Antenna dB gain.

The gain of an antenna is a measure of its performance as compared to a standard antenna. A gain figure is meaningless without the reference being stated. Common references are:

	dBi    -	Isotropic radiator. A theoretical antenna, which radiates signal equally in all directions (or would if it existed).
	dBd    -	Reference Dipole.

As an antenna cannot transmit more power than you put in, this gain means more power in one direction at the expense of less power in other directions.

Directivity
This is the ability of an antenna to direct more signal in one direction. The diagram on the right compares the signal from omni-directional and directional types, as viewed from above. While the directional type will transmit further, it can be seen that there are places it will not get your signal to.
All practical antennas are directional to some degree. Several factors can affect the directivity - including design, height above ground, and other nearby objects.
This kind of directivity is not always desirable. Although it is perfect for applications such as TV, where you know which direction the signal will come from, general communications could come from different directions.
As well as horizontal directivity, an antenna will exhibit some degree of vertical directivity. This may be referred to as the radiation angle, vertical angle or elevation.
The diagram on the left shows a view from the side, displaying how increasing the vertical directivity can improve the gain. As the intention is to communicate with or receive stations on earth, any upward signal is wasted. So it follows that the lower radiation pattern (light grey in the diagram) is far better.

Directional Beams
A directional beam is designed to give a large degree of horizontal directivity. This type is useful for directing the signal where you need it, but can also be used to reject unwanted signals or noise before they reach your receiver.
As well as dBi or dBd gain, a directional beam will also have a front to back ratio which is expressed as a dB figure. This is simply a comparison between the amount of signal transmitted in the desired direction, and that going out the back.

dB to gain conversion chart.

dB		power gain		dB		power gain		dB		power gain
1	=	1.26		3.5	=	2.24		5.5	=	3.55
1.5	=	1.41		4	=	2.5		6	=	3.98
2	=	1.58		4.5	=	2.82		6.5	=	4.46
2.5	=	1.78		4.77	=	3		7	=	5.01
3	=	2		5	=	3.16		8	=	6.3

Remember that, as the dB figure is a comparison, it must include details of what it is better than. Also remember that the antenna only puts out what goes in, so that extra power is in a certain direction - there will be a drop in some other direction.

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