# Are Elevated Radials Necessary for Elevated, 5/8-wave “Ground-Plane” Antennas?

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?

• 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. – tomnexus Oct 18 '18 at 2:59
• 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. – Richard Fry Oct 18 '18 at 11:16
• 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? – tomnexus Oct 18 '18 at 12:22
• 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). – Richard Fry Oct 18 '18 at 13:24
• 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. – Richard Fry Oct 18 '18 at 13:25