Gain of Antennas

It is difficult to talk about gain of antennas without pictures or plots. The only antenna that radiates equal power in all directions does not exist but its is used as a reference or baseline with which to compare real antennas.

Every antenna radiates its power in some pattern. More power in one or more directions and less power in other directions. Gain should be referenced in dBi. So one antenna may have a peak gain of 10 dBi and another may have a peak gain of 5 dBi. Which one is better? That will all depend on where you want your power to go. The one with the 10 dBi gain may radiate mostly straight up and be good for local communication on 40 meters during the day but have a low angle gain of negative 3 dBi.  The one with 5 dBi gain may radiate at a low angle and be great for long distance communication but very poor for local communication. Its gain of 5 dBi at a low angle means it will be 8 dB better than the antenna advertised with 10 dBi gain as the low angle gain is what is really important for long distance.  

The plots of antenna patterns and gain need to be considered when comparing or trying to choose an antenna.

The vertical is well know to have low angle radiation. That simply means that it radiated most of its power at a low angle. However the actual peak gain of a vertical is not much. Remember it radiates in all compass directions. Another antenna that has a peak gain at a high elevation angle may still have more gain than the vertical at low angles, but it will usually be directive in only one or two directions. 

In the classic case of the dipole vs the vertical, we have most verticals having zero dBi gain. Sometimes more but unless over seawater not much more than zero dBi gain. The low dipole has low gain at low angles where the vertical has its peak zero dBi gain. The vertical is best in that case, however as the dipole is raised above the ground its gain increases at the low angle and by the time the dipole is one half wavelength high it has probably 7 dB more gain than the single vertical. That gain is of course broadside to the dipole. Of the end of the dipole the gain drops maybe 5 dB or more if it is half wavelength high or higher. That makes the dipole off the end pretty close to the vertical for low angle radiation in that direction. In general any vertical array will have low angle gain and every time the number of elements in that array is doubled there can be an increase in gain of about 3 dB. So a two element vertical array may be reasonably considered to have 3 dBi gain, while the 4 Square array is near  

5 or 6 dBi gain.  In the case of the 4 square vertical array it definitely takes a dipole a halfway high to equal the gain and that is off the front of the dipole. Usually the 4 Square array is switched so that it covers four directions and in fact covers all directions of the compass quite good. In this case the 4 Square may only equal the dipole in two directions but in the directions off the end of the dipole the 4 Square should be 5 dB better! The dipole gain off the end is probably right at the zero dBi point while the 4 Square is at 5 or 6 dBi.

The above numbers are considered to be ball park for these antennas properly installed. Ground mounted verticals need to have good radial systems, the quality of the earth under and in front of them needs to be good, very good helps considerably, and dipoles need to be high in terms of wavelength. Ground under a dipole is not nearly as significant as with a vertical.