Copyright © 1999-2011 by Harold Melton KV5R. All Rights Reserved.
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Understanding Shortwave Antennas: Page 7
Typical Antenna Designs
- Longwire - any antenna which uses a single end-fed element which is several wavelengths long. A longwire is directional toward its far end: More wavelengths long = more directional.
- Monopole - like the single telescopic element used on portable radios. If the monopole is operated at one-half wavelength, it becomes a Zepp.
- Dipole - an antenna having two poles. Dipoles are usually ½ wave long, and have a narrow bandwidth. Dipoles radiate best off of the sides - perpendicular to the wire - assuming it is operating at the frequency for which it was designed. Dipoes are resonant at ¼ and ¾ wavelength. Dipoles are bi-directional. “Rabbit-ears” are simply “V” dipoles.
- Zepp - an antenna that is ½ wave long and fed at one end. It radiates like a dipole. If you shorten it or lower the frequency, it becomes a nonresonant monopole. If you lengthen it or raise the frequency, it becomes a nonresonant longwire. Zepps are bi-directional.
- Ground plane - an antenna which uses an artificial ground to act as an electrical mirror, making the antenna’s radiating element “think” it’s a vertical dipole. They are omnidirectional.
- Yagi - an antenna which uses a dipole with parasitic elements to make it directional.
- Log Periodic Dipole Array (LPDA) - like a TV antenna. The LPDA design is directional, and has an extremely wide bandwidth, because of all its different element lengths.
- Discone - an antenna that is a disc and a cone. They are omnidirectional and have an extremely wide bandwidth, so are very useful as scanner antennas.
- Feedline - the cable that carries signals between an antenna and a transmitter or receiver. This is usually coax or twinlead. The impedance of the feedline should match that of the radio and antenna, or the feedline will itself become part of the antenna, messing up your design plans.