Everyone knows that a 1/2 λ dipole antenna has on it a standing wave of RF energy the voltage and current of which are about 90° out of phase with each other. The standing wave is circulating reactive stored energy present due to the fact that the antenna is a resonant system.

Apparently there is a departure of phase difference away from 90° which is the in-phase component of the standing wave responsible for radiation, the out of phase energy of the standing wave remains in the antenna.

At the ends of the antenna where there is a complete open circuit, how can the phase difference between voltage and current of the standing wave be anything else other than exactly 90° ? But there must be some non-reactive component otherwise there will be no radiation !

So, does the phase difference between voltage and current of the standing wave start off as exactly 90° at the outer ends of the elements, and then gradually change to a lesser value along the antenna elements with a minimum value at the current maximum at the feed points, in which case the standing wave must not be a perfect sine wave ? or is the phase difference the same everywhere on the antenna, and so the reflection at the ends must not be exact ?


1 Answer 1


The total current (according to physics) across any region around the antenna is the sum of the conduction current within the elements plus (according to Maxwell's equation) the displacement current in free space. The displacement current represents the part of the current that generates any RF radiation. If you use the total current (conduction plus displacement) both in and around the antenna elements, even around the very end tips, you will find that the phase difference between voltage and current is closer to 90 degrees than just looking a portion of the current near what looks like an open circuit.


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