Understanding antenna specifications.

So many different antennas, which one is the best?
Manufacturers and sales staff may throw a load of figures at you, but do you understand the meanings, or just get impressed by high numbers?
The following list is designed to help you work out how much of the specification is useful for you, and how much is just to impress the unsuspecting customer.

Bandwidth
The amount of the band or channels the antenna can cover without being retuned.
A higher figure is better.

dBd gain
This is a comparison between how much signal an antenna puts out in it's best direction, when compared to a dipole. Higher figures are better.
See also gain.

dBi gain
This is a comparison between how much signal an antenna puts out in it's best direction, when compared to an Isotropic Radiator. An Isotropic Radiator is a theoretic antenna, which would (if it existed) put out the power equally in all directions. Higher figures are better.
See also gain.

Frequency range
This is the band of frequencies that the antenna can be tuned to.

Gain
All antennas will send more signal in one direction than in others. The gain is a comparison between how much power is sent out in it's best direction, when compared to some standard.
Unless the figure gives a reference (what it is compared to), then it is useless.
Higher figures are better, but beware of manufacturers claims, specially if they use dB without a third letter (such as with dBd or dBi).

Impedance
The input impedance of the antenna. The antenna impedance should match the impedance of the input or output impedance of the equipment.

Length
This is usually the physical length of the antenna although, for wound types, some manufacturers may be referring to electrical length (how much wire is inside it).
The physical length is a trade off between best performance and practical situations. A bigger antenna will usually work better, but you cannot drive about with a 22 foot antenna on your car (or motorbike).
If you are limited with space, then smaller may just fit your needs. If you want the best performance, the bigger is better.
You should be aware that some manufacturers may quote the electrical length, to make the antenna sound big and impressive. Remember that, if it is performance you want, then the actual physical size is what you need to know - don't be fooled by one that is only one or two feet long but boasts a big electrical length, it's still only small and not as efficient.

Polarization
Whether the signal from the antenna will be vertical or horizontal. This will usually be the same as the way it is mounted.
Some types, such as a dipole, can be mounted either way.
The best reception or communication distance will be obtained when both transmitter and receiver antennas are mounted the same way.

Power rating
The maximum power that may be put through the antenna before damage may occur. This only applies for transmitter and transceiver antennas, not for those used only for reception.

SWR
The is important for transmitter antennas, and is the lowest SWR figure to expect from the antenna.
You should be aware that this would be in perfect conditions, with no nearby obstructions or buildings and on the frequency it is tuned to. You will find that an antenna may give a higher figure and that the SWR will be higher on some channels than on others, this is all normal.
A lower is figure is better, although it is not critical.

Wind survival
How well the antenna will stand up to strong winds.
Many cheaper antennas will bend and break in strong winds. It should also be remembered that this is just the antenna - whether it will stay up will also depend upon the mountings used and if they are fitted well.
Higher wind speed figures are better, although it should be noted that their idea of "survival" may just mean that it remains in one piece - even if bent over.

Requirements
These are the minimum requirements for antennas for the specified uses. Figures in red are absolute, get as close to this figure as possible. Figures in green are maximum figures, lower is better. Figures in black are minimum, higher or wider range is better.

Equipment	Type	Bandwidth	Frequency range	Impedance	Power rating	SWR
European CB	Omnnidirectional	450 KHz	26.9 MHz to 27.5 MHz	50 Ohms	10 Watts	1.5:1
UK CB	Omnnidirectional	1 MHz	26.9 MHz to 28 MHz	50 Ohms	5 Watts	1.5:1
American CB	Omnnidirectional	450 KHz	26.9 MHz to 27.5 MHz	50 Ohms	20 Watts	1.5:1
VHF broadcast	Omnnidirectional	20 MHz	88 MHz to 108 MHz	75 Ohms	N/A	N/A
UK TV	Directional	Dependent on your location. Ofcom list shows transmitter information, colour of antenna end cap identifies which group antenna it is.		75 Ohms	N/A	N/A