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 Premium Subscriber 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.

    Eric
     
  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 Ham Member 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.
     
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