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Five eighths wavelength vertical antenna

- overview, or summary about the basics of a five eigths wavelength vertical antenna, a form of RF antenna widely used for mobile radio communications applications.


Vertical antennas find widespread use in applications where an "all round" radiation pattern is required. In these applications it is necessary to keep the maximum amount of radiation parallel to the earth. It is in applications such as these that the five eighths wavelength vertical antenna has become widely used. One particular applications where they are widely used is for mobile radio communications. They are partcularly suited to mobile radio communications because there is not need for the antenna to be reorientated as the mobile station moves, and in addition to this the antenna provides gain over a quarter wave vertical.


Development

The most straightforward vertical antenna is the quarter wavelength version. However it is found that by extending the length of the vertical element, the amount of power radiated at a low angle is increased. If a half wave dipole is extended in length the radiation at right angles to the antenna starts to increase before finally splitting into several lobes. The maximum level of radiation at right angles to the antenna is achieved when the dipole is about 1.2 times the wavelength.


RF antenna gain

When used as a vertical radiator against a ground plane this translates to a length of 5/8 wavelength. It is found that a five eighths vertical has a gain of close to 4 dBd. To achieve this gain the antenna must be constructed of the right materials so that losses are reduced to the absolute minimum and the overall performance is maintained, otherwise much of the advantage of using the additional length will be lost.


Matching to the RF antenna

For most applications, it is necessary to ensure that the antenna provides a good match to 50 ohm coaxial cable. It is found that a 3/4 wavelength vertical element provides a good match, and therefore the solution to the 5/8 wavelength antenna is to make it appear as a 5/8 radiator but have the electrical length of a 3/4 element. This is achieved by placing a small loading coil at the base of the antenna to increase its electrical length.


Mechanical considerations

Five eighths wavelength vertical antennas are often used on automobiles. Accordingly one of the main constraints is to ensure that the coil at the base of the antenna is be kept rigid and does not bend as the antenna flexes with the movement of the car. If there is too much flexing then the match to the feeder will change and the operation will be impaired.

By Ian Poole

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