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quick question on feeding a half-square?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K3UK, Aug 7, 2010.

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  1. WA4FNG

    WA4FNG Ham Member QRZ Page

    I think all of the important questions have been answered, but I'll chime in with "it's a great antenna..." My feedline leaves the corner at a 45deg angle between the vertical and horizontal sections -- directly inline with the antenna. I do plan to try bringing the coax out horizontally to see if there's any difference, but I don't expect there to be much at all.

    I am using a choke at the feedpoint. Had a ready-made 1:1 isolation balun laying around so I used it.

    73,
    Milt
     
  2. KI6J

    KI6J Ham Member QRZ Page

    I really like Steve's idea and now it is a must-do experiment. It's kind of like the halfsquare version of the Carolina Windom. I'm sad I have to wait until next weekend to give it a try.
     
  3. K7JBQ

    K7JBQ Moderator Volunteer Moderator QRZ Page

    Steve,

    Truth be told, never thought of it.

    Try it. If it works, name it.

    73,
    Bill


     
  4. W5DXP

    W5DXP Ham Member QRZ Page

    How many ohms of choking impedance would be required at that current node?
    Would a quarter-wave sleeve over the coax be better?
     
  5. G3TXQ

    G3TXQ Ham Member QRZ Page

    Cecil,

    I don't really know!

    If you "tune" the appropriate number of turns on a T240-61 for the frequency in use, you should achieve 30k ohms or more. I've used the technique on dipoles using ferrite chokes and been unable to detect any CM current beyond the choke.

    Steve G3TXQ
     
  6. G0GQK

    G0GQK Ham Member QRZ Page

    Andy..... make certain that the bottom of the vertical sections are at least 6 feet above the ground, or higher. I made one for 30 metre DX and there's been none for two years ! I took it down and made one for 15 metres, and it hasn't been used on 15 metres yet, and its been hanging up there for 12 months !

    G0GQK
     
  7. KI6J

    KI6J Ham Member QRZ Page

    I am working on the design suggested by TXQ and have a few questions.

    Should I cut the coax element to the physical length of the outside braid rather than the electrical length of the coax as a whole? Should I then shorten the half wave phasing line to compensate for the velocity factor of the center conductor of the coax element?

    Stu
     
  8. W5DXP

    W5DXP Ham Member QRZ Page

    The length of the coax from the feedpoint to the choke should be the same 1/4WL physical distance as the length of the other vertical element. The VF of the coax doesn't enter into it. The horizontal wire should be 1/2WL. The choke will have no effect on the differential transmission line signal.
     
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    The only compensation needed might be in the length of vertical from the feedpoint to the choke. That's because the coax has a PVC jacket which will slightly alter its Common-Mode velocity factor. But the difference in length compared to the vertical section at the other end will likely only be a couple of %.

    As Cecil said, the "normal" velocity factor of the coax (typically 0.66) is not relevant here.

    Do make sure to use a choke which has a very high impedance at your operating frequency - that's essential in this application. Put simply, at the feedpoint you are deliberately forcing Common-Mode current onto the coax braid, and at the bottom of the vertical section you are then trying to stop it :)

    Some choke ideas here: http://www.karinya.net/g3txq/chokes/

    One of the higher-Q designs on a FT240-61 core would probably be a good choice.

    Steve G3TXQ
     
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