Feed Line

Feed lines are specialized cables that connect an antenna to a transmitter or receiver of a radio or wireless communication system.

The most widely used types of feed lines are:

• Coaxial cable – This consists of four parts: the center conductive wire, plastic insulation around the wire, copper shielding around the insulation and a tough outer coating.
• Twin-lead – This is a wire encased in plastic and is demarcated at a uniform distance throughout the line.
• Ladder line – This is also called the parallel conductor feed line and consists of two conductors separated by insulating rods.
• Waveguide – This is used at microwave frequencies.

Feed lines carry the radio frequency voltage from one end to the other and hence they are made of specialized cables. Each feed line has its own characteristic impedance which must be matched with that of the antenna to transfer the RF power efficiently. If the impedance is not matched, the RF energy is reflected back to the transmitter, causing energy wastage and overheating of the transmitter. A device called an antenna tuner is used to make the necessary adjustments for efficient transfer of energy.

Some points to remember while using feed lines are:

• When the frequency increases, the amount of signal loss also increases.
• Signal loss is also high for longer feed lines as signal resistance increases with the length of the feed line.