Feedline radiation question

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W6OGC, Apr 9, 2012.

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  1. W6OGC

    W6OGC Ham Member QRZ Page

    I've been reading a bunch of articles this afternoon on feedline radiation in coax, and the use of ununs, baluns, beads etc. That has resulted in replacing ignorance with confusion.

    It looks to me like there are three options. 1. Build/buy a unun 2. Buy ferrite cores to put on the coax for a couple of feet. 3. Just leave it alone unless you get RF in the shack, etc.

    I have an AH-4 tuner which is coax fed. I can put a unun in between the transmitter and the tuner, at/near the tuner. It takes a maximum of 120 watts so I don't plan high power with this set up. I can also pop 10 or so ferrite beads/cores on the feedline up at the tuner.

    What approach is most satisfactory?
  2. KL7AJ

    KL7AJ Ham Member QRZ Page

    The most satisfactory approach is actually MEASURING feed line radiation and seeing if it's even a problem. It's easy to build a current probe to measure common mode currents on the line.

  3. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    With the AH4 you are unlikely to have problems since it is a matching unit at the feed point and the type of antennas that are used with it.

    Antennas that are problematic are ones fed with long lengths of coax or balanced line.

    We are much more aware of the "possibility" of feedline radiation than in the past and baluns are much more available and common. Frankly, I've used coax fed dipoles with and without baluns and could not tell any difference (though never measured for it).
  4. K1DNR

    K1DNR XML Subscriber QRZ Page

    Best advice, ever.

    The only solution for your confusion is to know what is happening through measurement.

    I just posted an article about my common mode current meter, and solving an issue with RF on my CAT control line - http://charlessocci.com/2012/04/07/...ontrol-common-mode-current-usb-cable-problem/

    A current meter is really, really easy to make. You probably already have all the parts.
  5. WX7G

    WX7G Ham Member QRZ Page

    To decouple the AH-4 you must add common-mode impedance to both the coaxial cable shield and the control cable. The easy way to do this is to route the coaxial cable and the control cable through the same ferrite core(s).

    Mouser stocks many ferrite cores. An example is the Fair-Rite 5943003801 ($3.79). Wrapping three turns through six stacked cores gives a choking impedance of roughly 1000 ohms at 3.5 MHz.
  6. W6OGC

    W6OGC Ham Member QRZ Page

    Confusion is merely a stopping point on the path to understanding all this. You start at ignorance, proceed through confusion, get some experience, build some judgment about what you are experiencing, while taking a few detours through superstition and despair, and eventually you arrive at knowing what you are doing, provided you don't get waylaid along the way,

    How does one build a current meter? I almost certainly don't have the parts but that is just a few hours of internet shopping away.
  7. G3TXQ

    G3TXQ Ham Member QRZ Page

    Just a couple of observations:

    1) The Common-Mode signal is a standing wave; just because the current measures low at some position doesn't mean it's low everywhere - you could just be "unlucky" and measure it at a current minimum!

    2) A feedline that radiates is also a feedline which receives! You may have no noticeable RFI problems on Tx but be suffering more noise pick-up than necessary on Rx.

    Steve G3TXQ
  8. G0GQK

    G0GQK Ham Member QRZ Page

    There's a little used saying today "ignorance is bliss"
  9. KD0CAC

    KD0CAC Ham Member QRZ Page

    GQK , someday I hope to be less ignorant .
    So the issue is I still want to be blissful .

    Ignorance maybe bliss , but what do you do when you are no longer ignorant , and still want to be blissful :)
  10. W6OGC

    W6OGC Ham Member QRZ Page

    I never saw it that way. I'm just trying to get to be as smart as I thought I was when I was 16.
  11. K1DNR

    K1DNR XML Subscriber QRZ Page




    If you follow the link in the first paragraph, there is a link to VK1OD's article on building a current meter. W8JI has a similar article on his website.

    Did you actually look at my link?

    Have you ever used Google?

    G3TXQ - I said in my write-up that common mode is a standing wave and not a universal indicator - however high common mode current measured anywhere it isn't supposed to be indicates a problem. I also mentioned it is responsible for receiving as well as radiating. I also referenced your table on chokes and impedance.

    G3GQK - ignorance is not bliss. It is just ignorance, and particularly annoying to those people looking for and sharing understanding.
  12. G3TXQ

    G3TXQ Ham Member QRZ Page


    I wasn't being critical of your write-up - simply trying to re-inforce those points for the benefit of members who read forum posts but don't read the linked articles :)

    Steve G3TXQ
  13. W5DXP

    W5DXP Ham Member QRZ Page

  14. AA7EJ

    AA7EJ Ham Member QRZ Page

    You could just ignore the fact that coax will radiate – physics of the beast.
    I would not ignore it or bother to measure it – it will be there!
    I prefer KISS approach – choke “balun” - unless you are using RG8 and than you have to do something about the weigh of the choke. Not a problem when the feed line is supported.
    Ferrites are my second choice, but the weight may also be a challenge.
    73 Vaclav
  15. W6OGC

    W6OGC Ham Member QRZ Page



    Some of us who are less technically proficient need more basic explanations, I'm afraid.
  16. K8JD

    K8JD Ham Member QRZ Page

    RF in shack?

    If you have high levels of RF in shack, getting loud noises in computer speakers, little RF burns off metal equipment housing, mic, key etc, I would start by placing the Ferrite beads or UnUn or BalUn on your coax at the feedpoint of the antenna and not inside the shack beteen tuner and rig etc.
  17. KA9JLM

    KA9JLM Platinum Subscriber QRZ Page

    You can do a lot with a field strength meter.

    You can check stuff that is in conduit without needing to direct couple to the wire.

    How to fix a problem, may be harder than finding a problem.
  18. AA7EJ

    AA7EJ Ham Member QRZ Page

    The OP "What approach is most satisfactory?"

    And your answer is.....

    Just curious - did you watch the video ?

    Nothing personnal, but this is why US does not have people interested in engineering.
    It is easier to BS than to be factual.
    Last edited: Apr 10, 2012
  19. KA4DPO

    KA4DPO Ham Member QRZ Page

    As an AH4 owner, user, and experimenter I can tell you that the only situations that ever gave me any significant common mode on the feed line was a. not having an adequate RF ground, and b. using an end fed antenna that was very close to a half wavelength.

    If you have a good RF ground at the tuner and that means a very short run to a good RF ground, you should never have any significant RF on the coax or the control line. This only applies to end fed wires and verticals.

    For loops and dipoles (balanced antennas), unless you are close to one half wavelength at the frequency of operation or, the antenna is not balanced (like a dipole with one side shorter than the other or hanging over a metal roof) common mode RF current should be insignificant. I like Eric's suggestion of measuring the return current on the coaxial shield, it may not be enough to worry about.
  20. K1DNR

    K1DNR XML Subscriber QRZ Page

    I don't consider myself technically proficient. I was trained as a musician, who went into computer support in my 30's, and then got a masters degree in management. There is little about me that is formally technical.

    I have spent some time over the past year trying to understand fundamental ham radio theory concepts, like basic AC theory.

    What part don't you understand? I see you are a lawyer and have done a lot of volunteer work, etc - so clearly you are up to task from an intellectual standpoint.

    You also have an Advanced ticket, which means at some time you learned basic concepts like reactance and resistance = impedance.

    All I presented to you were:

    1 - a suggestion to make a rather simple device to measure the current
    2 - a little bit of info on chokes, and how to measure their effect.

    There is another approach which is to throw ferrite at stuff until the problem goes away.

    I think it is more fun to measure, and know exactly what works, where, and just a little bit about why.

    No equations have been presented. No trig or calculus.

    I don't follow the reference to the admittedly funny video, even If I do consider it a bit insulting.
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