Elevation Radiation Patterns and their uses...

Changes in radiation characteristics at VHF frequencies and above happen quite slowly, so one pattern will be valid over quite a range of frequencies. What is more important is whether the antenna is half wave or a collinear array. Below VHF, however, radiation characteristics can and do vary quite considerably from frequency to frequency. For this reason it may be prudent to check what the radiation looks like at the frequency or frequencies you intend to communicate on.

Because an antenna radiates in three dimensions it is necessary to have two types of plot in order to represent the pattern of radiation for a given frequency: from the angle of elevation (vertical plane) looking side on, and the angle of azimuth (horizontal plane) looking from above. It is more important to find the gain at the elevation and azimuth angles of interest and to use that as a reference to compare other antennas against rather than maximum gain, which may be straight up in the air.

The Elevation pattern indicates the angle or angles at which the signal is propagated at different frequencies. With this type of plot you can determine what angles of elevation give the most gain, and the useful beamwidth of the lobe or lobes.

The maximum gain is shown as 0 dB. The useful area of gain can be estimated by measuring the beamwidth. The limits of the beamwidth are usually measured at minus 3dB, that is 3dB less than whatever the maximum gain of the lobe is. These are commonly referred to as the half power points or 3 dB down, as for every 3dB loss, the radiated power is effectively halved.

This example shows a slanted wire type which produces both a vertical and a horizontal pattern which combine to give a total radiation pattern.

The radiation beam may be primarily directed towards low angles favouring communications over longer distances, yet at certain frequencies lobes may develop that are advantageous to higher frequencies, or multiple lobes on the one frequency. Such antennas would be multi purpose, favouring long range communications on some frequencies and shorter distances on others.

As variation from frequency to frequency can be great, if the higher frequencies 9-30 MHz are used for very long distances, the radiation angle is usually lowered down to a minimum of 3 degrees to reduce the number of hops. Lower angles are impractical.

However, it is not always desirable to have very low Angles of Elevation at HF. When communicating over short to medium distances, narrow angles of refraction and higher angles of elevation are required. In this case it is sometimes better to have a horizontally polarised antenna. This is especially so when using high frequencies which are harder to bend than low frequencies, and therefore have wider angles of refraction.

While elevation plots are normally shown at 90 degrees, it is possible also to produce them at 0 degrees. This is especially useful where antennas are not omnidirectional, as can be seen from this end fed type where the directional bias become evident in the 0 degree elevation plot.