An antenna has to be connected to something in order to be useful. The means by which power is sent to and from an antenna to a device is a crucial part of the configuration. All antennas, except for those operating at low frequencies, cannot be connected to a device with just a few loose wires. The reason is simple. A signal line, whether it is a coax cable, a twin lead cable, or loose wires, has an impedance.

Special circuitry is often used to match the impedance of the antenna to that of the device it is used with. In the same way, the signal line between the antenna and the device must have an impedance matching the two units it is connecting, or loss and poor VSWR is the invariable result.

Types of Feed Lines

Twin Lead

There are several different feed lines used to carry signal to and from an antenna. One is the twin lead. This consists of two parallel wires with spacers between them to keep the correct spacing. This spacing between the two wires sets the impedance of the line.

This type of line is balanced, meaning that, when signal is flowing through it, the identical flow between the two identical wires causes two identical magnetic fields. As these fields are going in opposite directions, they cancel. This balanced characteristic makes the twin lead perfect for dipoles and other balanced antennas, but it is not suited for unbalanced antennas like a monopole.

Coax Cables

Coax cables are probably the best-known RF feed lines. They consist of an inner conductor (center conductor) surrounded by an outer conductor (the shield). They are unbalanced, which is not surprising considering the asymmetrical nature of the two wires. The signal is fed on both the inner conductor and the shield. With an antenna such as a monopole that consists of the antenna proper and a large metal ground plane, the center conductor is connected to the antenna and the shield to the ground plane. A balun is required between the coax and the antenna when it is used with balanced antennas. More on that later.

Choosing and Using a Cable Type

Twin Lead

Twin lead, as it happens, is often undesirable. For twin lead to remain balanced and retain the proper impedance, nothing can interfere with the wires. This means that twin lead cannot be run near metal objects or anything that will interact with the signal. This precludes most scenarios save for running the wires up a pole held away from anything conductive.

Coax Cables

The coax cable, unlike twin lead, is essentially independent of its environment. Being unbalanced, there is no balance to lose. Furthermore, its impedance is largely set by the spacing between the inner and outer conductors. As this spacing is set by the insulators of the cable itself, it cannot be altered by most external influences.

For a coax cable to be used with a balanced antenna, a balun must be used. For more details on specific types of baluns, see our post on Balanced and Unbalanced Antennas. The purpose of the balun is essentially to isolate the coax from the antenna. The balun gives the antenna the balanced signal it needs to operate correctly, and then sends this signal to the cable in such a way that the cable has little to no effect on the antenna side of things.

The balun is another important piece of an antenna feed to keep in mind when using a balanced antenna like a dipole with a coax or most microstrip designs.

Applications in PC Boards

PC board feeds operate similarly to their cable counterparts. That said, whereas a cable is already designed to be the right impedance, in a PC board feed, the designer has to set the parameters (dielectric, spacing, etc.) to achieve the correct impedance. We will address PC board feeds in the next article in this series.

Treating Cables Properly

One final point worth mentioning about RF cables is their relative fragility. Cables have their weaknesses, primarily due to the inherent impedance of the cable. Anything that alters this spacing will cause a localized impedance change, which will cause loss and worse VSWR.

For instance, if a coax is crushed or kinked, at the point of damage it will no longer be the correct impedance. Sometimes, in extreme cases of very carelessly run cables, enough damage can accumulate to prevent the antenna and its associated device from working at all. Care should be taken when running cables to ensure maximum performance.