Understanding Antenna Gain - SARS


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Understanding Antenna Gain

Antenna Gain is 'the relative increase in radiation at the maximum point expressed as a value in decibels (dB) above a reference antenna'

Phew, let's try and decipher that.

The basic reference antenna is a ˝-wavelength dipole by which all other antennas are measured. The reference figure is known as 0dBd (zero decibel referenced to a dipole).
Sometimes the quarter wave ground plane is used as a reference, especially for mobile antennas, as it is equivalent to the quarter wave dipole.

Vertical antennas achieve gain by varying the wavelength and can have omni-directional radiation gains of up to 9dB. Figures beyond 9dB are impractical owing to the fact that the focusing is directly related to the length (in wavelengths) of the antenna. A quarter wave ground plane antenna has a radiation pattern that produces maximum gain at about 25 degrees and a half wave antenna drops that angle to 20 degrees, and the 5/8 wave antenna further drops that angle to 16 degrees. Increasing beyond 5/8 wave gives very high angles of radiation so we've reached the maximum gain at this point.

Some manufacturer’s state gain figures without giving the reference source from which their figures were calculated. (i.e. 3dB gain, etc.)
This can be deceiving as the figure could be obtained by referencing an isotropic antenna. The isotropic antenna is a mathematical representation of an imaginary antenna that radiates equally in all directions in a spherical pattern. This gain figure is expressed as dBi

Unfortunately dBi figures are approximately 2dB more than dBd figures for an identical antenna. This is because a ˝-wavelength dipole antenna has a gain of 2.14 dBi 
Therefore an antenna referenced as having a 3dBd gain by one manufacturer could be referenced as as having a 5dBi gain by another. Yet overall gain is the same.

So how does an antenna have gain?

Firstly, gain is a relative term. No power increase is actually produced by the antenna. Assuming no losses, power in equals power out.

To start with let us take a standard ˝-wavelength dipole and "suspend" it in free space (i.e. ignore all possible surroundings that could affect the antenna). The radiation pattern of this antenna is typically referred to as the "doughnut".
In order to achieve gain, the radiation pattern is focus into a more useful direction, typically achieved by flattening out the "doughnut" in a horizontal plane rather than towards the sky and into the ground.

Gain, or focusing, can be intensified further by focusing the radiation in only one direction.

If a reflector is placed next to a dipole, most of the energy that would have radiated in the direction of the reflector is now reflected back in the direction of the dipole. This makes all the energy appear in only one hemisphere and thus results in a doubling of radiated energy in this direction or a 3dB gain.

This radiated energy can be increased further with the use of "directors". Introducing more directors makes the angle of radiation smaller and narrower by packing all the radiation into one direction. By this method, gains as high as 20dB can be achieved.

Because you can never reflect 100% of the signal, some radiation will escape, this gives rise to the term Front-Back Ratio.
This is the ratio of the maximum directivity of an antenna to its directivity in the opposite direction.

To summarise.

Antennas do not create power. Assuming no losses, 50w in equals 50w out. The radiated RF power is simply focused into narrower patterns such that there appears to be more power coming from the antenna in the required direction.

An antenna with the effective radiated power of twice the input power would therefore have a gain of 10*log(2/1) = 3dBD.

Using 50 watts as an example:

0dB gain = 50w radiated power
3dB gain = 100w effective radiated power or 2 times
6dB gain = 200w effective radiated power or 4 times
9dB gain = 400w effective radiated power or 8 times

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