Antenna Gain, dBi, and Radiation Angle tidbits

 Antenna Gain, dBi, and Radiation Angle tidbits

I compare all antennas to a full size halfwave dipole if possible. It’s very hard to beat a dipole if it’s is reasonably high in terms of wavelength. I also like to use the term dBi for antenna gain.

There is a very good reason for doing this. First of all, for example, if you start with 100 watts at a point source it is very easy to calculate what signal strength you would have in any direction and at any distance. Although this omnidirectional   point source cannot exist in the real world, we can calculate what the signal would be if it did. That becomes our well defined reference point. It is also relatively easy to calculate what the signal strength would be from a dipole antenna in free space where it is unaffected by any reflections from the earth or anything else for that matter. Consequently we know with almost complete certainty what the gain of a dipole in free space is relative to that theoretical point source which we call an isotopic point source it isotopic antenna.  We also know the overall radiation pattern of such a dipole in free space. We know that the dipole has directivity. The gain off the broadside is 2.15 dBi or 2.15 dB stronger than from the isotopic point source.

Many people incorrectly state that gain of a dipole is always 2.15 dBi. The gain of a dipole is only 2.15 dBi in free space. The gain of a dipole near the ground is usually considerably more due to ground reflections. I have heard one well known top contester state that with a horizontal antenna near the earth you get 6 dB free ground gain! What he means is that the maximum gain of a dipole off its broadside is about 8 dBi and not 2.15 dBi. This is because at some point, well away from the antenna, the energy reflected from the ground will add in phase to the direct wave from the dipole and cause the signal to be reinforced or significantly stronger that it would have been with out the ground reflection. 

Since horizontally polarized waves are known to be reflected from ground with a phase shift of 180 degrees, it becomes a simple geometry problem to determine the peak radiation angle from a dipole at any height above ground. Indeed the maximum radiation angle or angles from a horizontal antenna depends only on the height above ground in terms of wavelength. While these calculations can be done by hand and were done by hand in the not too distant past, with the aid of computers we can instantly perform all such calculations to give us the gain if every possible direction from a dipole antenna. Such calculations are quite good for horizontal antennas. The same cannot be said for vertically polarized waves as from vertical antennas. Such calculations are very exact for “ perfectly conducting ground”.  However, due to the complex way vertically polarized waves react in contact with real ground, calculations, either by man or computer, are not so exact. This is particularly true at the low radiation angles ( below 10 degrees). It has been my experience that most vertical low angle gain figures are somewhat inflated. The one exception is where seawater is the actual ground in front of the vertical. When vertically polarized waves are reflected from earth at low angles the exact composition of the earth can and does cause great variation in the signals phase shift. The result is the direct wave and the ground reflected wave are not adding in phase at these low angles over real earth but are actually out of phase there by tending to cancel instead of adding in phase. 

Typically with a vertically polarized wave being reflected from ground there is an insignificant phase shift but at low angles, depending on the ground conductivity, there is usually a significant phase shift. We tend to use the image antenna theory to explain the difference in vertical and horizontal reflections from earth. With a horizontal antenna the image antenna is shown with a 180 degree phase shift while the vertical image antenna is not shown with a phase shift, the vertical antenna and its image are in phase, however that is only for perfectly conducting earth. It must be remembered that the concept of image antennas is only a tool to help understand what happens. In reality the wave that comes from the image antenna is really a reflected wave from the real antenna that only acts like it came from the direction of the image antenna. So with the vertical antenna reflected waves from all but the low angles are reflected from real ground with minimal phase shift just like they came from an image antenna in phase with the real ( vertical) antenna. However when the vertical waves strike the ground from angles of 10 degrees and below ( the exact angle depending on ground composition) there will be a phase shift that increases significantly as the angle decreases. For this reason most if not all verticals over real ground do not have the extreme low angle performance that is sometimes attributed to verticals which are modeled either in free space or over perfect ground. Seawater is the exception because the angle at which the phase begins to change is extremely low. This angle where the phase changes is commonly called the Brewster angle or Pseudo Brewster angle in most antenna books.