv 

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