juniorXJ said:
i know this thread is kinda old but this needs to be known. if that mount is bolted to the plastic tail light you arent getting any ground (seeing how plastic is an isulator.) an antenna system needs to be grounded to the body of the vehicle. the metal of the body is half the antenna system. its called a ground plane. the RF travels along the ground plane. im sure if WheelinJR could chime in on this he could explain it better than i. seein how he has his ticket. and probably has a better understanding of this sort.
the antenna system needs the ground to achieve the proper Ohm load of 50 Ohms. without that ground it basicly becomes an open curcit. the SWR (Standing Wave Ratio) is a measure of how effitiantly your antenna, coax, and radio work together. the radio wants to see 50 Ohms, the coax is and wants to give the radio 50 Ohms. the antenna is adjustable (all of them) some easier than others. all mobile antennas for CB, also refered to as 11 meter (physical wavelenght). are 108", or 1/4 wavelenth. a full wave beeing 36 feet. for the antenna to resonate properly it needs to be 1/2, 5/8, 3/4, or full wave. the groundplane of the vehicle is what gives you the other 1/4 wave to be a 1/2 wave. anyway i confused myself, again. fire away with the questions.
You'll still get a ground through the screw - but it won't be as effective as a direct ground (also, he's using the OEM "oxided" screw, switching to a galvanised/Cd-plated screw would also be an improvement.)
You also don't "get power" from your antenna - it's a passive device. However, the description of SWR tuning is spot-on - if it's not set up correctly, you transmit and receive will both suck mightily. Most production CB antennas need to be trimmed a bit for the specific application - which is where the SWR meter comes into play (the closer you can get to a radiating element with 1/4, 1/2, or full wavelength "length" (sometimes, 1/8-wave will serve - but only for HF/LF radio.) In this case, the average wavelength of the CB radio signal is 11 metres. Half of that is 5.5 metres, and one-fourth of that is 2.25 metres (~36.2 feet, ~18.1 feet, and ~9 feet respectively. If you could get it to work, then 4.5 feet, or 1/8-wave, could also happen.)
Note that I said "effective radiating length" - this isn't an absolute measurement of the radiating element. The ground plane has a good deal to do with effective radiating length - so once your antenna is set up, you may as well replace it if you're going to move it to another vehicle, or even to a drastically different location on the same vehicle (if you want it to stay tuned well. Roof to roof would be a minimal change, but could be affected by the change in vehicle mass, roofline surface area, ...) That's why antennae are tuned using SWR as a standard - since you'll need a different length of radiating element for a roof-mount as for a side-mount, as for a hood-mount (akin to a BFM/BAM antenna.)
The antenna has rather more to do with clean radiation than the transmitter itself - which is why antenna tuning is so important. You can make a cheap radio sound much better when the antenna is set up properly, and an expensive radio can sound a good deal cheaper if the antenna is off...