Join Date: Nov 2000
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Re: CB question
If you want as close to a 1:1 SWR you have to tune the antenna. Length produces inductance, proximity to sheet metal produces capacitance. You need to find out whether your antenna is too short or too long. THe length of the feed cable has no effect on the actual SWR, just your perception of it, as you may find the cable length positions your radio at a node or a peak in the waves. Here's how to start. The high channels are higher in frequency, and consequently shorter in wavelength. The low channels are lower in frequency and longer in wavelenght. Antenna work best when they are some multiple fo the wavelenght. 1/4 wave whip is the simplest and that is what you are building. CB is 11 meter, roughly 33 feet or 1/4 wave is 8.25 feet. Roughly. As it's a multiple of the wavelenght, high frequencies use short antennas, and low frequencies use long antenna, generally. Test the SWR at both upper and lower extremes and compare the SWR. If the antenna is too short, the SWR will be lower at higher frequencies. Test the theory by either adding length to the antenna with an extension (the fiberglas whips have a wire down the middle) such as a spring base, or you can disconnect the connection at the base of hte antenna and add a coil of wire to the centre terminal. Use heavy copper wire wound around a large diameter approx toilet paper roll size. Wind a dozen widely spaced turns around a core like a piece of plastic pipe. connect one end to the base of the antenna centre, and connect an alligator clip to the centre of the coax cable on a short lead. Tune the antenna by positioning the clip closer or further from the end connected to the antenna. When you add more turns to the coil, it's like having a longer antenna, more inductance. You can either fabricate a longer antenna, or build a base coil having the same no. of turns and spacing as your tuning coil. You should be able to get it down to close to 1:1 by this method and greatly improve the efficiency of your antenna. You may also try positioning the antenna elsewhere. It needs a ground plane and works best on the centre of a metal roof to radiate equally in all directions, but will work in other locations but the proximity of the ground plane will affect the transmission pattern. You might want to try other locations from where you have it now.
First point though is to make sure you can proper connections on all points. Perhaps as other poster pointed out, a quality factory crimped cable might help. As I said, length plays not role other than fooling you into thinking it is better because the length of the cable happens to produce a position in a node of waves cancelling each other out. Also, excessively long cables produce losses in signal that both reduce output and diminish the return of the standing waves (so you are once again fooled into thinking the swr is lower). SWR has to do with reflected power back from the antenna. A poor antenna will radiate less and reflect back down the cable more signal. High swr means the antenna is working poorly and can even damage your equipment (not likely on CB;'s having low wattage).
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