AM/FM Radio Antenna
It's hard to say which is more important, the radio or the AM/FM radio antenna. A good AM/FM radio antenna can make a weak radio sound better more so than the other way around. With that in mind, you need to give the selection of your AM/FM radio antenna just as much consideration as the choice of radio. HF antennas can be fairly large. An effective AM/FM radio antenna is usually at least 1/4-wavelength in some dimension. On 40-meters, for example, a 1/4-wavelength vertical antenna is a metal tube or wire 33 feet high. At the higher HF frequencies, antenna sizes drop to 8-16 feet but are still larger than even a big TV antenna. Your physical circumstances have a great effect on what antenna you can put up.
The simplest AM/FM radio antenna is a wire antenna that is a dipole which is a piece of wire cut in the middle and attached to a feedline. The dipole gives much better performance than you may expect from such a simple antenna. To construct a dipole, use a 10 to 18 gauge copper wire. It can be stranded or sold, bare or insulated. Vertical AM/FM radio antennas are nearly as popular as wire antennas. The 1/4-wavelength and the 1/2 wavelength antennas are two common designs. Verticals don't require tall supports, keep a low profile, and are easy to move or carry. Vertical antennas radiate fairly equally in all horizontal directions so they are considered omnidirectional antennas. Any AM/FM radio antenna that uses more than one element to focus or steer its listening and transmitting capabilities toward one direction is called a beam, which is short for beamforming. Beams can be as simple as two wires or as complicated as more than a dozen pieces of tubing. Most AM/FM radio antennas used above 50 MHz at fixed stations are either verticals or beams. Verticals are used almost exclusively for FM operation, while beams are used for VHF/UHF DXing on SSV and CW. For VHF, antennas for FM use are almost always vertical. The wavelength at these frequencies is such that a full-sized antenna is the norm. Mobile HF antennas, however, are generally reduced-size versions of verticals used at fixed stations. Horizontally polarized mobile HF antennas are rare due to their size and mechanical considerations. The challenge for mobile HFers is to get the most efficiency out of a short antenna. Antennas radiate and receive electromagnetic fields composed of an electric and magnetic field. Electrons in the conducting surfaces of an antenna move back and forth (creating a current) in the same direction as the oscillations of the electric field. This action causes currents to flow in the antenna, which either radiates a signal if the field is applied from a transmitter via a feedline or receives a signal if the field is caused by a distant transmitter. |