On Jan 31, 11:12 am, "Suzy" not@valid wrote:
Hello Tom

My responses**

"K7ITM" wrote in message
....
OK, I gave this some thought last night. I see a couple problems...

Though you could use 1mA meter movements, that puts you at a detected
power level high enough that the meter scale won't be linear in power,
assuming Schottky or germanium diode detectors.

**Pardon my ignorance, but isn't it just a case of using an op amp or
whatever to suit whatever meter movement I have? ANyway, I have now sourced
a 100 microamp meter (MU65) with a 3.9K resistance (sounds strange as the 1
mA one has a 210 ohm resistance.

Ah, OK. I had assumed you wanted to use just the meters, with no
amplifier. With amplifiers, the 1mA meters will be fine. But we need
op amps that have very low offset voltage and drift--I would prefer to
set the meter full scale to correspond to around a millivolt or two of
detected DC. I suppose if you have a way to zero the offset and it
doesn't drift, that'll be OK. Then we need to make sure the amplifier
is reasonably immune to 450MHz signals floating around... This is all
"do-able" but there are some details you'll have to pay attention to.
I'd suggest using a couple of the RF power detector ICs available from
Analog Devices or Linear Technology, but we're back to surface mount
stuff again at that point.


BTW, I'n not bothered about the linear issue. I will be having two meters to
show forward and relected powers simultaneously. I don't want to clculate
actual SWR.

Well, yes, but wouldn't you want to know whether "0.1" on the meter
represented 1/10 the power of "1.0" on the meter, rather than 1/100 of
the power?? If you don't pay a little attention to the level of RF
the detector is actually detecting, you're liable to have that
problem.


....
I suppose there's still the possibility of cutting the trace a bit
narrow on purpose and adjusting the impedance by adding a grounded
plate above the board. It could be spaced an adjustable distance away
by mounting it with threaded rods (long screws), and adjusted to make
the traces 50 ohms.

**But how do you check that in a workshop with no test gear?

What do you mean "no" test gear? You'll have the directional coupler
with meters itself, and a power source. The only other thing you need
is a 50 ohm load to put on the coupler output. Is it not worth having
at least a load you can trust? With a known good load, you feed some
power through the coupler and adjust for zero indicated return; turn
the coupler around and make sure the other port also reads zero. With
an open or short load you should get equal readings on the forward and
reverse meters. (I suppose you need two couplers and a good load to
insure that the through line of the coupler is also the same impedance
as the load...) I have a good network analyzer on my bench at work,
but without a calibration load to test and calibrate it with, I don't
know how good its readings really are.

Cheers,
Tom