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Meauring voltage from a transmitter's directional coupler - strange?

R

radiofish

I've measured DC voltage directly from a directional coupler BNC socket (the BNC to transmitter meter amplifier disconnected to do this). With the digital voltmeter set to 4v DC, putting the positive probe tip on the center pin, and the negative probe on the metal outer part of the BNC socket (which I assume to be ground) I read about 0.6 VDC, then reversing the positions of the probes (so the negative probe is on the center BNC pin), I then get -1.4 VDC.

This is repeatable and consistent.

This seems strange, if I'm measuring a DC voltage, I expect the same value, just a reversed polarity (positive to negative). What does this observation suggest is going on? Is my "ground" not really ground??
 
Assuming you've checked the other side of the BNC and thereby determined there's nothing on it but a pickup loop (no sampling diode, for example), it's likely that the RF bypassing inside the meter is less than the best, and you're rectifying a bit of the RF inside the meter. Could we have a bit more information:

What make and model transmitter?
What make and model meter?
Frequency involved?

It's an interesting circumstance. Lesse if we can figure it out.
 
First off, its a Connecticut Microwave type directional coupler, incident, reflect, and monitoring, 3 1/8", attached to a SWR harmonic filter, fed by an Energy-Onix ECO 30 on 91.7 MHz. Measurements done using an IDEAL brand true RMS digital multimeter set to 4V DC. The other side of the BNC should and does have a diode behind it.
 
I take 'the other side' to mean the side you aren't measuring. I'd bet the meter is rectifying the RF coming into the digitizer somewhere along the line. The digital meters usually have screaming high input impedances, on the order of many many megohms. They've also got (usually) a pair of diodes affixed in both polarities across the input of the A/D converter, arranged such that any overvoltage will make one or the other conduct and thereby protect the digitizer. If RF gets into these, they may well rectify it, and if their characteristics aren't identical, you'd expect a different reading when the probes are reversed. You might try a different meter from a different maker; or you might bypass the sample by shunting it with perhaps a thousand ohms or so and a good mica .01 cap and see if this removes the DC. If it doesn't, then something inside the sampler is rectifying the RF. I'd look for a cold joint where the sample button (or loop) is soldered to the center pin of the BNC. It's a fun exercise to try to figure this sort of thing out! Keep us posted on what you find.
 
Try changing the battery in your meter, I have had the same thing happen to me when the battery is low on my Fluke.

Also I think the coupler output should should see a load, disconnecting the metering and measuring with a hi z meter may cause problems, try a Simpson 260.

What are you looking for anyway?

Pete
 
The diode went bad in the directional coupler. I had previous measurements that I took months ago between the transmitter and the directional coupler so if the transmitter's meter board was replaced, we'd have a reference curve to match the new meter calibration to.
 
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