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A NEC4.2 analysis of this configuration may reveal an unexpected truth about this...

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Below is a NEC4.2 comparison of the v-plane patterns of a 1/4-wave ground plane antenna mounted 5m above level Earth and using 4 x 1/4-wave, horizontal radials, with / without a coax transmission line having the OD of the shield "grounded" to the Earth.

Two observations:

  • The 4 x 1/4-wave horizontal conductors do not effectively decouple the transmission line from currents flowing along the vertical conductor of the ground plane, and

  • The net, peak radiation pattern gain of the ground plane antenna is reduced by about 1.8 dB for these conditions, due to the effects of the radiation from the OD of the outer conductor of that transmission line.

enter image description here

Added below are my comments and a graphic in response to the comment of hotpaw2:

The feedpoints for the GP as modeled attach the center conductor of the coax feedline to the base of the vertical conductor, and the feedline shield to the common-point of the four radials. The vertical conductor is one pole of the antenna system; the radials are the other.

If the radials are attached to the base of the vertical conductor and then driven together by the center conductor of the coax feedline, that would force all of the r-f current flowing on the ID of the shield to migrate to the OD of the shield at the top of the feedline, and produce radiation from its outer surface.

enter image description here

| improve this answer | |
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  • $\begingroup$ I wonder if these effects would still be present if the feedline was electrically detached from the antenna (but left in place) and the antenna was fed at it's base (ground plane intersections)? $\endgroup$ – hotpaw2 Apr 11 at 15:15
  • $\begingroup$ The feedpoints for the GP as modeled attach the center conductor of the coax feedline to the base of the vertical conductor, and the feedline shield connected to the common-point of the four radials. The radials are one pole of the antenna system, and the radials are the other — IOW they are already detached from each other. $\endgroup$ – Richard Fry Apr 11 at 17:32
  • $\begingroup$ Final edit: The feedpoints for the GP as modeled attach the center conductor of the coax feedline to the base of the vertical conductor, and the feedline shield to the common-point of the four radials. The vertical conductor is one pole of the antenna system; the radials are the other. If the radials are attached to the base of the vertical conductor and then driven by the center conductor of the feedline, that would force all of the r-f current flowing on the ID of the shield to migrate to the OD of the shield at the top of the feedline, and radiate from its outer surface. $\endgroup$ – Richard Fry Apr 11 at 17:41
  • $\begingroup$ Very good, this agrees with what W8JI has long stated about 90° ground-plane radials. Also, shouldn't your comments be moved to your answer, so that everything is in one place (and searchable? ;-) $\endgroup$ – Mike Waters Apr 11 at 18:44
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    $\begingroup$ Thanks for the suggestion, which was followed. $\endgroup$ – Richard Fry Apr 11 at 19:53

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