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______________________________________________ ______________________________________________ AN OLD MYTH REVIVED? _______________________________________________
There are a number of myths that are widely held among radio communications hobbyists – and amateur radio is no less infested with some of these myths than others (CB, for example). Twenty-five years ago I worked in a CB shop in Virginia, and we kept hearing one old saw over and over again: you can ‘cut your coax to reduce the VSWR to 1’ (actually, they meant ‘1:1’ but routinely called it ‘1’). Hoards of CBers have ‘cut the coax’ and watched the VSWR reduce to 1:1, so they cannot be talked out of the error. I even know of one shop that kept 30 cm lengths of coaxial cable, with connectors on both ends, so they could insert them into the line at the transmitter in order to find the correct length that would reduce the VSWR to 1:1. What actually happens in that case is a measurement artifact that makes it appear to be true. Of course, hams are superior to CBers and so do not believe that error, right? I would like to think so; but having been in both the CB and the amateur worlds, and ‘Elmered’ (mentored) more than a few CBers studying for amateur licenses, I have to admit that at least as many amateurs believe the ‘cut the coax’ error as CBers (sorry, fellows, but that’s my observation). The only really proper way to reduce the VSWR to 1:1 is to tune the antenna to resonance and then match the impedance. For a center-fed halfwavelength dipole, or a bottom-fed quarter-wavelength vertical, the proper way to resonate the antenna is to adjust its length to the correct point. The formulas in the books and magazines only give approximate lengths – the real length is found from experimentation on the particular antenna after it is installed. Even commercial antennas are adjusted this way. On certain CB mobile antennas, for example, this trick is done by raising (or lowering) the radiator while watching the VSWR meter. On amateur antennas similar tuning procedures are used. Even when the resonant point is found, the feedpoint impedance may not be a good match to the transmission line. A VSWR will result in that case. The impedance matching should be done between the far end of the transmission line (i.e. away from the receiver or rig) at the feedpoint of the antenna. Antenna tuners intended for strictly coaxial cable are little more than line flatteners. They do not really ‘tune’ the antenna, but rather they reduce the VSWR looking into the transmission line so that the transmitter will work properly. If the antenna tuner is not a high-pass filter (as some are), then it will also provide some harmonic attenuation. An approach used by many amateurs (including myself) is to connect an antenna-matching unit (tuner) at the output of the transmitter. For my Kenwood TS-430, I use either a Heath SA-2060A or an MFJ Differential Tuner to ‘tune-out’ the VSWR presented by my Hustler 4BTV and 23 m of coaxial cable. But I do not even pretend to be tuning the antenna. The
34 ANTENNA TOOLKIT