The gain required depends on system margins and how far the system must operate. Generally, the more gain, the more cost and the longer the antenna. Systems where the antenna can be a low performance type will have less cost and less noticeable antennas at any particular frequency. Installations with a suitable ground plane will have the same performance at less cost than those without adequate ground plane.
To determine the best mobile antenna types to use in a particular installation, you should first be acquainted with all the electrical types available. Keep in mind not all types are available in all mechanical configurations and frequencies.
Loaded 1/4 Wave
The loaded 1/4 wave antenna is electrically a 1/4 wave but is shorter than a full size 1/4 wave antenna. This is accomplished with a loading coil which places a portion of the electrical length of the antenna in a coil located at the base of the radiating element. The efficiency of the antenna depends on how much of the electrical length is inside the coil (and therefore not radiating). Typical gain is comparable to a full 1/4 wave where the full 1/4 wave is mounted on the fender and the loaded 1/4 wave is mounted on the roof. Typical length at the lowest recommended frequency is 49”.
1/4 Wave
A single radiating element 1/4 wavelength long. It is the simplest and least expensive type of antenna. Length varies from 20” at 144 MHz to 3” at 900 MHz. A loading or matching coil is not required. Typical gain is unity when mounted on a suitable ground plane.
1/2 Wave
The 1/2 wave antenna is a single radiating element 1/2 wavelength long. Because the end fed impedance of the antenna is not suitable for matching the radio, an impedance matching transformer is used at the base of the radiating element. Length varies from 49” at 120 MHz to 13” at 440 MHz. The 1/2 wave antenna is suitable for use where no ground plane exists. The gain with no ground plane is unity. Gain with a suitable ground plane is 2.4 dBi.
5/8 Wave
The 5/8 wave antenna is a single radiating element 5/8 wavelength long. In single element antennas, the 5/8 wave antenna has the best performance (3 dB) when mounted on a suitable ground plane. Element length varies from 49” at 144 MHz to 18” at 440 MHz. Since the end fed impedance of a 5/8 wave antenna is not suitable with a radio, an impedance transformer is used at the base of the rod. Must be mounted on a suitable ground plane.
Collinear Two elements separated by a phasing coil for increased gain. Three styles are common:
5/8 over 1/2
These collinear designs have two elements separated by a phasing coil.
5/8 over 1/4
The top element is 5/8 wave and the bottom element is either a 1/2 wave or a 1/4 wave. Gain is typically 5 dB for a 1/2 wave lower element and 3-4 dB with a 1/4 wave lower element when mounted on a suitable ground plane. Antenna length is 23” to 29” at 440 MHz depending on the lower element. The end feed impedance matches the transmitter’s impedance, so no transformer is used.
5/8 over 5/8
This collinear design has two elements separated by a phasing coil. Both top and bottom elements are 5/8 wave. Gain is typically 5 dB when mount¬ed on a suitable ground plane. Collinear element length is 33” at 440 MHz. The end fed impedance does not match the transmitter’s impedance, so a transformer is used.
Source: Larsen Antenna Products
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