... Antenna Gain on trial
When antennas are advertised figures are often given for the amount of gain they exhibit. Unfortunately there is no universal format for specifying performance gain. Suppliers and manufacturers often quote this in different ways which makes it difficult to make comparisons. You will find: unity gain, gain relative to a dipole, dBi. dBd or just plain dB! The trouble is you get very different figures in each case.
The problem with choosing the dipole as a standard for measuring gain is that it is a real antenna and as such will vary slightly from antenna to antenna, depending on its design and what it is made from and how made. Choosing a theoretical model known as the Isotropic Radiator as a standard means that any antenna can be measured against the standard and give you results you can compare with reasonable confidence.
The hypothetical isotropic radiator exists only in free space and radiates uniformly in all directions simultaneously. It is a sphere of radiation surrounding the antenna. Its gain is defined as 0 dBi (decibels over isotropic).
The 1/2 wave dipole in free space has this donut shaped with deep nulls off the ends of the wire. It is said to have a gain of 2.15 dBi in its favoured directions. Gain measured in dBd refers to this. Therefore 2.15 dBi is normally added to dBd for purposes of comparison (eg 2.85dBd = 5dBi).
Right: Circular isotropic, dipole figure 8 and directional patterns in free space superimposed, showing the different gains in relation to one another.
Antennas can be modelled in free space, and over perfect ground or real ground. Free space is a term used to model antennas in a place where the ground does not exist in any form whatsoever. Real ground can be any type of ground from the earth itself, which varies greatly in conductivity, to the sea or river water and man-made earth mats and elevated ground planes.
Obviously, if you want to get a realistic comparison, neither of these will do. Free space, although providing a common ground, will give totally unrealistic performance expectations, and, with antennas modelled on real ground, comparison is meaningless unless modelled on the exact same ground.
Antenna gain measured in dBi over perfect ground, an agreed standard ground with defined characteristics, is the most useful method for making meaningful comparisons in antenna performance providing a theoretical baseline so comparisons can be made. However, if accurate predictions are required it will be necessary to plot patterns over real ground.
Because antenna location has a critical influence on performance, height above ground, proximity to other objects, type of ground and surrounding terrain all come into play. Naturally, the performance of vertical antennas is more affected, especially ¼ wave grounded antennas, common at HF frequencies. Here the ground plane acts like a mirror reflecting the quarter wave above to make a total ½ wave.
If you model a vertical 1/2 wave dipole over perfect ground, at some angles the theoretical gain is increased to +5dBi, due to a phenomenon known as ground-reflection gain. Direct and reflected waves at favoured elevation angles add together in phase. Over real ground, however, not all of this translates into actual ground reflection. Some will be absorbed, the amount depending primarily on the type of ground and, to a lesser extend on the degree of shift in phase.
Waves radiated from an angle below the horizon are reflected from the ground like light reflected from a mirror at the same angle that they hit the ground. The reflected waves combine with the direct waves in different ways influenced by the orientation of the antenna with respect to the ground, the height of the antenna and the critical ground characteristics.
At one extreme, when the direct and reflected waves are exactly in phase (ie the maximum field strengths of both waves are reached at the same time at the same point in space, and the directions of the fields are the same) the field strengths add together (ie are doubled). At the other extreme, when the waves are totally out of phase (ie fields are maximum at the same instance and directions are opposite at the same spot) field strengths subtract from one another. Then there are all the values in between.
In a vertical antenna the currents of the direct and reflected waves are in phase with currents flowing in the same direction with the top ends positively charged. In a sloping antenna currents also flow in the same direction but are subject to differing effects depending on the tilt of the antenna. In a horizontal antenna the currents flow in opposite directions, ie 180( out of phase
How does this affect radiation patterns?
Often the low angle radiation predicted for vertical antennas over perfect earth does not occur over real ground for the following reason.
When the radiation is close to the horizon, the paths of the two waves in free space are nearly the same. The field strengths add together producing maximum radiation at 0 degrees. However over real earth the amplitude and phase are changed on reflection.
Where the phase shift is less than 90 degrees, the field strengths add together and some gain is achieved. When the waves are as much as 90 degrees out of phase some degree of cancellation becomes evident. However at 180 degrees out of phase the waves cancel out completely so that no signals can be transmitted or received at this angle. The angle at which cancellation begins to occur varies mainly due to the conductivity of the ground itself.
Thus, the type of soil will affect both the amount of gain and the degree of low angle radiation. The following radiation pattern will give you an idea of how much the elevation pattern of a vertical can be affected.
While the radiation patterns of horizontal antennas will be affected similarly by soil conductivity, the phase angle does not depart greatly from 0( so that low angle radiation remains unaffected.
In the case of horizontal antennas near perfect ground, if you change the height of the antenna from 1/4 to 1/2 wave, the high angle radiation is affected quite considerably.
However, over real earth the high angle nulls are filled in to some degree. This is because over real ground the reflected waves that would otherwise cancel out the high angle direct waves are attenuated to a significant degree by ground absorption.
By way of conclusion of what is a highly complex topic, we would offer the following suggestions. If you are looking to compare the performance of two antennas, dBi in free space is the fairest and most accurate method. If you need to get a better idea of antenna radiation performance, you will need to look at patterns over real ground (average/perfect earth). If your requirement is such that you need to predict what will occur at your site, only patterns over your ground will suffice.