RF cables are, for frequencies higher than HF, almost exclusively coaxial cables (or coax for short, derived from the words “of common axis”). Coax cables have a core conductor wire surrounded by a non-conductive material called dielectric, or simply insulation. The dielectric is then surrounded by an encompassing shielding which is often made of braided wires. The dielectric prevents an electrical connection between the core and the shielding. Finally, the coax is protected by an outer casing which is generally made from a PVC material. The inner conductor carries the RF signal, and the outer shield prevents the RF signal from radiating to the atmosphere, and also prevents outside signals from interfering with the signal carried by the core. Another interesting fact is that the electrical signal always travels along the outer layer of the central conductor: the larger the central conductor, the better signal will flow. This is called the "skin effect".
Even though the coaxial construction is good at containing the signal on the core wire, there is some resistance to the electrical flow: as the signal travels down the core, it will fade away. This fading is known as attenuation, and for transmission lines it is measured in decibels per meter (dB/m). Therateof attenuation is a function of the signal frequency and the physical construction of the cable itself. As the signal frequency increases, so does its attenuation. Obviously, we need to minimize the cable attenuation as much as possible by keeping the cable very short and using high quality cables.
|Figure 4.1: Coaxial cable with jacket, shield, dielectric, and core conductor.|
Here are some points to consider when choosing a cable for use with microwave devices:
- “The shorter the better!” The first rule when you install a piece of cable is to try to keep it as short as possible. The power loss is not linear, so doubling the cable length means that you are going to lose much more than twice the power. In the same way, reducing the cable length by half gives you more than twice the power at the antenna. The best solution is to place the transmitter as close as possible to the antenna, even when this means placing it on a tower.
- “The cheaper the worse!” The second golden rule is that any money you invest in buying a good quality cable is a bargain. Cheap cables are intended to be used at low frequencies, such as VHF. Microwaves require the highest quality cables available. All other options are nothing but a dummy load1.
- Always avoid RG-58. It is intended for thin Ethernet networking, CB or VHF radio, not for microwave.
- Always avoid RG-213. It is intended for CB and HF radio. In this case the cable diameter does not imply a high quality, or low attenuation.
- Whenever possible, use Heliax (also called “Foam”) cables for connecting the transmitter to the antenna. When Heliax is unavailable, use the best rated LMR cable you can find. Heliax cables have a solid or tubular center conductor with a corrugated solid outer conductor to enable them to flex. Heliax can be built in two ways, using either air or foam as a dielectric. Air dielectric heliax is the most expensive and guarantees the minimum loss, but it is very difficult to handle. Foam dielectric heliax is slightly more lossy, but is less expensive and easier to install. A special procedure is required when soldering connectors in order to keep the foam dielectric dry and uncorrupted. LMR is a brand of coax cable available in various diameters that works well at microwave frequencies. LMR-400 and LMR-600 are a commonly used alternative to Heliax.
- Whenever possible, use cables that are pre-crimped and tested in a proper lab. Installing connectors to cable is a tricky business, and is difficult to do properly even with the proper tools. Unless you have access to equipment that can verify a cable you make yourself (such as a spectrum analyzer and signal generator, or time domain reflectometer), troubleshooting a network that uses homemade cable can be difficult.
- Don't abuse your transmission line. Never step over a cable, bend it too much, or try to unplug a connector by pulling directly the cable. All of those behaviors may change the mechanical characteristic of the cable and therefore its impedance, short the inner conductor to the shield, or even break the line. Those problems are difficult to track and recognize and can lead to unpredictable behavior on the radio link.