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.
This type of antenna is often referred to as a monopole antenna, as opposed to a dipole. It has a single radiating element, and then relies on the ground or ground simulating system for the remainder of the antenna.
In fact the quarter wave dipole can be considered as a dipole in which one half is the radiating monopole, and the other half is a reflection seen in the ground.
In terms of its operation, the reflection in the ground means that it appears as if there is an image of the top half of the antenna below the ground – in other words it appears like a vertical dipole.
For this to work satisfactorily, the ground system for the vertical monopole antenna must work efficiently, otherwise the overall antenna system will be compromised and the efficient will fall. The ideal is that the ground system should be a perfectly conducting and infinite RF ground.
When operating well, the radiation pattern of a monopole antenna will be identical to the top half of a dipole pattern, with its maximum radiation in the horizontal direction, perpendicular to the antenna.
As this antenna can only radiate above the ground plane, a monopole antenna will have a gain of 3 dB over an equivalent dipole. This assumes that there are no earth or grounding losses. In reality it is very difficult to get an antenna RF grounding system that is lossless and it is accordingly difficult to fully realise this gain.
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 for the 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.
Quarter wavelength vertical feed impedance
We have seen that a quarter wave vertical antenna uses the real, ie.e quarter wavelength vertical section and an image of this in the ground system for the basis of its operation.
Since a half-wave dipole has a radiation resistance of 73Ω, a quarter-wave monopole will have a radiation resistance of about 36.8Ω if it is mounted above a good ground plane. However for this to be true, the antenna RF earth must be perfectly conducting. This means that an exceedingly good antenna RF ground is required.
In reality the ground will introduce some additional resistance and this will increase the overall impedance.
That said, vertical antennas, especially for HF where separate ground or radial system is used will have a matching assembly in the base feed point to accommodate the mismatch as they are normally fed with 50Ω coaxial feeder.
This matching arrangement normally consists of a tapped coil which gives the required impedance transformation.
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. Alternatively a matching coil can be accommodated in the base.
Quarter wave vertical directional pattern
The radiation pattern or polar diagram of a quarter wave vertical is important. Often for vertical antennas it is an essential part of their performance that the maximum level of radiation emanates at an angle close to the ground.
This is particularly true if they are to be used for local two way radio communications or for HF ionospheric long distance amateur radio. In both these cases a low angle of radiation is needed.
The radiation pattern of a dipole antenna has a figure of "8" pattern with the maximum radiation occurring at right angles to the axis of the antenna.
It might be thought that for a ground mounted vertical dipole, this pattern may be cut in two as a result of the action of the ground - only the upper half of the pattern being present.
In reality, it is not possible to get the maximum level of radiation exactly parallel to the ground and instead, the pattern is distorted slightly with the maximum radiation occurring at an angle slightly above 0°. The actual angle being dependent upon many factors including the Earth connection, local objects, the local Earth conductivity, etc.
This shows the radiation pattern in the vertical plane. In the horizontal plane the radiation pattern is equal all around the antenna element.
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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