Graham wrote in news:329ab6b5-154c-4ee2-a6b7-77f841f27cf6
@f36g2000hsa.googlegroups.com:

On Apr 18, 11:26*pm, bz wrote:
" wrote innews
:

I have a need for a power meter for general low band and VHF work. I
would like it to have at least two scales. One, *a 0 - 10 W or so
scale and if possible another which would enable it to measure up to
around 125 W as well. The immediate need to satisfy the requirements
of a job we're doing is for an instrument that can measure 1.0 W at
72.0 MHZ. The signal is AM with a duration of .50 sec. and there is
some type of digital alarm transmission which modulates the carrier.
The only way I think that I can do this now is to measure the RMS
voltage accross a 52 ohm dummy load with my Boonton, and then
calculate the power. I feel though that this is clumsy and may be
potentially inaccurate. I'd love to have a Bird with all the bells and
whistles but I really can't afford one. Does anyone know of a home
brew project for doing this or even an inexpensive accurate kit?
Thanks, Lenny.

http://www.elecraft.com/mini_module_...ni_modules.htm

The signal is AM with a duration of .50 sec. and there is
some type of digital alarm transmission which modulates the carrier

That sounds like a complex transmission , measuring the incident
voltage over the 50 ohm load may be the only way , may be hire
something expensive to get a calibration chart ?


One of the kits on the page I referenced could be the basis for his power
meter. There are others in _The Radio Amateur's Handbook_ that would work.

What he may need is probably to use a computer A/D card
(possibly the audio card) to capture the waveform. Then to numerically
integrate that over the time period of interest.

A simpler way would be to read the peak voltage across the load.
That can be done with a simple diode detector probe.
That would give the peak power with a simple calculation.

Nothing expensive is needed to get an accurate peak power reading.

A diode, a couple of small capacitors and a resistor will make a fairly
accurate peak voltage probe. Elecraft has a low price kit.

A couple of 100 Ohm surface mount resistors of the appropriate power
rating, soldered in parallel will give a very good, low reactance 50 ohm
dummy load that should cover the frequencies of interest.

Or 20 each, 1000 ohm resistors, 10 watt resistors (non inductive) in
parallel for a 200 watt 50 ohm dummy load.

P=V^2/R; V=sqrt(P*R);
easy enough to make a calibration chart:
v p
15.81 5
22.36 10
27.39 15
31.62 20
38.73 30
44.72 40
50.00 50
61.24 75
70.71 100
74.16 110
77.46 120





--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

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