  We have talked about using shielded cable to get the signal fromthe antenna to the receiver, and we've talked about achievinga nominal 50-ohm impedance at the antenna feedpoint. What do wemean by coaxial, shielded cable? A coaxial cable is made up of a solid or stranded center conductor,wrapped with a non-conductive white dielectric, then a braidedshield and finally a vinyl jacket. There are various grades ofcable and each type and grade of cable has a standard number.Some have a double shield, and some have vinyl jackets that aremore impervious to the elements -- sun and liquids -- than theinexpensive cable. Coaxial cable is available in 50 and 75 ohm impedances, and thecable size (diameter) varies. The standard notation of 50-ohmcable is RG-8X, RG-8, RG-58, RG-213 and RG-214. The smallest diametercable is the RG-58, next up in size is RG-8X. RG-213 is single-shield,RG-214 double-shield. Most table-top receivers have an SO-239 connector on the rearapron, and the matching PL-259 connector must be soldered to theend of the coaxial cable to connect the antenna to the radio.Be careful that you do not short-circuit the center conductorto the braided shield. As the RG-58 and RG-8X cable diametersare less than the PL-259 inside diameter, a reducer is required. Unlike radio amateurs that require better grade cable capableof withstanding high transmitting voltages, the SWL can probablyuse the less expensive RG-58 or RG-8X cable (unless the environment,such as exposure to salt air or chemicals warrent the investmentin a more impervious cable). For short runs under 30 meters (100feet) it probably doesn't matter which cable you use. For maximum transfer of signals, the feedline should be odd multiplesof a one, half or quarter wavelength. The physical wavelengthon a coaxial cable is less than on a wire. The "velocityfactor" varies among cable types, varying between 66 and80 percent. Apply the following values to the measurements inthe previous table.
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