Currents at the base of an elevated, 5/8-wave, vertical ground-plane (GP) transmit antenna are lower than when that vertical conductor is 1/4-wavelength. Does this mean that the performance of the elevated radials is less important in producing optimal radiation from a 5/8-wave, elevated GP antenna?
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$\begingroup$ Currents are the same for all antennas with radials, for a certain transmitter power - in all cases, all the feed (braid) current goes into the radials. But compared to E fields in the far field, they're lower, so you can say that their influence is smaller. $\endgroup$– tomnexusOct 18, 2018 at 2:59
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$\begingroup$ RE: "Currents are the same for all antennas with radials, for a certain transmitter power - " — A NEC4.2 comparison of the r-f currents at the feedpoints of a 1/4-WL GP and a 5/8-WL GP both modeled in free space shows that for equal applied powers, the 5/8-WL GP has about 14% of the base current present in the 1/4-WL GP. The currents at the inputs of networks needed to match the (different) feedpoint Zs of those two GPs to a given Zo of a transmission line may be about the same, but not the currents at the feedpoints of those two GPs. $\endgroup$– Richard FryOct 18, 2018 at 11:16
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$\begingroup$ Interesting. 1) but the matching component for a 5/8 monopole is a series inductor, so the current is the same at feedpoint and at coax point (no parallel element). 2) I assume the 1/4 has no significant mismatch. 3) NEC usually uses a fixed voltage /applied E field source, which is not constant power; with an unmatched 5/8 the current (and power) would be some 5x smaller due to the -j270 ohms, iirc. It may have a constant power mode too, when it scales the currents to get a certain power. Could you check the total input power in the OUT file, and ideally the integrated radiated power too? $\endgroup$– tomnexusOct 18, 2018 at 12:22
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$\begingroup$ RE: "1) the matching component for a 5/8 monopole is a series inductor" — An inductor can offset the Xc at the base of a 5/8 GP, but won't match its ~ 91 Ω Rr to any (conventional) feedline Zo. "2) I assume the 1/4 has no significant mismatch" — NEC4.2 shows the feedpoint Z of my 1/4 GP as 23.1 +j6 Ω, so that wouldn't be a very good match to conventional source Zs. See the next comment to continue to item 3). $\endgroup$– Richard FryOct 18, 2018 at 13:24
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$\begingroup$ RE "3) NEC usually uses a fixed voltage /applied E field source, which is not constant power" — In my NEC software, setting the power applied to the model to a value other than zero (watts), which I did, supersedes the power produced when using only voltage or current to drive the model. $\endgroup$– Richard FryOct 18, 2018 at 13:25
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Elevated radials are required in order to increase/optimise the radiation possible from any such elevated "GP" monopole or whip antenna.
If the elevated radials are symmetrically arranged and lie in the horizontal plane, then their net far-field radiation is zero. However the r-f current at the common point of those radials is not zero — it is the same as the current that flows into the base of the vertical antenna conductor above that common point. All of the useful far-field radiation from such antennas is produced by the single vertical conductor, not by elevated horizontal radials just below it.
Currents at the common-point of the radials for a given, Z-matched r-f power at the feedpoint are smaller for such 1/2-wavelength and 5/8-wavelength monopole/whip antennas. But those currents cannot be zero if the antenna is to radiate anything at all.
Regardless of their absolute value, any percentage reduction in the sum of those radial currents due to the elevated radials being "sub-optimal" reduces the e-m radiation that antenna (alone) will produce, by that same percentage.