Coaxial Current Monitor for High Frequencies - Page 1
Forum section: electronics / projects

author: Plasmateur
author_rank: Regular Contributor
timestamp: September 28, 2016, 12:48:04 am
content: I'm looking to build or buy an equivalent current monitor such as the one found here:http://www.pearsonelectronics.com/news/130This is the only product online that I can find, and I was quoted between $1000-$2000 for this item.Are there other ways of measuring HF currents w/o dramatically changing the impedance of the circuit?

author: Marco
author_rank: Super Contributor
timestamp: September 28, 2016, 12:50:14 am
content: At low power and low side a small resistor(Or in other words, specify your requirements better.)

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 28, 2016, 12:52:55 am
content: http://www.ebay.ca/itm/Tektronix-CT1-CT-1-Current-Probe-1-GHz-/162209500403?hash=item25c4708cf3:g:C28AAOSwTA9X4Jo3

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 28, 2016, 12:55:12 am
content: http://www.ebay.ca/itm/Tektronix-CT3-Passive-Signal-Pickoff-/300744971497?hash=item4605cc14e9:m:md9ZRKy8xr0fEaakw2kuYOghttp://w140.com/tekwiki/wiki/CT-3Huh, might get one myself. Of course, you'll probably want GR-874 adapters. GR-874 are the big connectors on there.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2016, 01:43:15 am
content: Yeah. You're looking for a directional coupler: a traditional bit of RF kit, it couples a fraction of the RF power (flowing in a given direction) out sideways.Since some power is diverted, there is a necessary loss associated with it. Disturbingly, the current sensor in question does not document this, nor do they provide scattering parameters or a frequency response plot. Rather a shame, seeing as we're talking aboutPearson Electronicshere...Not that we should expect the minimum power loss is very much: this is a 1/50th current transformer, so it manifests as a series resistor of 1/50 ohm, giving a whopping minimum loss of: 0.0035dB.The losses due to connector resistance, and reflection from reactances in the part, will be much more significant. But it would've been nice to see that.Note that it's not meaningful to speak of wideband current measurementsANDBNC connectors (and the assumed coaxial cables associated with them). If you're using any length of cable, then your impedance must be very close to 50 ohms, otherwise the equivalent stray inductance (Zsrc < 50 ohms) or capacitance (Zsrc > 50 ohms) totally defeats the point of having so much bandwidth. I mean, not that you can't or anything -- but you'll simply be measuring the power, as a nondirectional coupler would do.So, all this to say -- they probably didn't intend that part to be used in RF circuits anyway. But it's not made for low impedances either, which is where you usually need a wideband current transformer. So it's really not very optimal for anything, sadly.If you need a low impedance measurement, with high bandwidth, like what you'd need to probe a switching supply -- you will need different methods: perhaps a very thin Rogowski coil, or qualitative measurements with a near-field H probe and integrator.Tim

author: Plasmateur
author_rank: Regular Contributor
timestamp: September 28, 2016, 04:48:12 am
content: Thanks for the response Tim.I have previously split a coaxial cable and ran the center conductor through a Rogowski coil, which is one of Pearson's green current donuts. The length of my cables run about 6-10 meters. The frequencies I need to operate in are between 10-200MHz give or take some from the top or bottom.The purpose I need the current monitor is for a diagnostic for a simple loop antenna. The antenna is in an environment which cannot have other devices introduced into.I have a coaxial cable where I have made a 50mm diameter simple loop antenna by shorting the inner conductor to the ground shield. By using the device I have listed, I thought this would be a decent means as to monitor the current into the loop antenna. As of now, I have -8.8dBm going into a 50ohm coaxial cable, into a an amplifier(43dB) with length of 2 meters to which the loop antenna is soldered to end.I figured then, my current to be at the very least, with no imaginary component to be10*ln^((-8.8+31)/10)/1000 = 2.63 Watts50Ohms = RCurrent = (Watts/R)^(1/2) = .23 ampsIs this okay for an estimation? I assumed the loop at the end would cause some form of mismatching, however I see a very clean sinusoid on a Bdot probe which may indicate very low reflections on the mismatched antenna. Perhaps a SWR meter would allow me to infer what the current is?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2016, 05:25:07 am
content: Yabbut...You're ignoring SWR. (And, I'm pleased to see thisisan RF application, which makes a directional coupler all the more relevant.)If this is a resonant loop at 200MHz, then it's a "small loop" at ~50MHz, and merely a poor short circuit at 10MHz. With 2 meters of cable and a maximum frequency of 1.5m wavelength, you have more than enough room to observe standing waves in the 30-150MHz range. Which means, where you place the current transformer, matters.At 25MHz, where the 2m line is a 1/4 wave impedance transformer, you will get maximum current at one end, and minimum current at the other. And the other way around, for voltage. (With a low inductive impedance at the antenna, there should be a high capacitive impedance at the transmitter, or something like that. But it depends on the transmitter's equivalent output impedance, and also, success at all depends on the SWR it can tolerate...)With a directional coupler, you can measure transmitted and reflected power directly, so you can calculate SWR and impedances.I think you made a typo (31 --> 43?), but the figures are right (2.63W, and 0.23A at SWR=1). What's missing is SWR -- any guesses what that will be for the poor antenna over such a wide range? Will the transmitter handle that without blowing up or splattering throughout the dial?A clean waveform tells you nothing about SWR; if anything it could be a counter-indication, because reflections imply frequency selective filtering action, so you might well expect clean sinusoids, if anything. But the levels of distortion that are important are in the -60dB range, so you shouldn't expect to tell anything about that from an oscilloscope.An SWR meter contains a directional coupler, so it's a natural fit for this. The only way you can address SWR is by adding a tuner; for limited cases, special-length transmission lines work, but the only possible way you can get anything to work, over that wide a range, without adjustment or selection, is with a very wideband antenna (spiral or log periodic, perhaps?). Which from the sound of it, you can't fit due to size or accessibility.There's a well proven tradeoff between minimum antenna size, instantaneous* bandwidth and directionality. So we can very easily do a sanity check and see if you're asking for the impossible, at least.*That is, how much bandwidth you can get from a particular setup. An antenna might be tunable over a wide range, but its bandwidth at any particular tuning setting will be poor. At best, tuning affords you a slow TDFM function, I suppose you could say. Which needn't be all that bad of a limit; it would be more than practical for a HAM station (as long as it's more automated and not painfully tedious!). But less practical for a cellphone, which might need to hop frequencies quite rapidly to support the various channels and modulations supported.Tim

author: Zero999
author_rank: Super Contributor
timestamp: September 28, 2016, 08:07:42 am
content: In summary: this is more complicated than just measuring the current to something like a DC motor because it takes time for a change in voltage & current to propagate down a piece of cable and at high frequencies this delay becomes significant.

author: dmills
author_rank: Super Contributor
timestamp: September 28, 2016, 10:50:23 am
content: 10 - 200MHz you say.That is significantly easier then the few hundred Hz to a few hundred MHz that part is specified for....Take one ferrite (NOT iron powder!) toroid, one small diecast box, 3 RF connectors to suit, a short piece of hardline and some magnet wire.Bolt all three connectors to the box in a T arrangement, one on each side, there will be RF flowing in the box walls so this needs to be electrically solid.Wind 33 turns of magnet wire evenly around the toroid, it is important that the toroid be small enough that the winding is shorter then 1/10th wavelength @ 200MHz, so say shorter then 10cm total.Connect the ends to the sample output socket.Now take the coax line and pass it thru the centre of the toroid, solder the centre of the coax to the centre pin on the input and output connectors and the screen to the screen connection at ONE END ONLY.What you have created is an electrostatically screened current transformer having a 33:1 ratio (so in a matched impedance situation it will have a coupling factor of -30dB), current on the sample port will be 1/33 of the RF current on the main line, and if terminated into 50 ohms it will appear as a series resistance of 0.05 ohms approximately.By picking a suitable ferrite and making the thing as small as you can get away with it is possible to get surprisingly wide bandwidth and good accuracy even without calibration.Regards, Dan.