Re: Milliwatting the Norcal 40


From: Alan Kaul (
Date: Sat Sep 24 1994 - 03:54:00 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.

        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.

        Anyway, the test proceeded til the drive control was turned to
        minimum. The Vrms across the dummy load was 0.08 V and
        (0.08 * 0.08)/50 = .000128 W --- or significantly less than
        a milliwatt. Whatever the accurate number is, it is way down
        there. My Daiwa Powermeter (15W full scale) stopped reading
        early in the game!!!

        If we use approx. 50 percent as the error figure on
        the high power measurement (output power closer to 2W than 3W)
        AND presuming the error is linear -- i.e. the same at both ends
        of the scale, then the output is somewhere between .00007 and
        .0002 Watts. I guess the next thing is to have QSO's at that
        level (it won't take anyone too far away to earn the 1000 miles
        per watt award).

        One of these days I'm going to get some accurate test gear so
        I can be certain of these things.

                But one thing does seem assured, the
        Norcal 40 would be an ideal rig TO MIX in a TRANSVERTER
        circuit to either 6 or 2 Meters (or 160-80-30-17-12-Meters
        for that matter [harmonic problems at 20-15-10M], because the
        transmit voltage probably can be set low enough not to damage
        the MIXER. If there was any question, a simple 3-or-6-or-9 dB
        pad could be added between the Norcal 40 and the Transverter
        to make sure the transmitting power doesn't cause any damage.

        You've designed a terrific rig, Wayne. Thanks! And 72, de alan

        W6RCL, Alan S. Kaul ----------- kaul @ netcom . com

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