Basic half-wave antennas. At A, dipole antenna. At B, foldeddipole antenna. At C, zepp antenna.
A half wavelength in free space is given by the equation:
Lft = 492/fo
where Lft is the linear distance in feet, and fo is the fundamental frequency, in megahertz, at which the antenna exhibits resonance. A half wavelength in meters, Lm, is given by:
Lm = 150/fo
For ordinary wire, the results as obtained above should be multiplied by a velocity factor, v, of 0.95 (95 percent). For tubing or large-diameter wire, v can range down to about 0.90 (90 percent).
An open dipole or doublet is a half-wavelength (λ/2) radiator fed at the center (top Figure A). Each leg of the antenna is a quarter wavelength (λ/4) long. For a straight wire radiator, the length Lft, in feet, at a design frequency fo, in megahertz, for a center-fed, λ/2 dipole is approximately:
Lft = 468/fo
The length in meters is approximately:
Lm = 143/fo
These values assume v = 0.95. In free space, the impedance at the feed point is a pure resistance of approximately 73 Ω. This represents the radiation resistance alone, in the absence of reactance at the resonant frequency.
A folded dipole antenna is a λ/2, center-fed antenna constructed of two parallel wires with their ends connected together (above figure B). The feed-point impedance of the folded dipole is a pure resistance of approximately 290 Ω. This makes the folded dipole ideal for use with high-impedance, parallelwire transmission lines in applications where gain and directivity are not especially important.
A radiator measuring λ/2 can be stood up, fed at the base (the bottom end) against an earth ground with a transmatch or antenna tuner designed for high impedances, and connected to a radio by a coaxial cable feed line. This type of antenna is an efficient radiator even in the presence of considerable loss resistance, because the radiation resistance is extremely high.
A zeppelin antenna, also called a zepp, is a λ/2 radiator, fed at one end with a λ/4 section of openwire line (top figure C). The impedance at the feed point is an extremely high, pure resistance. At the transmitter end of the line, the impedance is a low, pure resistance. A zeppelin antenna can operate well at all harmonics of the design frequency. If an antenna tuner, also called a transmatch, is available to tune out reactance, the feed line can be of any length. Feed-line radiation can be minimized by carefully cutting the radiator to λ/2 at the fundamental frequency, and by using the antenna only at this frequency or one of its harmonics. Zepp antennas are rarely used at frequencies above 30 MHz, except when modified to form a J pole.
A zepp can be oriented vertically, and the feed line placed so it lies in the same line as the radiating element. This antenna, called a J pole, radiates equally well in all horizontal directions. The J pole is used as a low-budget antenna at frequencies from approximately 10 MHz up through 300 MHz. It is, in effect, a λ/2 vertical fed with an impedance matching section consisting of a length of transmission line measuring λ/4. It does not require any radials, and this makes it convenient in locations where space is at a premium.
Some radio amateurs hang long J poles, cut for 3.5 MHz or 1.8 MHz, from kites or heliumfilled balloons. Such antennas work well, but they are dangerous if they are not properly tethered to prevent them from breaking off and flying away, or if they are placed where they might fall on power lines. They can develop considerable electrostatic charge, even in clear weather. They are deadly if flown in or near thunderstorms.