Rob's web

Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - Radio Communication - Gain of antennas over ground


The question keeps arising about the quoted gains of antennas mounted over reflecting ground. At 144MHz and above, antenna gains are always quoted relative to a reference antenna in free space, and professional gain measurements take great pains to control or eliminate ground effects. But at HF (and also the 'border country' of 50 and 70MHz) the effect of ground cannot possibly be ignored.

At certain wave angles above the horizontal, the signal from any antenna placed over reflecting ground will be enhanced by the ground reflection. At other vertical angles the signal will be cancelled, and the effects of ground reflections will totally dominate the antenna's radiation pattern in the vertical plane. Fig 1 is a typical example, though the angles of the maxima and minima in the pattern will change with height.

Fig 1
Fig 1: Vertical-plane radiation pattern of a dipole one wavelength above groud. Maximum gain is almost 6dB over the same dipole in free space.

Up to 6dB of 'ground gain' is available at the best angles, so if you compare the gain of an antenna over ground with the standard reference antenna, a dipole in free space, you immediately pick up an extra 6dB of `free' gain! Unless you're aware of this, it can lead to absurdities such as "The gain of a dipole over ground is 6dBd." More importantly, it can lead some antenna manufacturers to inflate the gains of their products. For example, a 3-element HF yagi may have a true gain of about 5dBd (dB over a dipole, both antennas considered as being in free space), but the manufacturer adds another 6dB to account for the ground reflection, inflating the 'catalog gain' to 11 dBd.

The only way to avoid falling into this trap is to know your antennas, and have some idea of the gain you can expect from other antennas of a similar type. I keep saying this, but evidently can't repeat it often enough: Antenna technology is sufficiently mature that it's no longer possible for new designs to produce a lot more gain than the competition. Comparing antennas of similar size and appearance, you're most unlikely to see more than 1-2dB of difference in real gain - and that's an outside estimate. Anything higher than that is either over-enthusiasm or calculated commercial hype.