Voltage probes for high power RF? - Page 1
Forum section: electronics / testgear

author: qwaarjet
author_rank: Regular Contributor
timestamp: August 14, 2014, 01:47:49 am
content: I am working on a RF project in using 13.5Mhz at fairly high SWRs and powers (~500W). Currently I am measuring voltage on one of my cables using a N type T fitting inline and attaching a 1:100 scope probe in the unused port on the T. While this works ok, it is somewhat delicate and I would prefer something more secure than a friction fit. Ideally I would like a scope probe that was terminated with BNC or some other RF fitting instead of the usual metal tip. I have been unable to find something like via the quick search, and am ask the community for ideas.Probe performance requirements:+100Mhz bandwidthDC coupling+-1500Vp

author: tautech
author_rank: Super Contributor
timestamp: August 14, 2014, 02:06:15 am
content: The cheap 100:1 probe I have is only rated to 1200 V @ 2 MHz the falls to 750 V for your frequency of interest. I am not sure how much better spec is available in 100:1 probes.Is your application HV?You could look at the next level of attenuation up, 200:1, 500:1 1000:1Check with the suppliers as many come with a BNC slip-on adapter, the all you will need is a N to BNC adapter

author: qwaarjet
author_rank: Regular Contributor
timestamp: August 14, 2014, 02:57:15 am
content: voltage of 1300Vpp are commonthe exact attenuation is not critical

author: VK5RC
author_rank: Unknown
timestamp: August 14, 2014, 03:48:36 am
content: Won't the T fitting cause a standing wave unless the side fitting is terminated in 50ohms? Typically a rf sampler (Directional Coupler) is used, they have frequency and power limits. There are lots of home brew samplers that cover the HF range on the amateur radio websitesEghttp://n9sja.com/2013/10/06/rf-oscilloscope-sampler/

author: qwaarjet
author_rank: Regular Contributor
timestamp: August 14, 2014, 04:01:56 am
content: Haven't had a problem yet with that , no noticeable SWR changes, probably because the T connector is so much shorter than the wavelength. I have some N type connector fail in some rather cool (read terrifying) ways though.

author: David Hess
author_rank: Super Contributor
timestamp: August 14, 2014, 03:42:22 pm
content: Coaxial BNC to probe tip adapters are available which work well for this when combined with a T-adapter. With BNCs the stub length is short enough not to affect operation up to and beyond 50 MHz but I have not seen N to probe tip adapters. This is how I normally make this type of measurement.I have also seen sampling adapters which allowed the coaxial probe tip to be directly inserted into the line. I know they had them for GR-874 connectors and I am pretty sure I have seen them for N connectors as well.500 watts through 50 ohms is about 160 volts RMS or 225 volts peak and a good oscilloscope probe will just start to derate near 12 MHz so any good x10 500 or 600 volt probe will work at any SWR at this frequency. Standard high voltage x100 probes derate at a lower frequency so at 12 MHz, they have a similar peak voltage limit to the x10 probes.A direct connection to the oscilloscope input is certainly feasible at 12 MHz with a home built tap using a capacitive or compensated resistive divider. Essentially this would be a low-Z attenuating oscilloscope probe operating into a 50 ohm instead of 1 MOhm oscilloscope input. The input side of the probe needs to use a capacitor or resistor/capacitor capable of withstanding high voltage.I would overbuild as much as possible (two or more resistors and capacitors in series on the input side so a short of any one is not catastrophic) this type of sampling circuit and include overload protection (shunt transient voltage suppressors if not something more sophisticated) on the output because failure is dangerous to the test instrument. Professional designs include active overload protection and their are some clever ways to do this. Of course passive high impedance oscilloscope probes can also fail this way and a 1 MOhm oscilloscope input is not going to stand up to 160+ volts at 12 MHz very well either.

author: KJDS
author_rank: Super Contributor
timestamp: August 14, 2014, 04:26:41 pm
content: I'd build and calibrate a simple vestigial coupler. I've just done a quick google and there is very little public information about them in the public domain.The easiest way to make one is to strip the outer insulation from a length of coax. Cut a short slot in the outer conductor and use kapton tape to stick the inner wire from some thin coax over the slot. wrap some PTFE over it and wrap the lot with some copper foil to seal the structure.This approach gives a very weak coupling, but that's fine when you've got high power and don't want to let much out.

author: Sebastian
author_rank: Regular Contributor
timestamp: August 14, 2014, 06:29:12 pm
content: I am not an expert on that kind of stuff, but I think ideally, you should use a power divider to couple out a small amount of your signal power and than calculate the actual power by multiplying it with the attenuation of the divider.

author: w2aew
author_rank: Super Contributor
timestamp: August 14, 2014, 09:26:58 pm
content: Quote from: David Hess on August 14, 2014, 03:42:22 pmDitto to David's good advice above. You might also get a little something out of this video I did 2 years ago on RF Samplers:

author: Co6aka
author_rank: Unknown
timestamp: August 16, 2014, 04:23:52 pm
content: Quote from: qwaarjet on August 14, 2014, 02:57:15 amYou lost me there... If you're not working with an exact "attenuation" your measurements will be floobydust.(RIP Bob)Quote from: David Hess on August 14, 2014, 03:42:22 pmIFthe line is properly terminated with a resistive/absorptive 50-Ohm load. If not, find a guy named R. F. Burns and he'll measure the voltage fer ya!The problem with couplers and samplers is coupling factor and "calibration" or accuracy/stability and so on. If you can directly measure voltage without altering ("introducing significant error") then directly measure the voltage. Otherwise you'll need to calibrate your coupler before, during, and after... otherwise you're not an official RF-measurement-phreak.If you hack up some coaxial Tees you'll find some where the stub-pin can simply be unscrewed. You can then make up a non-inductive-resistor tap at that point -- think "voltage divider" and etc. Observe max voltage ratings of resistors used. Alternatively, you can use some connectors and a short length of copper pipe (or a Tee fitting) and solder up whatever kind of Tee you need -- just calculate the necessary diameter of center conductor for 50 Ohms, or whatever impedance you're working with, in relation to the ID of the pipe. Small brass rod or tube works quite well. (FWIW, some of my homemade "plumbing fixtures" even look pretty good at 2.4GHz.)You can also make a pretty decent RF-current probe along the same lines.

author: David Hess
author_rank: Super Contributor
timestamp: August 16, 2014, 05:10:57 pm
content: Quote from: Co6aka on August 16, 2014, 04:23:52 pmThe attenuation needs to be accurately known but not a specific exact value as desirable as that may be. One reason I like working with compensated resistive dividers is that they are so easy to compensate and calibrate. Dielectric absorption and hook are not problems because they are easy to observe if present.QuoteIn the worst case the peak voltage will only double. Double 225 volts peak is below the conservatively rated 500 or 600 volts of a good probe.For home built probes, I would use a much larger derating factor.On the subject of Mr. R. F. Burns, the worst I have gotten was with a well matched system including the match to me. When Mr. Helical Resonator is carrying 1 watt or more, he is not your friend.

author: larry42
author_rank: Regular Contributor
timestamp: August 16, 2014, 09:35:28 pm
content: Quote from: Co6aka on August 16, 2014, 04:23:52 pmThe coupling factor of a coaxial type (backward wave) coupler does not change with power level or time. Measurement of the coupling factor can easily be made at low levels.To the OP - What accuracy is needed and does it need to be directional. Ie what are you trying to find out?Sent from my One S using Tapatalk

author: owiecc
author_rank: Frequent Contributor
timestamp: August 16, 2014, 10:17:06 pm
content: Quote from: qwaarjet on August 14, 2014, 01:47:49 amWhat about aT&M research voltage divider?

author: David Hess
author_rank: Super Contributor
timestamp: August 16, 2014, 10:23:14 pm
content: Quote from: owiecc on August 16, 2014, 10:17:06 pmIt would be overkill for anything at 30 MHz and below but would certainly work. The coaxial voltage sampler delivers the same result as a capacitively compensated resistive voltage divider.