Monopole antennas such as quarter wave vertical antennas rely on a good earth system for their operation.
Also antennas like end fed wires also often rely on a good ground connection for their proper operation.
In reality a good antenna RF ground system should present a low impedance for radio frequencies within the band of frequencies of interest.
Operation of a monopole antenna with a ground
An RF ground system forms an integral part of the operation of a monopole antenna.
In terms of the operation of a monopole, the reflection in the ground means that it appears that there is an image of the top half of the antenna below the ground – in other words like a vertical dipole. For this it appears that the ground plane is large enough.
As a result, the radiation pattern of a monopole antenna with a perfectly conducting and infinite RF ground is 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.
Since a half-wave dipole has a radiation resistance of 73 ohms, a quarter-wave monopole will have a radiation resistance of about 36.8 ohms 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.
Effect of RF antenna ground on radiation efficiency
The resistance if the RF ground system will naturally introduce losses into the overall antenna system. If the resistance is high, then this will absorb a significant proportion of the antenna power supplied to it.
It is possible to determine the efficiency of an antenna system by looking at the resistance of the antenna RF ground and radiation resistance of the antenna.
Neglecting the resistance of the antenna wire, which for most systems is low and can be ignored it is found that the radiation efficiency of an antenna is:
R = radiation resistance of the antenna
Re = resistance of the earth connection
If the antenna RF earth connection had a resistance of 100Ω and the radiation resistance of the vertical antenna was 36Ω (the radiation resistance for a λ/4 vertical), then the loss would be 6dB.
The power loss caused by a poor antenna RF ground system is of particular importance when signals are to be transmitted as a poor ground resulting in real power losses as seen above. For the receiving aspects it can be less important. RF grounds of this nature tend to be used at MF and HF. Here the limit in sensitivity of the receiver is not an issue, but instead it is the level of atmospheric and other noise received via the antenna. As the poor earth or ground will result in the attenuation of all signals equally, it is unlikely that the receiver gain will not be able to make up the loss without any noticeable effect.
Where problems can arise is if the coaxial feeder used itself picks up unwanted noise, particularly that which is generated locally. This can travel along the outer of the coax and enter the receiver, causing increased levels of interference to the required signals.
Practical antenna RF ground system
It has often been said that the ore metal that is buried in the ground the better. Broadly speaking this is true, but with a little more planning and insight, it is possible to install a very effective antenna RF ground system more easily. It is also possible to follow some hints and tips and ensure that the operation of the antenna RF ground system is as good as it can be for any given location.
There are many points to note when installing an antenna RF ground system:
- Local earth conductivity: It is obvious to say that the better the earth conductivity in a given region, the better that the earth connection will be. Areas that are on sandstone are very poor. It is very difficult to get a sufficiently good antenna RF ground system when on a sandstone base. However areas that are wet, and even salty provide a very much better opportunity for a ground system.
- Large conductive surface area: The traditional method of installing a DC ground connection is to have an earth rod and drive this into the ground. Earth rods may be anywhere between a metre or a little more up to 2 metres. The longer rods can be difficult to drive into the ground if done manually, but this can nevertheless be achieved.
An alternative or additional approach can be to bury discarded or surplus metal into the ground. Copper sheet or sheets of other metals can have a large surface area which can make good contact with the ground.
A combination of both methods may be suitable to adopt.
- Use buried radials: An effective RF earth or ground system can be created by burying radials. The more radials buried, generally the better. One approach suggested by Les Moxon is to bury between 50 and 100 wires of length up to 3λ/2. A general rule of thumb that is often used is that the more radials the better, and it is better to have more short radials than a few long ones.
Where radiation is required to be optimised in a particular direction additional and longer radials can be added in that general direction.
In reality a combination of all methods of making the antenna RF ground will be used. By employing all techniques, the best overall solution can be implemented.
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