I Might Just Know What I'm Talking About
Join Date: Dec 2006
Location: S. CA
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Re: C.B. radio antennas
What Babypoo says is correct....if you have the bucks, pays someone to set it up if your electrical skills are of the newbie skill.
But to add to the confusion....a few points of interest....
Antenna are designed by wavelength. The wavelength is the distance the signal travels in one cycle....for the CB frequency range, this would be around 39'....as you can see, that would be kinda long for an antenna...hence, most antenna are in fractions of a wavelength with 1/4 being the most common. This would mean a 10' antenna.
As most of us know, a 10' antenna is still too long...that is where we get into fully loaded, top loaded, bottom loaded and middle loaded antenna. Basically, a coil of wire is added to the antenna to make the CB 'think' the length is longer than it is. It keeps the impedance correct but the effective propagation are...or in other words...effeciency...
I think the best type is the fully loaded....you can recognize it by the wire that is wound around the shaft (typically fiberglass).
Next is the propagation pattern. As noted above, the jeep body plays a very important part of the ground plane for the CB. The best analogy I can give is comparing it to jumping into the water...if you do it from a small boat, you don't get much distance because the boat pushes away from you...but if you do it from land, you can jump further. Where you place the antenna dictates the pattern. If you put it on the right rear corner, then a majority of the energy is projected in a direction from the antenna towards the left front. Basically, the propagation pattern looks like an long oval. If you place the antenna on the roof of the jeep, the pattern takes on a more round shape.
One point (and we will most likely hear exceptions to this)...the length of the coax from the CB to the antenna does not make a significant effect on the SWR (Standing Wave Ratio)
SWR...the effective length of the antenna and the frequency being used determines this. In a perfect world, if the length of the antenna is exactly a multiple of the wavelength (1/4, 1/2, etc), then all energy is being sent to the antenna and there is no reflected waves...which is a ratio of 1:1. Change the length of the antenna and the SWR starts going up (too complicated to explain here). Hence, when you tune the antenna it needs to be done on the middle freq (chan 20). Yes, this means your SWR goes up at chan 1 and 40. Not much you can do about it unless you have a variable antenna.
So....have I confused you enough? And I didn't even talk about characteristic impedance, mw or watts.