# Aerial Gain

Introduction
Many radio amateurs have difficulty understanding the mechanism of “aerial gain”. But first, what is meant by the “Gain of an Aerial”? For a transmitting aerial it is its ability to concentrate the radiated power in a given direction so as to increase the signal strength at a receiver in that direction. For a receiving aerial it is the ability to capture more signal from a given direction than a simple aerial such as a dipole or a theoretical “isotropic radiator”, (i.e. an aerial which would radiate equally in all directions). In both cases the gain, (measured as a factor or in dBs), is always referenced to a dipole or to a theoretical isotropic aerial. (You cannot actually make an isotropic aerial but it is a useful concept). An aerial is both passive and reciprocal and its gain is the same on both transmission and reception. Gain is sometimes referred to as “Directivity”, but for the purposes of this article the terms are synonymous. “Gain” when applied to an aerial does not mean it actually amplifies like a transistor, but it behaves as if it were somewhat bigger. It is the purpose of this note to explain how it accomplishes this.

In order to understand the reasoning, it is necessary to accept a couple of essentials. Firstly, that radiated electric and magnetic fields can pass through each other without disturbing each other. Secondly, that the resultant field at any point is the sum or their constituents, taking into account the relative phases of the constituents.

Consider a plan view of the radiation from a single radiator, for example a vertical dipole. It radiates in all horizontal directions, (i.e. in the plane of the plan), equally. If, (at any instant of time as the waves spread out), successive positive peaks of the electric field are joined by “contour lines”, then a “snap-shot” representation of the dipole and its radiated waves would appear as in figure 1, where the spot marked “A” is the position of the dipole seen end-on from above. .

If now a second radiator, designated “B” is placed a short distance from A, and fed with an equal RF power in the same phase, (and provided the field from B does not affect the radiation from A and vice-versa), the combined fields will appear as in figure 2.

It can be seen that the only directions in which the fields add constructively are those marked “X” and “Y”. (Putting a metal screen a quarter of a wavelength to the left (say) of the dipoles will eliminate radiation in the “X” direction and give a single beam in direction “Y”). Very little radiation will occur in directions “P” and “Q” because the positive peaks from one aerial will fall near the negative peaks from the other. This principle may be applied to any number of equally fed radiators, and the result is known as a “broadside array”.

Gain on transmission.
Consider that, when RF power is fed to a single radiator, (A), the power received at a distant point is measured and used as a reference. Now consider that the same amount of power is fed to two radiators, (A and B above), so that the power radiated by each is 3dB down on that originally supplied to the single radiator. The electric field radiated by each will be 1/√2 times or 0.707 of the reference case because power is proportional to the square of the electric field.