I am not a radio guru but antenna are different lengths for different freqs.
You're absolutely right. The antenna
shaft itself is used for
TRANSMITTING the 4 watts of RF energy that a stock CB radio is regulated to by the FCC. On a GROUND PLANE antenna, the bike (frame, forks, fenders, bars, etc.) is used to
receive the signal and thus must have a good attachment to the frame. In order to effectively transmit, the SWR (Standing Wave Ratio) must be matched by varying the length of the antenna. This is adjusted with a SWR meter hooked into the antenna system where the antenna attaches to the radio. After calibration you should have as close to a 1:1 SWR (yeah, right...maybe a 1.5:1 is the best you can hope for) as possible on channel 20. It will change as you change frequencies (channels). Transmit with your radio mis-matched to your antenna and
you will blow the RF finals out of the radio.
If you do any TRANSMITTING with your radio, I highly recommend you get with a CB shop and have them adjust your SWR to a proper match.
Antenna's have come a LONG way from the 102" twangy-twangy redneck stainless steel whip that I used to drive around with on my 68 LeMans. Now, because of base coil loading, you can use some relatively short antennas and obtain as good a DB gain as with the old 102" whips. Wilson and Antenna Specialists makes a good antenna, as does Shakespeare. There are even NGP (Non-Ground Plane) antennas that can be mounted on fiberglass and don't require a chassis ground.
Modifying your antenna can do some serious eventual damage to your radio if you don't know what you're doing. Radios aren't cheap!