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.

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.

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		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.

Over the years, many antenna have been available in the shops. They often give a gain figure, which can vary from close to zero to double figures. But, just how reliable are these figures and can you rely on them to decide which one will perform best for you?

There is a saying regarding "special deals" that applies equally to manufacturers claims...
  If it seems to good to be true, it probably is.
There can be no doubt that some of these figures look quite incredible, such as 18 Inch antennas claiming a high dB gain, but these are not exactly lies. They are calculated and written in ways that may lead non-technical people to think they mean more than they really do.
It can be a bit of trickery, smoke and mirrors, spin or simply being economic with the truth - it's not only what they tell you, it's what they don't tell you that makes the difference.

Before calling dB figures false or accusing a company of lying, consider whether you can prove it. Unless a company does not have the finances to obtain real figures, there is no reason to lie. With a few tricks they can manipulate the truth, make the figures look better.
I suppose it's no different to companies adding colouring to food and drink in order to make it look better. Those additives do nothing to enhance the product, just your perception of it. Marketing people know how you will judge a product, their job is to do whatever it takes to improve sales.
If the target customer judges a product purely on it's gain figure, then that has to be improved. After years of designing antennas, it cannot be easy to improve on the true gain already achieved by existing product.

What effects gain figures?

The radiation pattern and gain is greatly changed by the ground. A lossy ground will lose signal and gain, costing you in distance obtained. On a vehicle you may well see a difference between signal with different mounts.
If, as with most mobile types, the antenna is designed to be mounted on a ground plane the size of that ground and the position of the antenna on it can make a big difference.

A high SWR can have an effect on the radiation pattern. It's not just the amount of reflected signal, but what effects that signal can have on the system.

The radiation pattern changes with the height above ground. Unless you fit the antenna at the same height as they did, then you will get a different result.
The picture on the right shows two radiation patterns, one in light grey and one dark grey. These are both patterns from the same antenna, the difference being the height above ground that this antenna is mounted at.

Any object near to the antenna will have an effect on performance. Such objects can absorb or reflect signal. In the real World, you are usually unable to avoid such obstructions.

Gain figures are dependant upon many variables. Unless you match the test conditions, you cannot hope to get the same results as the manufacturer claims.
However, some figures seem to beat the laws of nature. But there are tricks that can be used to get these figures...
  So, now that you have seen how your setup may change your gain, let's get on to ways that the manufacturer may have got the figures higher than you will ever see in the real World. There are many ways to get high gain figures on paper, even if that antenna will perform poorly in the real World.
This can involve several methods...

The exact performance depends upon the conditions at the time. As stated on the previous page, many things can affect the performance.
The height above ground is a major factor, as it has a big effect on the radiation pattern. At certain heights an antenna will radiate more power upwards, while at other heights it may increase the power sent across the ground.
Arranging the perfect test conditions is a great way to get the best figure, maybe selecting the conditions which favor your design while working against the comparison type, even if those conditions will not be matched in use.

As gain is a comparison, a figure must state what the antenna has been compared to. If an antenna claims 3dB gain, that's twice the power - but twice as much as what? For all you know they may have compared it to having no antenna or using a dummy load.
If they state dBi or dBd, then they have told you what they compared it to. However, dB on it's own is meaningless. They could be comparing to anything, whatever gives them the highest figure to put on their sales literature or packaging.
Never pay too much attention to any gain figure that does not tell you what they compared to, it tells you nothing.

The common standards are dBi and dBd, but they are not the same. One antenna specification may quote gain as dBi, another as dBd.
To compare a dBi to a dBd figure, simply subtract 1.64 from the dBi figure.

Gain is measured in the direction of maximum signal for the test antenna. It is important to note that this is not necessarily the direction which works best in the real World. It is purely about which direction that model favors, whether or not that is useful in real use.
Consider the diagram on the right. antenna 'a' clearly has the higher gain. However, considering where your contact is (also on planet Earth), is that the best one for the job?
If an antenna puts out most of it's signal towards the sky, it is of very little use to you but it could still have an impressive gain figure.