Wire Antenna

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Revision as of 16:43, 13 April 2008 by YL (Talk | contribs) (Added more text. Ultimately I think the content of this section doesn't belong under "Wire", since it applies to all antennas. We should think about moving it.)

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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. When a dipole oscillates current, in sync with the radio's RF output during a transmission, 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 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. This works but there are losses at the connection due to impedance mismatch.

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

For example an 80m horizontal dipole 132 feet long and 66 feet high (1/4 wavelength above ground) has a feedpoint impedance of 84 ohms. This would create a mismatch of 1.68:1 VSWR if fed with 50 ohm coax. The same antenna installed only 18 feet above ground (7 percent of a wavelength) has an impedance of 45 ohms with a mismatch of only 1.1:1 VSWR. (However, the radiation pattern of the lower antenna would put more signal at high elevation angles, suitable for local QSOs, but it would be inferior for DX.)

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