Saturday, May 31, 2014

Feedline and Measurements


Most radios and antenna tuners have the ability to evaluate the electrical conditions inside the feed line, measured as the standing wave ratio (SWR).

SWR is a ratio of voltages and tells you how much of the power supplied by the transmitter is getting radiated by the antenna. Most radios have a built-in meter that shows feed line SWR. Having a stand-alone SWR sensor, called an SWR meter or an SWR bridge, to measure SWR is very handy when working on antennas or operating in a portable situation. You can also measure feed line conditions by using a power meter, which measures the actual power flowing back and forth. SWR meters are inexpensive, while power meters are more accurate. These devices are typically used right at the transmitter output.

Feed lines
Two common types of electrical feed lines connect the antennas to the station
and carry RF energy between pieces of equipment. The most common is coaxial
cable, or just coax, so named because it is constructed of a hollow tube surrounding central wire concentrically. The outer conductor is called the shield and also braid, if made from fine woven wire. The wire in the middle is called the center conductor and is surrounded by insulation that holds it right in the center of the cable.The outer conductor is covered by a plastic coating called the jacket. The other kind of feed line is open-wire, also called twin-lead or ladder line, which is made from two parallel wires. The wires may be exposed, only held together with insulating spacers, or plastic insulation may cover them.

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