- Prediction Patterns for HF Horizontals

Predicting the theoretical radiation characteristic of an HF antenna is very useful for determining how actual performance will be...

HF wire antennas when erected horizontally exhibit a certain radiation pattern irrespective of length. This pattern is dependent on the height the antenna is erected above ground. We have already looked into the affects of erecting wire antennas at lower heights than normally indicated to give high angle NVIS (near vertical incidence skywave) close-in communications. Following on from this it is also useful to know what happens at the other end of the scale. Horizontal antennas exhibit radiation characteristics in the same way as HF vertical whip antennas.

These elevation patterns can be taken as representative of all horizontal antennas, as pattern development follows the same progression as the height above ground is raised. This is because, by raising the height, the antenna operates at a different wavelength. The following are indicative patterns for horizontal antennas erected over average ground at from 1/8 to 2 wavelengths high.

As can be seen, antennas operating at 1/8 wavelength and 1/4 wavelength exhibit high angle radiation which is very suitable for NVIS communications. From 3/8 wavelength the higher angles are gradually lost as two lobes begin to develop.

At a half wavelength these lobes are at around 30 degrees favouring medium to longer distances. At 5/8 wavelength to 1 wavelength, the two lobes are now at around 15-20 degrees favouring long distances while a third lobe develops providing high angles for short distances. At 1 wavelength, this high angle lobe then divides to provide two lobes at 50 degrees for medium distances as well as the long distance lobes. At this point the pattern breaks down into more lobes.

This typical elevation pattern can be seen in the Moonraker type FD230 half wave dipole antenna when erected horizontally.

Generally broadband wire dipole style antennas, when mounted at practical heights above ground, will exhibit higher radiation angles at the lower frequencies, ideal for medium to short distance communications. Erected horizontally at a height of about 10-15m (33-50ft), the radiation is omnidirectional up to around 10 MHz but above this it becomes increasingly directional, providing gain in favoured directions as the number of lobes grows.

By lowering the height of the wire above ground, more omnidirectional radiation can be achieved at higher frequencies. Erected in an inverted V configuration, the effective height of the wire is brought closer to ground and the radiation is essentially omnidirectional throughout the band. Tilting the wire gives horizontal and vertical patterns as well as both high angle and vertical radiation at lower angles. This makes them good for very close in communications via the sky wave, minimising the skip distance, as well as for medium distances.

Of course, the effect of slanting the wire applies to all sloping wire antennas, not just to inverted Vs. Slanted wires give both high angle radiation, permitting short distance communications with minimum or no skip distance, and vertical radiation at lower angles for longer distances. These characteristics can be exploited to effect, as in the type HFB D/S .

In addition, when a wire (or wires in parallel) are tilted in one direction, an unsymmetrical pattern favouring one particular direction can occur, depending on the degree of tilt.

The sloping triangle type is designed to give directional low angle radiation at high frequencies. Systems like the broadband HFB S/T provide omnidirectional radiation below 8 MHz, above which the signal becomes increasingly more directional with low angle radiation characteristics.

The position of the antenna feed will also influence the radiation characteristics. If the antenna is end fed rather than centre fed (as above), it acquires a directional bias. In an end fed inverted V antenna , the bias will be in the direction of the feed point from 2-8 MHz. Above this the bias moves towards the opposite direction.

Vertical whips may sometimes be able to be lowered to a horizontal position, as our type 23L/D . Radiation characteristics will then depend on height above ground, which may consist of a metal building roof/ground plane, metal deck or the ground /sea.

The advantages of power gain, however, do not become great compared with the 1/2 l dipole until antenna length (in terms of wavelength) is really long.

Long wire antenna theoretical increase in gain
from added length over a half wave dipole

These properties can be best utilised when the antenna is end fed, providing gain in a favoured direction.

Radiation patterns are usually complex. When the wire is several wavelengths long, a hollow cone of radiation forms about it in free space generated by rotating the wire on its axis. Fields combine to form an intense main lobe which grows in intensity and becomes sharper in all planes as wire length is increased. As a result, the angle at which maximum radiation occurs becomes smaller favouring longer distance communications.

This quality can be exploited to good effect using the Moonraker type OV220 . At the same time a series of minor lobes develops and grows in number as antenna length is increased, thus decreasing the intensity of the main lobe. While this tends to make the long wire antenna less directive, the possibility of communicating in other directions arises, making it a good all rounder. Thus, while arrays may be more directive, the long wire antenna will achieve both high gain in the direction of the main lobe and similar gains to a half wave dipole in other directions.