The Antenna.

Antennas.
This must be a special CB antenna - do NOT use an antenna that was not designed for use on CB.
The type you use is one of the most important choices to make when buying a CB system. The reason for this is that the radio itself is limited by technical and legal restrictions and is usually designed to work to those restrictions - so they all perform about the same.
The antenna, however, is very different. Although there are legal restrictions on size and certain spec, they may be designed for different requirements.
If you have limited money, save on the radio and spend on the antenna.

Size.
In the UK, the maximum size is restricted to 1/2 wave (about 18 foot / 5.46Metres) for a dipole type or 5/8 wave (about 22 foot / 6.8Metres) for a monopole type. The width is also restricted - 55mm maximum width. Within these limits, there are a wide range of different sizes available.

The size will affect performance - take no notice of a manufacturers claim that their 1 foot wonder has a gain of 5 or 6dB. A small antenna can be useful for limited space and will be less obtrusive, but will not perform as well as the larger ones.

Gain.
Each antenna performs differently. They can only put out only the power you put in, but some may be more efficient and some may concentrate the signal more in certain directions than in others.

The amount of signal it puts out in it's best direction, compared with a reference (usually a standard type of antenna) is the gain. The best way to make a gain in this way, is to limit the amount of signal that goes upwards, and direct more of it horizontally, across the face of the earth.

Be careful of the gain claims made by the manufacturer. They rarely make it clear what the figure is referenced to - what it is that dB better than. Also, they do not state the test conditions for each antenna (their one and the one they compared it with). Manufacturers figure are not to be relied on too much.

Directivity.
Antennas can be made to send more signal in one direction than in others. This can be either vertical or horizontal directivity.
All antennas will be directional to a certain degree.

Antenna tuning.
All antennas should be tuned for the frequency they are going to be used on. As CB uses different frequencies in different countries, they will not usually come ready tuned.
Tuning usually just involves adjusting the length and checking the tuning with a SWR meter.