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I run a λ/2 vertical dipole for 20M that's fed to a 1.5:1 balun at its center with coax that is perpendicular to the dipole (parallel to the ground), for about 30 feet before it comes down to the shack. I wanted some opinions about what would happen if I ran the coax into the center of the lower λ/4 radiating element, if say that element were a copper pipe? I use a balun and get excellent VSWR across the entire band currently, but I wonder if running the coax through the lower element would induce current on the coax shield, and cause the shield to radiate outside the lower element, and of course, back to the radio. The other problem I was thinking about is changing the radiation pattern substantially. Here's a picture of the current setup, now picture the orange wires are copper tubes, and the lower element has the coax going through the tube to connect to the balun box. enter image description here

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    $\begingroup$ Hello Louis, and welcome to this site! We look forward to seeing more of you here. :-) $\endgroup$
    – Mike Waters
    Jul 13 at 18:50
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If a feedline isn't centered in a balanced antenna's near field, it can couple into the EM near field, pick up induced currents, and thus distort the pattern.

If you feed the feedline into a quarter wave length pipe segment of a half wave dipole, the end exit of that pipe will be a very high voltage node at resonance that can capacitively couple to the feed line shield at the pipe exit. Even a disconnected wire around that high voltage node can pick up induced currents, so a choke in that vicinity may not be very effective.

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  • $\begingroup$ You are partially right, but I upvoted this before Phil posted his answer. (Duh!) Running the coax through the hollow λ/4 lower section is much different than if the coax were outside it. $\endgroup$
    – Mike Waters
    Jul 13 at 19:43
  • $\begingroup$ Much different inside, yes. But the near portion of the coax outside the hollow section after the exit can still allow far greater EM coupling problems than an orthogonal dipole center exit. $\endgroup$
    – hotpaw2
    Jul 13 at 20:01
  • $\begingroup$ About how much would "far greater" be? $\endgroup$
    – Mike Waters
    Jul 13 at 20:02
  • $\begingroup$ I suspected this would be the case. And I suppose that my current horizontal coax run which is about a λ/2, is sufficient to minimize induced currents since I can achieve about 1.2:1 VSWR. $\endgroup$ Jul 13 at 20:25
  • $\begingroup$ @hotpaw2 Seems? There's that word again. Stack Exchange is all about accurate references with sound information. :) $\endgroup$
    – Mike Waters
    Jul 14 at 12:51
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That is a great idea. In fact antennas of this design are quite common, and you probably have a much smaller antenna of similar construction on your Wi-Fi router:

enter image description here

Effectively, the lower dipole element, which is a tube, forms a "bazooka" or "sleeve" balun. See also W8JI's description of sleeve baluns. I'll let you follow the links for more detail, but briefly this construction creates another coaxial transmission line around the coax, with the coax shield being the "center" conductor, and the dipole element being the shield. By virtue of being a quarter wavelength long this transmission line is able to convert the short at one end (the feedpoint) into a high impedance at the other end.

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  • $\begingroup$ Oops, I forgot about that! I've done this very same thing for a λ/2 2-meter vertical dipole. But I seem to remember some info at w8ji.com that shows that the coax is not perfectly decoupled unless further decoupling methods are used. He gave the AEA Isopole as a good example. $\endgroup$
    – Mike Waters
    Jul 13 at 19:36
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    $\begingroup$ @MikeWaters yeah, I just added a link to a w8ji article that might be the one you're thinking about. $\endgroup$ Jul 13 at 19:39
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    $\begingroup$ Thanks Phil, I am familiar with the "coax dipole", but I've never been a fan of not using a balun with a dipole/coax arrangement. To my knowledge, without a balun you don't get equal currents on both legs of the dipole and you wind up with a poor counterpoise that doesn't have the same radiation pattern as a λ/2 antenna. I was curious if doing a "tube in radiator" would still have a problem, even though the coax shield was decoupled from the currents via the balun. Thanks! $\endgroup$ Jul 13 at 20:18

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