Text: Types Of Shortwave Antennas Many different types of antennas are used by hams depending on the band of frequencies in which they are operating. No matter what amateur band is considered, the basic antenna theory discussed in the previous chapters still applies. For example, the free space radiation pattern and other characteristics of a half-wave 3 GHz dipole is the same as a half-wave dipole at 3 MHz. Only the size is different. The 3 megahertz dipole is 165 feet long, whereas the 3 gigahertz dipole is about 2 inches long! Electrical Length vs Physical Length Because it gives the half-wave length in feet, a somewhat easier equation for the half-wavelength of an antennas is: Half-wave Length (in feet) = 492 / Frequency (in Megahertz) However, the half-wave length that one actually uses for the physical length is about 5% shorter than that calculated with the above equation. The reason for this is that the electrical length of the antenna is different than its physical length. The antenna wire is usually fastened to insulators by making several twists or wraps at the ends. This produces what is known as ³end effect.² End effect, and the fact that energy travels along the antenna wire at slightly less than its free space velocity, makes the effective electrical length about 5% longer than the physical length. Long Wire Antennas Probably the most common type of antenna used by early radio amateurs was a long wire antenna. This type of antenna is a horizontal wire antenna which is at least one wavelength long, sometimes much longer. It may or may not have a terminating resistor, as shown in Figure 9-2. When no terminating resistor is used, it is bidirectional, with the maximum response off both ends. If the terminating resistor is used, maximum response is off the terminated end. Zepp (or Zeppelin) A type of end-fed antenna shown in Figure 9-3 is called a Zepp antenna. They get their name because they were first used on the German lighter-than-air dirigibles called Zeppelins. This antenna is one-half wavelength at the lowest frequency. The impedance at the end is much higher (e.g. 1000 ohms or more) than the -impedance at the center of a resonant dipole (75 ohms). Usually widely spaced 2-wire transmission line is used to couple to the antenna, and a matching box must be used between the transmitter and the transmission line.

See Also:

Source: