Coaxial Cable

A coaxial cable is made up of a solid or stranded center conductor, wrapped with a non-conductive white dielectric, then a braided shield and finally a vinyl jacket. There are various grades of cable and each type and grade of cable has a standard number. Some have a double shield, and some have vinyl jackets that are more impervious to the elements -- sun and liquids -- than the inexpensive cable.

Coaxial cable is available in 50 and 75 ohm impedances, and the cable size (diameter) varies. The standard notation of 50-ohm cable is RG-8X, RG-8, RG-58, RG-213 and RG-214. The smallest diameter cable is the RG-58, next up in size is RG-8X. RG-213 is single-shield, RG-214 double-shield.

coaxMost table-top receivers have an SO-239 connector on the rear apron, and the matching PL-259 connector must be soldered to the end of the coaxial cable to connect the antenna to the radio. Be careful that you do not short-circuit the center conductor to the braided shield. As the RG -58 and RG-8X cable diameters are less than the PL-259 inside diameter, a reducer is required.

Unlike radio amateurs that require better grade cable capable of withstanding high transmitting voltages, the SWL can probably use the less expensive RG-58 or RG-8X cable (unless the environment, such as exposure to salt air or chemicals warrent the investment in a more impervious cable). For short runs under 30 meters (100 feet) it probably doesn't matter which cable you use.

For maximum transfer of signals, the feedline should be odd multiples of a one, half or quarter wavelength. The physical wavelength on a coaxial cable is less than on a wire. The 'velocity factor' varies among cable types, varying between 66 and 80 percent.


Coax Cable Types
Cable VF %
RG-8X 75
RG-8 66
RG-8 foam 80
RG-59 66
RG-59 foam 79
RG-213 66
RG-214 66