Re: Milliwatting the Norcal 40


From: Kevin Purcell (
Date: Mon Sep 26 1994 - 20:19:11 EDT

>On Thu, 22 Sep 1994, Wayne Burdick wrote:
>> I think it will go all the way to zero; at some point the drive level gets
>> too low to forward-bias the final amp. Connect a hi-Z voltmeter (such as a
>> DMM) and an RF probe to the antenna jack and measure the RF voltage (Vrms).
>> You can then calculate the power from:
>> P = Vrms * Vrms / 50
>> assuming you have a 50-ohm dummy load or well-matched antenna.
>> I'd like to know your results.
>> 72,
>> Wayne
>Wayne, et. al.
> I took your advice and wired up an RF probe ala ARRL Handbook
> (.01 mfd cap, 1n34a diode, 4.7m resistor) and my Radio Shack
> DMM, a 4-W, 50 Ohm dummy load and ran the tests.

Whats the input impedence of your meter?

That makes a difference in selecting the 4.7M ohm resitor (to convert from
V pk to V rms). The 4.7 M ohm is for a 10 M ohm input impedence meter.

This resitor forms a voltage divider using the resitance of the DMM. The
resitor should have a value of 0.414 x Z(DMM) to convert V pk to V rms
(sqrt(2):1). So for a 10 M ohm input resitance meter then 4.1 M, but 4.7 is
close enough.

Took me a while to figure this one out (even though its simple!).

> The Handbook suggests such a method will measure Vrms to within
> about 10%. It might be the vagaries of my peculiar system, but
> I question the exact precision in my case because a full power
> measurement on the output of the 2SC799 reads 12.25 Vrms (which
> calculates to 3W output) ----- not likely with 13.5V to the rig
> at keydown 200 mils.

Before the filter?

If so you are looking at the harmonics (its class C PA after all).

The simple calculation of power using the pk to pk value from a RF probe
only works for a sine wave. Harmonics in the input will give a much higher
pk to pk voltage and an incorrect apparent power.

Kevin Purcell, N7WIM / G8UDP 206/649-6489
Seattle dBug Mac Developers SIG organiser

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