A dipole is perhaps the most basic radio antenna. The most commonly used dipole is the half-wave, so named because the total length is approximately one half of a wavelength long at the desired center frequency. The antenna is made up of two wire elements, each about a quarter of a wavelength long.
Dipoles are excellent antennas for general purpose shortwave listening, including pirate radio listening. It is easy to find straightforward instructions on building a dipole, and the materials are cheap and readily available. The ARRL has very straightforward explanations and plans at the following url: http://www.arrl.org/single-band-dipoles
It's useful to note that you do not need to use 50 ohm coaxial cable, which is generally more expensive and harder to source. A simple dipole can be fed with the inexpensive television type 75 ohm coax found at your local home improvement or department store.
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