The quarter wave vertical antenna is used at all frequency bands including LF, MF, HF, VHF and beyond. VHF and beyond.
The quarter wave vertical antenna possesses the attributes of many vertical antennas including the omnidirectional radiation, and vertically polarised signals.
Basic quarter wavelength vertical antenna
As the name suggests the quarter wave vertical antenna consists of a quarter wavelength vertical element. The antenna is what is termed "un-balanced" having one connection to the vertical element and using an earth connection or simulated earth connection to provide an image for the other connection.
The voltage and current waveforms show that at the end the voltage rises to a maximum whereas the current falls to a minimum. Then at the base of the antenna at the feed point, the voltage is at a minimum and the current is at its maximum. This gives the antenna a low feed impedance. Typically this is around 20Ω.
The ground is obviously an important part of the RF antenna. Many MF and HF installations use a ground connection for this. These ground systems need to be very effective fort he antenna to perform satisfactorily. They must obviously have a very low resistance, and often utilise large "mats" of radials extending out from the base of the antenna to ensure excellent RF performance.
For VHF and UHF installations, height is obviously important and antennas need to be raised to ensure they are above the nearby obstructions. Also for mobile installations it is clearly not possible to use a true earth connection. In these cases a simulated earth is used. For mobile applications this consists of the body of the vehicle. The antenna mounting will normally enable a suitable connection to be made to the vehicle body, sometimes using a capacitive connection. However it is necessary to ensure that the vehicle body is metal, and not plastic in the vicinity of the antenna mounting.
For fixed stations a set of radials simulating a ground plane is used. In theory the ground plane should extend out to infinity, but in practice a number of radials a quarter wavelength long is used. Typically for many VHF applications four radials is sufficient.
If the radials are bent downwards from the horizontal then the feed impedance will be raised. A 50Ω match is achieved when the angle between the ground plane rods and the horizontal is 42 degrees. Another solution is to include an impedance matching element in the antenna. Normally this is in the form of a tapped coil that can be conveniently housed in the base of the antenna.
Folded quarter wave vertical antenna
In view of the low impedance presented to the feeder by the quarter wave vertical, it is necessary to implement a matching scheme to ensure that the antenna presents a good match to the feeder.
Angling the radials in a downward fashion has been outlined above. Another is to use a folded element. In the same way that a folded dipole increases the feed impedance of the antenna, so a folded vertical element can be used. If the diameter of both sections is the same, then an increase by a ratio of 4:1 is achieved. This would bring the impedance to 80Ω and will provide an acceptable match to 75Ω feeder. By using a smaller diameter grounded element the feed impedance can be reduced so that a good match to 50 ohm coax can be achieved.
The quarter wave vertical antenna is widely used in view of its simplicity and convenience. To improve on its performance other types of vertical are available. It is also possible to use further verticals and feed them with different phases to provide gain to the overall antenna system.
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