Feed 40m wire vert with open line?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AK3W, Nov 17, 2009.

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  1. G3TXQ

    G3TXQ Ham Member QRZ Page

    At right angles to the radiating element and close to ground would help. Use other chokes along the line if there is still some induced signal.

    Something like this would be useful:
    http://www.balundesigns.com/catalog/1-1 5kw Choking Impedance.jpg

    It doesn't really matter - the losses in the coax will be so great that there wont be enough RF to cause a common mode problem ;)

    Steve G3TXQ
     
  2. K4SAV

    K4SAV Ham Member QRZ Page

    Steve was probably thinking FIRST harmonic (14 MHz) when he said that. Second harmonic (21 MHz) is fairly low impedance but most of the signal goes up at a high angle so it won't be very good there either.

    You can mostly keep the currents balanced on a 600 ohm feedline by using a 1 to 12 balun at the transmitter and a 12 to 1 balun (or approximate) at the antenna provided you add extra radials to provide good decoupling. Of course the currents that get put there by radiation from the antenna will still be there. But why bother? The antenna is unbalanced and is a good match for coax, although with only 4 radials, a choke at the antenna will be needed. This may not work well on bands other than 40 meters. The impedance of the elevated radials is so high on those bands that it will be almost impossible to keep common mode currents off a balanced feedline. I suppose you could add radials for those other bands to provide feedline decoupling on those bands. Also at any frequency above 17 meters most of the signal will go up at high angles anyway. I guess you could add more vertical elements to solve that problem, but if you did that you could eliminate the two baluns and the balanced feedline and all the problems with routing that feedline which will be a real PITA.

    Nothing like making it difficult.

    You will find that many people have tried to feed this antenna directly with balanced line and use a tuner. Most will say it works OK, but most never compare it to anything else. I guess you just have to accept whatever RFI is created and ignore any ground loss that happens because of common mode currents on a wire that is close to the ground, or maybe they put the feedline up in the air and get a little extra RFI.

    Jerry, K4SAV
     
  3. G3TXQ

    G3TXQ Ham Member QRZ Page

    Jerry,

    Like you I always thought it logical that the "second harmonic" of 7MHz would be 21MHz; but I'm not sure that's the way most folk think of it, so I used what I judged to be the "common parlance". I pretty sure that was also Rege's meaning when he raised the topic.

    I noted the same ambiguity in one of my music theory books:

    I can't recall ever seeing a definition in RF circles.

    73,
    Steve G3TXQ
     
  4. G7VQE

    G7VQE Ham Member QRZ Page

    Would it help if we called them odd and even harmonics?

    I have to say I'm still very happy with my coax fed plus big ugly balun 40 metre vertical combination Steve,thanlks again.
     
  5. K4SAV

    K4SAV Ham Member QRZ Page

    Steve

    I used to get confused about the use of the harmonic number, and it seems I still am. :)
    I looked at the ARRL Antenna Book and they use harmonic #1 is the same as the fundamental.
    In the past, I remember changing the way these were counted but I can't remember why I did that. Maybe it was just a bad dream.
    Hope I didn't confuse anyone.

    Jerry, K4SAV
     
  6. K8JD

    K8JD Ham Member QRZ Page

    Shouldn't the existing radials prevent this by decoupling effect from the coax line? The SWR should be rather low on the feedline and unless the op goes way out of band it should work fine without any choke at the feedpoint !!!
    .
     
  7. K8JD

    K8JD Ham Member QRZ Page

    What harmonic

    It's easy to remember the proper harmonic...2nd harmonic= 2Xfundamental, 3rd harmonic= 3X fundamental etc....

     
  8. K8JD

    K8JD Ham Member QRZ Page

    What makes someone want to hook up 600 ohm balanced line to a 40 some ohm UnBalanced antenna in the first place ????
    There is sure a lot of hype of the ladder lines on these forums !
    Too much disinformation around...
     
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    Not necessarily. Depending on the length of the coax line, what it's connected to, and how it is routed, it could appear a lower impedance path than the radials.

    Low SWR has nothing to do with common-mode current. You can have 1:1 VSWR and lots of common-mode current; you can also have a very high VSWR and no common-mode current.

    But some music theory books say fundamental=f, 1st harmonic=2f, 3rd harmonic=2f etc :)

    Steve G3TXQ
     
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