**Next message:**Dick G0BPS: "ARCI DX MEMBERSHIP"**Previous message:**wynn c c: "RE: QRP for starting ham"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

*>
*

*>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 xenolith@halcyon.com 206/649-6489

Seattle dBug Mac Developers SIG organiser

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