On the subject of the performance of short antennas

Stewart D. Personick, AB2EZ

Many discussions take place on the subject of the performance of antennas which are physically short compared to the wavelength of operation… i.e., less than ½ of a wavelength for balanced antennas, and less than ¼ of a wavelength for unbalanced antennas which use ground as a portion of the antenna.

In most of these discussions, there is an emphasis on the importance of using very low loss components to build the feed line, the antenna tuner, and the conductors that make up the radiating elements of the antenna. 

However, very few of these discussions recognize the role of the non-radiating electromagnetic modes of the total field produced by the antenna in determining the performance of the antenna as a radiator of useful power for communication.

As is well known, if the feed lines (e.g. open wire balanced line or coaxial cable) between the transmitter and the tuner (if there is a separate tuner) and between the tuner and the antenna have very low losses under the conditions of operation (typically a very high SWR on the feed line between the tuner and the antenna), then, by definition, very little of the transmitter’s output power will be lost as heat produced by the high currents and high voltages present in those feed lines.

As is also well known, if the tuner (if there is a separate tuner) or the transmitter’s output “tank” circuit are made of very low loss components (inductors and capacitors), one can ensure that power that is reflected by the antenna, back toward the transmitter, is re-directed back to the antenna, while keeping losses due to high circulating currents and high voltages to moderate levels.

Thus the combination of low loss feed lines and low loss tuner components can ensure that most of the power that is generated by the transmitter is coupled to the conductors of the antenna.

If the conductors of the antenna have sufficiently low resistance (taking into account the "skin effect" at the frequency of operation), then the currents in the antenna’s conductors will produce only moderate heat losses.

Thus, under the above conditions, most of the power produced by the transmitter will reach the antenna, and be radiated.

However, the total electromagnetic field leaving the antenna consists of two types of fields.

One portion of the total electromagnetic field that is produced by the antenna is made up of “propagating modes”, which propagate through space with the familiar 1/r reduction of field strength with distance. The particular propagating modes, which the antenna couples to, may or may not travel in the most desirable directions for communication… but they do, at least, propagate for long distances in whatever directions they travel.

The other portion of the total electromagnetic field that is produced by the antenna is made up of non-propagating “modes”, also called the “evanescent” field (made up of evanescent modes). The field strengths of these evanescent modes fall off exponentially with distance, r, from the antenna… unlike the propagating field, whose field strength falls off as 1/r.

Any antenna will couple into both propagating modes and evanescent modes… but antennas which are short (compared to a wavelength) couple much more strongly into non-propagating, evanescent, modes; and much less strongly into propagating modes.

Typically, the evanescent modes interact with nearby objects, such as the ground, buildings, trees, etc. The evanescent mode fields produce currents in these objects… which will be converted to heat unless the objects are perfect conductors. Evanescent modes can also produce magnetic fields in objects which also produce heat through hysteresis effects.

The shorter the antenna is compared to a wavelength, the larger the strength of the non-propagating, evanescent field near the antenna (the sum of the propagating and the non-propagating fields is often referred to as the “near field” of the antenna), compared to the strength of the propagating field near the antenna… and thus the larger the percentage of power that is converted to heat by losses associated with the evanescent modes.

In theory, if the evanescent fields do not interact with objects near the antenna to produce large amounts of loss (conversion of electromagnetic energy into heat)… then most of the power leaving the antenna can still be carried away by the propagating modes. However, keeping the heat losses associated with the evanescent mode fields to a moderate percentage of the total power leaving the antenna becomes more and more difficult as the size of the antenna becomes smaller compared to the wavelength of operation.

The losses associated with evanescent fields are a limiting factor in the performance of short (compared to a wavelength) antennas… and can also explain why the performance of a physically short antenna can be dramatically influenced by the nature of nearby objects… when compared to the influence of nearby objects on the performance of a dipole.