Brad,
Most cheaper antennas are really only half an antenna, the other half being the metal ground plane to which they are bolted, like the car roof or wing etc. A "Ground-independent" antenna is both halves together, so a relatively easy way to recognise such an antenna is that they tend to have a fat, elongated base unit, or have a chunky coil assembly half-way up. Any antenna that looks like a thin stick which ends in a small unobtrusive screw connector is almost certainly an 1/4 wave antenna, i.e. the half-an-antenna version. The 1/4 wave antennas rely on the metal body-work to work properly, the Ground-independent one's don't.
Take a look at the following URL for pictures and explanations of differing types of antenna, they explain it far better than I can;
http://www.mobileone.com.au/antennas/cb477.htmlThe Ground-independent antennas are good for bikes because there really isn't enough metalwork under a 1/4 wave antenna to effectively act as the 'other-half' so they can be horribly inefficient. A measure of efficiency of the antenna is called VSWR which stands for Voltage Standing Wave Ratio, and a VSWR of 1:1 it the absolute ideal, anything up to about 2:1 is acceptable but getting progressively closer to the limit, and anything over 2.5:1 will probably stuff the transmitter because it's a measure of how much power is reflected back into the output stage of the transmitter rather than going out of the antenna.
Hope this helps
