Balun measurements

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by VK2FXXX, Feb 13, 2011.

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  1. VK2TIL

    VK2TIL Ham Member QRZ Page

    I believe that any measurement, however crude, is better than no measurement at all; I am just following W7ZOI and Lord Kelvin in my thinking.

    To extend the "torch" analogy;

    I have two unknown resistors; I place each in turn in the circuit and note the brightness of the lamp in each case.

    Now I know which one has the higher resistance.

    The line used need not be any particular length; just a short piece would do.

    The termination at the load end should be acceptable to the transmitter; perhaps SWR of 2:1. So a suitable combination of load & "leak" resistors might be two x 50-ohms.

    Alternatively, build an attenuator (say 10dB) at the transmitter end; this will "set" the impedance seen by the transmitter and what goes on at the load end of the line won't bother it.

    Combinations of resistors should get the required 10w rating; in a 10dB pi attenuator about 50% is dissipated in the input resistor, about 35% in the series one and about 5% in the output one.

    There are a few current meters on the 'net including (but not limited to); Radio/Experimentation/RFProbe.htm

    Have fun!

    Incidentally 'FXXX; are you going to Wyong? I know it's a long way but I used to do it from Coffs each year. I think a group from Summerland comes-down each year.
  2. G3TXQ

    G3TXQ Ham Member QRZ Page

    I don't understand how your proposed test arrangement is creating a controlled common-mode current environment. Could you explain it some more?

    Steve G3TXQ
  3. VK2FXXX

    VK2FXXX Ham Member QRZ Page

    Ok Im back from work.

    Steve I understand that the experiment would not give me the choking impedance of the balun.As Owen says its not a simple ohms law calc.
    However could the test and measurements be used to determine the effectiveness of a given balun in a realistic environment.

    To use your torch analogy, we know that the resistor is 90% effective at extinguishing the light, if extinguishing the light is the goal.,
    Or the current on the coax dropped from 1000ma to 100 ma with the balun in place ie the balun is good for 10db of current mode attenuation.
    How can the average amateur"measure" the effectiveness of a balun away from the bench and VNA.

    Kerry I wont be at wyong this year ,I just moved to the area in October from Kurri Kurri! cant believe Ive never met you at westlakes or spoke on the repeaters there. Some good info in your last post too ,thanks.
    I also would like to know how to intentionally create common mode currents on a feedline ,it seems we can do it quite well accidently!:confused:
  4. VK2TIL

    VK2TIL Ham Member QRZ Page

    "I also would like to know how to intentionally create common mode currents on a feedline ,it seems we can do it quite well accidently!".

    This is exactly what's been puzzling me; you have said it very succintly.

    My earlier post was intended to provoke some positive discussion as to how this could be done; I failed. :(

    I puddled-around today with a current transformer on a length of co-ax terminated in various ways and fed with 13dBm; I got a substantial difference (about 10dB) between RF-on and RF-off but I think that was from cable leakage (I used RG-58; I'm not prepared to cut up my good calibrated test-bench cables!) rather than from common-mode current on the shield outer.

    I moved (home) to Newcastle a little under two years ago; I "camped" until I bought a house at Maryland about nine months ago.

    I'm not a "transmitting" amateur; I prefer the workbench.

    I have some nice radios, including a complete TS-830S station, but they are all stored away at present; I prefer the smell of solder flux. :)

    Kurri is a town that I like very much; I looked there for a house.

    The problem was that there are few new houses there; many lovely old houses but I am in my twilight years so I wanted low-maintenance, level block etc.

    I settled for a ten-year-old brick-veneer in Maryland.

    Pity you aren't going to Wyong; I would like to meet you.

    You seem to have an enquiring mind as far as things-RF are concerned.

    If you don't already have a copy of Experimental Methods In RF Design you should get one before it becomes a rare book; I think that you will like it.
  5. VK2FXXX

    VK2FXXX Ham Member QRZ Page

    Gday again Kerry

    Maybe K1BQT who posted on the first page could help us.
    I re-read his posts and there are some interesting bench tests there.

    Maybe a dipole on the end of coax ,with 1 leg over a metal roof will encourage some current down the coax?
    I could do that here easily ,but that would require repeated carriers over the air, probably best done with more power than 10w.
    I really need to upgrade my licence!

    I have other hobbies ,restoring an old Valiant ,golfing,plus a young family ,and a business to run,Time for such experiments is limited unfortunately. I believe you are retired,so you may have more spare time to play radios. Good for you, Im a touch jealous .The time between an idea for a test ,and actually doing the test can be months here.
    Im still making some loads for an ATU loss measurement experiment ,using the calorimeter method .I will get it done.

    Patience Kerry ,I think they`re still asleep on the other side of the Globe!

    Have you seen this current meter,I think it would be ideal for this type of test.

    I will look for that book ,other people have also recommended it to me .

    Have a look on my QRZ profile for my email address,I believe its there,shoot me an email If the discussion stalls here.

    Heres my email address without the asterix brendansbag***@yahoo*** will that fool the robots?
    Last edited: Feb 14, 2011
  6. VK2TIL

    VK2TIL Ham Member QRZ Page

    I've seen 'JI's current meter; it's good.

    For today's rough experiments I just took an FT50-77 (I think it was 77) and wound six turns of 0.63 ecw on it; I then slipped it over the bit of co-ax I was using and connected my AD8307 50-ohm power meter to the two wires.

    I have long lusted for a Tek 6022 current probe for this kind of measurement; there was one on a table at a 'fest a while ago but the bloke just in front of me was one of the few who knew what it was and bought it for $20. :(

    I will send you an e-mail; I think that we have a bit to talk-about.

    Real experimenters seem to be a bit thin on the ground these days; modelling has taken-over but there's no substitute for solder smoke. :)
  7. G3TXQ

    G3TXQ Ham Member QRZ Page


    It's not that simple because the torch analogy doesn't illustrate the effects of a balun's complex impedance.

    You could have a Resistive balun and a Reactive balun produce very similar CM current reduction in one test set up, yet perform very differently in other environments.

    Of course, if your test set up happens also to be your operational antenna system then the measurements would be valid!

    Steve G3TXQ
  8. G3TXQ

    G3TXQ Ham Member QRZ Page


    From "bitter experience", here's one way.

    Take a symmetrical dipole and feed it with coax. Make the dipole electrically very short at your test frequency, so that the feedpoint has a high Reactive component. Ensure that the length of the coax (including the CM velocity factor) is a half-wavelength (or even multiple) at the test frequency. Ground the coax at the source end.

    Now you'll have lots of CM current to experiment with! But note my earlier comment about what conclusions you draw from various tests.

    Steve G3TXQ
  9. W8JI

    W8JI Ham Member QRZ Page

    A more proper test with no equipment would be a balun with a floating load, with a good center tapped RF transformer to ground on the output. The center tap could be taken to ground through an adjustable series resonant circuit with various resistors in series with the system.

    The load balance could be measured with a meter or some other indicator.

    This would show almost all balance problems with baluns.
  10. VK2FXXX

    VK2FXXX Ham Member QRZ Page

    Hello again Steve.
    After reading Owens post No #16 again about 5 times.and thinking about the point you have been making about the test environment, I am starting to understand what you are saying! maybe.

    The complex impedance of the balun is determined by the common mode current presented to it,which causes magnetic flux,which is multiplied by the permeability of the toroid,
    As the antenna system environment is changed ,balance and common mode currents vary ,therefore altering the c m current to the balun, altering the flux ,therefore varying the choking impedance !?
    Somethings not quite right here but I think its close !?

    Is this basically why it is best to measure an antenna feed-line for cm current in its final operating position?
    Was this the point I was missing?
    Thanks for your patience.
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