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.,/p>
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.
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