[AMRadio] watt meter

John E. Coleman (ARS WA5BXO) wa5bxo2005 at pctechref.com
Thu Mar 2 10:44:12 EST 2006

The Best measurement I ever got was using a dummy load of know resistive
accuracy.  I used a new "Cantenna" from heath kit.  I said "new" because if
over heated once they can change resistance.  The older one seemed to be
better.  Anyway, I removed the little box cover on the top and stripped out
the circuitry in it.  Then I mounted a 7 pin socket in the top cover of the
box. Plugged in a 6AL5 and connected a plate to the top connection of the 50
ohm resistor dummy load brought the filament wires out to a 6.3v XFMR and
connected to corresponding cathode to the RCA phono plug that was on the
side of the little box.  I then bypassed the cathode connection with a .01uf
ceramic capacitor rated at 1000v and no bleader.  

	The theory is that the 6AL5 diode has very little plate to cathode
capacitance (used a lot in the older RF probes by HP).  It provides a very
accurate rectification of the RF and will charge the capacitor to a very
accurate peak of the RF voltage.  The DC voltage can be measured by any good
accurate DC voltmeter and will be a representation of the peak RF. 

Calculation as follows:

Convert the measured DC to RMS of RF by multiplying time 0.707

Power formula

(Erms * Erms) / 50ohms = Power

Somebody check me on this, because it has been awhile, but I think this all
reduces to

Epeak squared divided by 100

(DC * DC) / 100 = Power

 I have checked this all the way up to 10 meters against several of the bird
watt meters owned by FCC commercial licensed companies doing TYPE acceptance
checks.  It is as accurate as the dummy load and voltmeter that you have.

John, WA5BXO

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Brett gazdzinski
Sent: Wednesday, March 01, 2006 3:10 PM
To: 'Discussion of AM Radio'
Subject: RE: [AMRadio] watt meter

What is everyone doing for a watt meter?
My swan wm-3000 blew up last weekend, it reads no pep
and about 1/4 of the correct average power.
I could fix it if I could find out what diodes they used (3 gone),
and one of the little inducters broke, but it 
would not be calibrated.
It was a good meter, 4 scales going up to 2kw, real pep
reading, large meter, but its toast.

Looking around, not much out there that does high continuous power,
real pep readings, looks good, etc.
I went to ham radio outlet in Delaware today (working close by) and 
despite waiting for almost an hour, could not get much help.
They would not let me open a box of a daiwa meter, and I hate the cross
needle meters anyway, they had a diamond, not true pep, but I 
could not look at that, they wont unseal a box unless you buy it.
That and the poor service had me walk out of the store 
vowing never to buy anything from them, ever.

Some web searching turns up CB type meters, not good at lower
frequencies, or discontinued units.
Nye viking used to make a good one, but not anymore.

The 813 rig can do well over the 1500 watt pep level,
well over 2000 watts pep with the neg cycle loading 
deck hooked up, so I need something good for the power.
MFJ makes one, but the manual shows its only good for
500 watts 100% duty cycle, 600 or 700 watts of carrier means 
90 second transmissions (how cheap that mfj stuff is).

Not much on ebay but old heathket meters...


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