Re: Inexpensive fairly good quality home brew or kit RF power meter
- From: bz <bz+ser@xxxxxxxxxxxxxxxxxxxx>
- Date: Sat, 19 Apr 2008 02:24:39 +0000 (UTC)
Graham <g0nbd@xxxxxxxxxxx> wrote in news:329ab6b5-154c-4ee2-a6b7-77f841f27cf6
@f36g2000hsa.googlegroups.com:
On Apr 18, 11:26 pm, bz <bz+...@xxxxxxxxxxxxxxxxxxxx> wrote:
"captainvideo462...@xxxxxxxxx" <captainvideo462...@xxxxxxxxx> wrote innews:a0af529a-e358-4577-be1a-dd107caa9e25@xxxxxxxxxxxxxxxxxxxxxxxxxxxx:
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_kits/mini_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.
bz+ser@xxxxxxxxxxxxxxxxxxxx remove ch100-5 to avoid spam trap
.
- References:
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- Re: Inexpensive fairly good quality home brew or kit RF power meter
- From: bz
- Re: Inexpensive fairly good quality home brew or kit RF power meter
- From: Graham
- Inexpensive fairly good quality home brew or kit RF power meter
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