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Although the quarter wavelength vertical antenna provides a good level of performance, it is easy obtain some additional gain by extending the radiating element of the vertical antenna to five eighths of a wavelength, 5λ/8.
As a result the five eighths wavelength vertical is used in many instances. Using this antenna, more power is radiated at a low angle of radiation, enabling gain to be obtained in the required plane.
Although the five eighths wavelength vertical antenna can be used in many applications, one particular area where it is used is with mobile radio communications. For many frequencies the additional length is not a problem, and it provides additional gain at little increase in cost.
Five eighths wavelength antenna concept
The concept for the five eighths wavelength vertical antenna starts with the quarter wavelength antenna.
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.
As a vertical monopole antenna is half the length of a dipole, this equates to about a five eighths wavelength vertical monopole antenna.
5λ/8 vertical 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 wavelength vertical antenna has a gain of close to 4 dBd, i.e. 4 dB gain over a dipole.
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.
Antenna impedance & matching
For most applications, it is necessary to ensure that the antenna provides a good match to 50Ω coaxial cable.
It is found that a 3/4 wavelength vertical element provides a good match, and therefore one solution for providing a good impedance match for the 5/8 wavelength vertical antenna is to make it appear as a 5/8 radiator but have the electrical length of a 3/4 element. In this way it radiates like a 5λ/8 radiator, but has the impedance of a 3λ/4 antenna.
This is achieved by placing a small loading coil at the base of the antenna to increase its electrical length.
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 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.
The five eighths wavelength vertical antenna is mainly used at VHF or even UHF where its length means that it can easily be accommodated without the need for its size becoming too large. Here is can be easily accommodated on vehicles for applications like PMR or even amateur radio. It provides a useful amount of gain for an increase in size that can often be accommodated with no issues.