Coax Cable

Cable Loss

Every 3 dB of attenuation (loss) in the cable represents a loss of half of your transmit power (and half of the received signal’s power, too). 6 dB is a loss of half and then half again (3 dB + 3 dB), for a total of ¾ of your power being lost. This loss is usually not important when communicating via a repeater; the repeaters are usually located up high, with excellent antennas and filtering so they can hear even weak signals. But cable loss is very important in simplex operations, and simplex operations are critical for emergency communications.

For a given cable type, attenuation increases with frequency. What works fine for HF, may be just OK for VHF, and terrible for UHF. However, cables with lower attenuation typically cost more than cables with higher attenuation, and are less flexible and harder to work with. Choose the lowest loss cable that fits your needs and is within your budget.

Shielding

At VHF and UHF frequencies, it is important to have as close to 100% shield coverage as possible. This not only prevents leakage of your signal out of the cable, but also prevents intrusion of other signals into the cable.

LMR-type cables have a 100% coverage aluminum foil shield surrounded by a tinned copper braid shield. This gives 100% shield coverage with lower weight and cost than other 100% coverage options, making it an excellent choice for most home and field stations. However, over time, any imperfection in the cable can cause the braid and the foil to become oxidized. The oxidation creates very small disconnects between the two shields wherever the oxidation occurs. This is usually not a problem for simplex operations; installations can operate for 10 or 20 or more years without an issue. But this type of cable is not used for repeater installations, where both transmit and receive are happening at the same time. As the cable moves, such as when subjected to wind, those oxidized areas connect and disconnect, and the transmit power in the cable causes the cable to produce “crackling” sounds on the receive side. Well constructed repeater sites use hardline instead.

RG-type cables have one or more braided shields, usually silver-plated copper braid. Each braided shield has about 95% coverage. RG-214 is essentially RG-213 with two braided shields, yielding about 98-99% coverage. Oxidized silver is still a conductor, so even if some oxidation does occur, the “crackling” sounds usually don’t occur as with LMR-type cables. Therefore, RG-214 is often used in repeater site construction, where flexibility to make tighter turns is needed, such as when connecting duplexers together.

Hardline has a 100% solid copper or aluminum shield. (Heliax is a brand name.) As its name suggests, hardline is not very flexible and, in fact, has a limit to the number of times it can be bent/unbent “Superflex” hardline is more flexible than standard hardline, but is till very stiff compared to regular coax cable. It is also not meant to be constantly flexed, such as would happen at a field site. Its added flexibility is used for making tighter turns than are possible with regular hardline, such as inside a rack or cabinet.

Center Conductor

A solid center conductor will be lower loss, but the cable will be very stiff. A stranded center conductor will be higher loss, but the cable will be more flexible. LMR-type “Ultraflex” (UF) cables use a stranded center conductor.

Power Handling

Power handing for a given cable decreases with higher frequencies. For example, a given cable may be able to handle 1500 Watts at 50 MHz, but only 600 watts at 450 MHz.

Power handling for VHF/UHF FM is less of an issue for most feedlines since the power levels tend to be lower. For example, at a site with three radios, each transmitting at 50 watts, sharing the same tri-band antenna (144/220/440 MHz), the feedline (coax, connectors, adapters) needs to support at least 150 watts.

Power handling for HF is a problem for smaller coaxes. Verify that all parts of the feedline (coax, connectors, adapters) are capable of supporting the chosen power level, which could be up to 1.5 KW.