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If you put a vertical 1/4 wavelength antenna on a pole of about 15 feet and tie the base of the pole to a massive grid of radials and the ground itself how does it affect the radiation pattern and the feed point resistance? It is like an infinitely wide ground plane but the current in the lower pipe element supplements the field of the upper.

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  • $\begingroup$ At what frequency or wavelength is this? It all depends if the 15 feet is a small fraction of a wavelength or not. $\endgroup$
    – tomnexus
    Sep 5, 2022 at 3:30
  • $\begingroup$ How is the feedpoint (coax shield etc.) connected to ground? $\endgroup$
    – hotpaw2
    Sep 5, 2022 at 11:25
  • $\begingroup$ antenna is a whip for 14mhz (20meters) and the coax comes from it at a 90 degrees angle. It is essentialy a vertical dipole, $\endgroup$
    – david
    Sep 6, 2022 at 16:39
  • $\begingroup$ A 1/4-wavelength monopole antenna just over a ground plane creates a mirror image of the monopole and has the effect of creating a virtual 1/2-wavelength dipole. If you put the monopole well above the ground plane the virtual dipole is no longer "Hertzian" and the antenna pattern will have a large number of grating lobes with lots of nulls and peaks. I'll see if I can find a more rigorous treatment. $\endgroup$
    – AG5CI
    Sep 24, 2022 at 15:35

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The pattern of a 1/4 wave vertical monopole above perfect ground is the same as that of a 1/2 wave vertical dipole, so the pattern of a 1/4 wave+15' vertical monopole above perfect ground is the same as that of a 1/2 wave+30' vertical dipole — for 20 meters this will be about 0.93 wavelengths long, which is theoretically beneficial, since the gain of the main lobe (the one towards the horizon, for a vertical dipole) increases with length between 0.5 and 1.25 wavelengths. It's not much of an increase, though, and see below for a caveat. For bands shorter than 15m, the 15' of extra length makes the equivalent dipole more than 1.25 lambda, and so you start to get additional high-angle lobes that are probably unwanted.

The feedpoint location doesn't matter to the pattern, but it does matter to the impedance. Modeling with NEC2 above perfect ground, the impedance comes out to a rather unfriendly 1100-j2200 ohms, varying a bit across the band. Replacing perfect ground with NEC fast ground and adding a field of 8 1/4 wave radials does little to change that. The best result, matching-wise, comes from using real ground and having no radial field, in which case we have a vertical OCFD touching (or almost touching) the ground. If we allow the antenna to connect to ground (using ground rods, say) we get an impedance of about 360±j20 ohms, which could be matched to 50 ohms fairly easily (and in fact has a pretty nice SWR wrt 450 ohm ladder line), but has a calculated efficiency of only about 10%. If we elevate the whole thing by 10cm and disconnect it from the ground, we get an impedance of about 50-j475 ohms and the efficiency increases to 25%. But that still falls short of the performance of a quarter-wave vert fed at the ground.

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  • $\begingroup$ Great simulation thanks - it matches my instinctive feeling, and some experience, that adding a (almost) quarter-wave support to a monopole puts the feed at the worst possible position. The impedance "looking down" is very high, as you're at the tip of a resonant monopole already, so it's very difficult to feed the quarter-wave above. $\endgroup$
    – tomnexus
    Sep 8, 2022 at 16:48
  • $\begingroup$ "The pattern of a 1/4 wave vertical monopole above perfect ground is the same as that of a 1/2 wave vertical dipole ..." - I think the monopole needs to be very close to the ground plane for this to be true. When it gets fed far above the ground plane, the mirror image of the monopole gets displaced and the resulting antenna no longer behaves like a half-wave dipole. $\endgroup$
    – AG5CI
    Sep 24, 2022 at 15:41
  • $\begingroup$ @AG5CI yes. Read the rest of the sentence. :) $\endgroup$ Sep 24, 2022 at 16:31

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