Measuring common mode current on coaxial shield - Page 1
Forum section: electronics / rf-microwave

author: allenc
author_rank: Contributor
timestamp: July 23, 2022, 02:04:00 am
content: I've been thinking about a device to measure common mode current on coaxial cable. Today there are extremely small, cheap MCU boards with integrated ADCs that can be powered with small batteries for a useful amount of time. I am imaging a PCB attached to a split ferrite core snapped over a cable that couples common mode current into an ADC pin through a simple RF rectifier and perhaps a logamp. The MCU would record samples for some interval and dump the samples later for analysis, or perhaps even send the samples to a smartphone using Bluetooth or something.What I wonder about is if an MCU can be expected to operate in the RF field caused by the common mode current. Would a shield around the MCU be necessary or sufficient to make this work?This scheme seems like it is either obvious enough that someone has already tried it or it's obviously unworkable in some way that escapes me. Haven't found any examples on the interwebs that anyone has shared.

author: radiolistener
author_rank: Super Contributor
timestamp: July 23, 2022, 07:00:53 am
content: Quote from: allenc on July 23, 2022, 02:04:00 amyes, shielding is required, otherwise MCU will failBut I don't see that ADC and MCU is needed in this case, you can simply detect common mode currents with neon lamp for example. Just push 100-200 Watt RF power through coax cable and put neon lamp near to coax shield...

author: A.Z.
author_rank: Super Contributor
timestamp: July 23, 2022, 08:31:23 am
content: here, use a translatorhttps://www.iz2uuf.net/wp/index.php/2016/01/24/amperometro-rf/as simple as it goes

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 23, 2022, 11:10:59 am
content: In my experience, even with a good impedance match, at 100 Watts, there is enough common mode on the coax to show up an a frequency counter. Enough to cause the counter to indicate a freq.Wrap a wire about 6X around the outside of your coax near the transmitter and hook it up to a scope and see if you can get a trace with a CW or Tune signal.The Amperometro RF is cool.

author: A.Z.
author_rank: Super Contributor
timestamp: July 23, 2022, 01:43:27 pm
content: Quote from: Wallace Gasiewicz on July 23, 2022, 11:10:59 amWallace, that RF ammeter is a simple but useful tool imHo, one could slide it along a run of coax and find current min/max points (nodes/anti-nodes) this allows to check if and how much CMC flows on a given feedline, and also to find the best spots (lower current) where to place chokes

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 24, 2022, 11:45:26 pm
content: A.Z.I recently bought a Termaline milliwattmeter for really cheap, because I could not resist.It appears to work accurately at mid scale with the appropriate input.I don't think accuracy is of prime importance here, just relative strength.I think I will try out the sliding "transformer" along a line or two using my new ancient meter.It is a milli watt meter not a micro.If I am not careful though, I might learn something.Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 25, 2022, 02:27:47 am
content: Huh, what would an MCU do? You still need an RF detector (no MCU ADC has a short enough aperture to directly sample RF), and you get isolation for free with a current transformer (which is needed anyway). Just wire it back to any receiver (scope, spec, wattmeter..) and watch in real time. It's not going to be anything different with power level, just test with some standard power level in each band, something like that, right?If it's because it could be very remote e.g. up an antenna tower, and you don't want to run signal cables up to a sensor... I feel it would be much easier solving such problems mechanically in the first place: use a proper balanced antenna; use voltage baluns (or combinations of current and voltage baluns!), use grounding to the tower, use grounding where the feedlines reach ground, or again at the shack; put more current baluns inbetween ground points, etc. Don't forget to test and debug these at ground level: antennas and baluns can be tested, at least to whatever extent is feasible with respect to the size and height over ground (that is to say, given whatever response it should have in that environment), and baluns can be checked for balance with a simple jig, say precision resistors to GND, on a GND plane, and measure CM and DM with a LISN (current choke, one end grounded, other end shield tied to a CM RF port).Testing and trimming won't perfectly cancel CM anyway, nor account for proximity say if the elements are long and may approach structures/trees (especially if the antenna is rotating). So there's only so much prep you can do, and the rest can be dealt with by choking and grounding.Tim

author: allenc
author_rank: Contributor
timestamp: July 25, 2022, 05:19:06 pm
content: Quote from: A.Z. on July 23, 2022, 08:31:23 amYes, there are many designs like this around and I'm familiar with them. What I'd like is an instrument that does the same job as what you've linked, but delete the meter and large case, so the device is very small, light and can be installed on a line in awkward locations where it's difficult to read the meter. Some feed line systems are complex and it's not practical to get the meter to the point you want and still eyeball a needle or wave neon lamps around.Anyhow, someone else answered my actual question; use shielding. I'll just try it with a shield and see how it goes.

author: A.Z.
author_rank: Super Contributor
timestamp: July 25, 2022, 05:56:12 pm
content: Quote from: allenc on July 25, 2022, 05:19:06 pmit won't be so difficult to put together a simple probe like that one, just omit the analog instrument and use an ADC to turn the current reading into a digital value which may then be sent elsewhere, the problem is the fact that the digital circuitry won't be happy with all that RF around, also, adding shielding would probably further perturbate the field we're trying to measure and then if using an RF transmitter to send the data it may cause further issues

author: profdc9
author_rank: Frequent Contributor
timestamp: July 26, 2022, 02:13:34 am
content: Here's a small meter with low capacitance. I built one.https://www.w8ji.com/building_a_current_meter.htm

author: allenc
author_rank: Contributor
timestamp: July 26, 2022, 07:12:44 pm
content: Quote from: A.Z. on July 25, 2022, 05:56:12 pmThanks for the thoughtful response. On the shielding perturbing the field; my hope is that by using a really small MCU board (some recent boards are only slightly larger than a USB A type connector; much smaller than the ferrite involved,) this problem can be avoided as the shield would likewise be quite small.If I were to try communication using bluetooth I think I'd avoid doing it 'real time' for the reason you mentioned. The instrument's transceiver could be briefly powered off during the measurement.

author: A.Z.
author_rank: Super Contributor
timestamp: August 06, 2022, 08:11:57 am
content: any update on this ?