Though the first coax cable was developed in the19th century, it didn’t become popular with Hams until after WorldWar II when war surplus was plentiful. Hams liked it because it was easy toobtain, relatively inexpensive, and easy to install—runand done.
Right Cable,Right Application
You’ve probably noticed that most cable isidentified with the letters RG plus a number. The RG prefix on cable stands for“Radio Guide,” the original military specification for coax cable. The numberthat follows the RG was just a page in the radio guide—ithas no other significance.
TheRG designation is just a general description of coaxial cables that areavailable. Every manufacturer has their own variations, including differencesin shielding material, insulation, outer jackets, and other traits. Transmissionloss, power handling, and other specs will vary a bit from one brand toanother. An RG-8U cable from one manufacturer may be slightly different fromthat made by another.
ComparingCables
Whenreading the coaxial cable spec charts, most focus on attenuation (loss)figures, which are generally expressed in dB per 100 feet at a given frequency.Though this may be one deciding factor in choosing your cable, splitting hairsover a few tenths of a dB may not make much difference in real-worldapplications.
Cablejackets are the first line of defense for coax cable. They provide moisture,chemical, UV, and ozone protection. UV-resistant cable is preferable foroutdoor use, which will help extend the life of your coax. If you’re runningcable underground, be sure to choose one that is rated for direct burial.
Sizematters. Smaller diameter cables are OK for short runs, portable/mobile use, orfor low frequency antennas. At VHF/UHF frequencies, and for long cable runs,larger diameter cables will always be a better choice.
Powerlevels are also an important consideration, especially if you run an amplifieror continuous modes, such as AM or digital. It’s a good idea to useheavier-duty coax in these applications. Generally, the lower the frequency,the more power a cable will handle. For example, DX Engineering Low-Loss 50-ohm 400MAX cablewill handle 6.9 kW at 5 MHz, 4.8 kW at 10 MHz, and 2.8 kW at 30 MHz.
What Do YouReally Need?
Consider the following before you buy: operatingfrequencies, power level, length of cable run, and whether the cable will beinstalled inside, outside, or buried in the ground. Also think about if thecable will be subjected to frequent bending, such as a cable that connects toan antenna with a rotator.
The following are suggestions for using some ofthe most popular varieties of coaxial cable and their equivalents.
RG-58A/U: This flexible cable is about .195 inches OD with a single braided shield. It’s typically used for lower power applications, short patch cords, and mobile installations. The small diameter allows it to fit into tight spaces typically found in vehicles. Because of the relatively short cable distances involved in mobile installations, losses are minimal.
RG-8X: This .242 inch OD cable is extremely popular in the Ham radio community primarily because it’s super flexible, relatively low loss, and fairly inexpensive. It’s good for HF applications up to 30 MHz at 1.2 kW and is generally suitable for runs up to 100 feet. It’s also acceptable for short runs on 144/220/440 MHz, especially in mobile applications.
LMR 240: This .240 inch OD cable is an improvement over 8X, adding foil shielding to obtain lower loss figures. The Ultraflex version is easier to work with and suitable for use with antenna and rotator combinations.
RG-213/RG-8U: These .405 inch OD cables are best for high power use and providing low loss, especially for runs of more than 100 feet for HF use. The RG-8U foam dielectric has a slight edge over the solid dielectric RG-213 when it comes to losses.
400MAX /LMR400: Though this cable is generally the most expensive of those listed, it provides some of the lowest attenuation figures. The Ultraflex version is still pliable enough to use with antenna/rotator combos if you provide a generous bend radius. This is the preferred cable for VHF/UHF use and works very well with HF at 3 kW up to 30 MHz. The larger diameter LMR600 is sometimes used as an inexpensive substitute for hard line.
Don’tget caught short. Apply the old carpenter’s saying when buying coaxial cable—measuretwice, order once. For instance, if you need to run cable from your radio tothe antenna, make sure to measure the actual length you need, including bendsand turns. You can easily underestimate if you just measure the straight linedistance. It doesn’t hurt to order a little extra, since it’s easier to trim acable than splice one. I typically order an extra 10%, with any leftovers goingto making patch cables or doing mobile installations.
