Wire Antenna

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Revision as of 18:15, 20 May 2008 by YL (Talk | contribs) (Cleaned up text. Removed references to impedance mismatching, which is too complex for a general article and not limited to wire antennas. We need to move that to a separate detail article.)

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Many amateur radio antenna systems use a simple wire to carry the RF current in such a way as to radiate. One of the simplest is the half-wave center-fed dipole with coax feedline. When a radio-frequency oscillating current is applied to a dipole, the magnetic field generated around the wire expands and contracts very quickly, in most cases millions of times per second. It is the outer most part on the field that is radiated away.

The shape of a dipole resembles the letter "T". The middle leg, which consists of the coax feedline, connects the radio to the center of the upper, horizontal legs. There are several ways to make the connection. The simplest has the coax shield connected to one side and the center conductor connected to the other side. However, this can cause feedline radiation due to common-mode currents. It is often recommended that a balun be attached at the feedpoint, but many operators have good success with dipoles that have no baluns.

The feedpoint impedance of a horizontal dipole various dramatically depending on the electrical height above ground.

Refer to the article on impedance matching for more details on connecting transceivers to feedline and feedline to antennas.