RF current meters are good tools. Real handy for phased arrays and
debugging misc antenna problems. I just recently built one, since I had
this very nice 50 uA meter that needed a home. I took the simple
approach and then made it a little more complicated. As you said,
overcoming the diode threshold creates some problems, so I forward
biased it, and then added an opamp to subtract out the offset that
created. The opamp also made it possible to gain up the output and
calibrate it to the 50 uA meter. Since the diode is still somewhat
non-linear in the low current range, I added a circuit around the opamp
to correct that non-linearity, so I ended up with a very linear output
which was directly readable from the meter calibration.
The thing that surprised me when looking at the designs others have done
was that many people used the clamp-on ferrites that are intended for
RFI suppression. These add a large series impedance into what you are
trying to measure and will mess up the current you are trying to
measure. What was needed was a small iron powder ferrite in a clip-on
which will give low loss, but no one makes these in a clip-on (there is
no application for these). I ended up splitting a toroid and
fabricating a spring mechanism to make it function like a clip-on.
Worked pretty good. I have two calibrated ranges on the meter and can
measure from about 10 ma to 5 amps. There is also a super sensitive
uncalibrated (for now) range. Current drain on the 9V battery is less
than 1 ma.
The AD8310 is a nice idea also. I didn't think of that.
Ian White GM3SEK wrote:
>Coming back to the effectiveness of baluns, the final decider is the
>amount of unwanted RF current on the outside of the coax shield,
>compared to the wanted current in the antenna element. The only way to
>find that out for sure is to *measure* it, in the system as installed.
>I'm a big fan of RF current meters based on simple snap-on ferrite
>beads. Add a few turns of wire, one resistor, a diode detector, and you
>have a real measuring instrument. It's a real eye-opener to be able to
>snap the meter over any cable and *see* the common mode RF current.
>By coincidence, this afternoon I threw together an AD8310 wide-range RF
>detector for my own snap-on meter. Instead of being blind to small
>signals because of the diode-detector threshold, the meter now has a
>linear dB scale that goes all the way down to about 100uA. Once I have
>beefed up some of the resistors, the same scale will cover all the way
>up to 1A.
>Down at the low end, the sensitivity is scary - there's no such thing as
>a totally clean cable, anywhere in this shack! The general level of
>common-mode crud on computer, network and mains cables is on the order
>of a few milliamps. Pretty harmless from an RFI point of view, but it's
>nice to be able to see and measure down to those levels if needed.
>(Some reservations about absolute accuracy are in order here. The
>detector sees the combined common-mode currents at all frequencies, from
>VLF up to a few hundred MHz, and the current readings on various cables
>will also depend on the waveform of the signals. This means that a given
>level of measured current can have a widely varying potential to cause
>RFI, depending on the type of signal involved.)
>On the RG58 cable connecting the 100W rig to a very well screened 50 ohm
>dummy load, the key-down RF current inside the cable is obviously 1.4A.
>On the outside, the meter says about 10mA. However, that doesn't
>necessarily indicate leakage through the coax, because there is about
>the same level of common-mode RF current on the twin 13.8V power cable.
>Once the RF is out of the shielding envelope, it tends to crawl
>everywhere... with "interesting" implications for crossband duplex and
>Of course this is all anecdotal stuff, but it's so nice to be able to
>*see* what the RF currents are. It cuts out a load of guesswork.
>At the higher current levels that are more likely to cause trouble, even
>a simple diode detector will do the job:
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