Though the first coax cable was developed in the 19th century, it didn’t become popular with Hams until after World War II when war surplus was plentiful. Hams liked it because it was easy to obtain, relatively inexpensive, and easy to install—run and done.
Right Cable, Right Application
You’ve probably noticed that most cable is identified with the letters RG plus a number. The RG prefix on cable stands for “Radio Guide,” the original military specification for coax cable. The number that follows the RG was just a page in the radio guide—it has no other significance.
The RG designation is just a general description of coaxial cables that are available. Every manufacturer has their own variations, including differences in shielding material, insulation, outer jackets, and other traits. Transmission loss, power handling, and other specs will vary a bit from one brand to another. An RG-8U cable from one manufacturer may be slightly different from that made by another.
Comparing Cables
When reading 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 hairs over a few tenths of a dB may not make much difference in real-world applications.
Cable jackets are the first line of defense for coax cable. They provide moisture, chemical, UV, and ozone protection. UV-resistant cable is preferable for outdoor use, which will help extend the life of your coax. If you’re running cable underground, be sure to choose one that is rated for direct burial.
Size matters. Smaller diameter cables are OK for short runs, portable/mobile use, or for low frequency antennas. At VHF/UHF frequencies, and for long cable runs, larger diameter cables will always be a better choice.
Power levels are also an important consideration, especially if you run an amplifier or continuous modes, such as AM or digital. It’s a good idea to use heavier-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 cable will handle 6.9 kW at 5 MHz, 4.8 kW at 10 MHz, and 2.8 kW at 30 MHz.
What Do You Really Need?
Consider the following before you buy: operating frequencies, power level, length of cable run, and whether the cable will be installed inside, outside, or buried in the ground. Also think about if the cable will be subjected to frequent bending, such as a cable that connects to an antenna with a rotator.
The following are suggestions for using some of the 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’t get caught short. Apply the old carpenter’s saying when buying coaxial cable—measure twice, order once. For instance, if you need to run cable from your radio to the antenna, make sure to measure the actual length you need, including bends and turns. You can easily underestimate if you just measure the straight line distance. It doesn’t hurt to order a little extra, since it’s easier to trim a cable than splice one. I typically order an extra 10%, with any leftovers going to making patch cables or doing mobile installations.
