asymmetric shortened dipole

Question

It seems a resonant dipole can be shortened by either loading coils near the center feed point or capacitive "hats" at the ends (and still be resonant).

Say I run out of linear physical distance for a full-sized wire dipole by X.

What would be the difference (radiation pattern, radiation resistance, efficiency, SWR, etc.) if I use one of the above additions (loading coil or cap hat) to asymmetrically shorten only one side of the dipole by X, instead of symmetrically shortening both sides, each by X/2? Or how about one side short by X/3, the other by 2/3rds X?

(in all cases, "tweaking" the L or C additions to the shortened partition to optimize for the desired resonant frequency).

I the case of a vertical dipole, would it make a difference whether the upper half, or half closer to the ground was shortened? (in the extreme, replacing the bottom element with a large enough cap hat seems to be approaching being equivalent to a vertical over a ground plane).

Answer 1

We have a lot of available information and knowledge on a certain type of asymmetric shortened dipole, i.e. mobile antennas which are standing wave antennas. The vehicle body can be thought of as 1/2 of the dipole and the vertical part of the antenna as the other half. Since the legal height limit is something like 13.5 feet from the ground, mobile antennas are necessarily shortened for any wavelength above ~15 meters. What we know about such antennas is that the maximum radiation is associated with the point of maximum current on the standing wave antenna which is usually at the feedpoint. From mobile shootouts we know that loading coils are most efficient when located as far away from the feedpoint as feasible but not too close to a large top hat - that increases the radiation efficiency while lowering the feedpoint impedance. "The ARRL Antenna Book" has a chart for eight-foot base-loaded and center-loaded mobile antennas with the feedpoint impedance and radiation resistance listed. Dividing the radiation resistance by the feedpoint impedance gives an idea of radiation efficiency. I have the results of three 80m shootouts from the 1990s on my web page at:

http://www.w5dxp.com/shootout.htm

Please note that because of the self-resonance of loading coils, antennas with loading coils do not usually make good multi-band antennas although self-resonance is sometimes used as a trap function on the self-resonant frequency.