Antenna RF Ground: theory & practice

Antenna Grounding Includes:
How to ground an antenna     Antenna RF ground     Antenna ground plane    

Monopole antennas such as ground mounted quarter wave verticals rely on a good earth system for their operation.

End fed wires which are popular are also reliant on a good earth system for their proper operation. In fact any antenna system which uses the ground as part of the antenna needs a good grounding system if it is to operate to its best.

In reality a good antenna RF ground system should present a low impedance for radio frequencies within the band of frequencies of interest. It is not just the DC resistance that must be low, the impedance at radio frequencies should ideally be much lower.

Operation of a monopole antenna with a ground

An RF ground system forms an integral part of the operation of a monopole antenna. A low impedance ground system is essential if the antenna is to operate correctly.

As the ground system is actually part of the antenna system, it needs to be as good as it can be for the best performance of the overall antenna system.

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:

Where:
    R = radiation resistance of the antenna
    R_e = 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. If 50% of the power is absorbed in the ground connection, then the transmitted signal will be degraded by 3dB - an important amount, even if high power levels are available.

For the receiving aspects it may be a little 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 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 or several earth rods and drive these into the ground.

  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 to provide the greatest contact area with the ground.

• Use buried radials:   An effective RF earth or ground system can be created by burying radials that radiate outwards from the base of the antenna.

  Some of the practical considerations for a ground system for a monopole antenna are discussed in the section below.

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.

Reception and transmission

When developing a good ground system, the main benefit by far is for transmitting and there is often little benefit for reception.

The limiting factor for noise for a receiver at the lower frequencies where ground mounted verticals are used is the received static noise and not the receiver front end noise.

This means that if the ground is not as good and the signal level is reduced at the receiver, the RF gain control can be increased to accommodate the loss in signal level without any undue impairment of the received signal.

Ideal grounding system for a monopole antenna

When planning the use of a monopole antenna system it is wise to also plan the grounding system as well as there are a number of practical considerations.

It is important to consider the ground conductivity in the region because if this is poor then it is unlikely that the antenna wil work well.

There are two main elements to creating a good ground system. The DC connection is made by burying meal such as copper which is connected to the wired earth connection.

  •   DC connection

When doing this, be aware that the metre or 3ft long spikes often used for electrical installations will only give an earth resistance of 20Ω at best, and it could be much higher: - 100Ω or more dependent upon the local earth conductivity. It takes a lot of metal and large surface area to create a good ground.

One arrangement could be to drive several rods into the ground space a few metres apart and connected with stout cable, but keeping the main earth connection as close to the base of the antenna as possible. When using this approach the earth resistance is only roughly divided by the number of rods use - it will be higher than might be expected because of the local resistance of the earth itself.

The rods used for electrical installations are designed to be driven into ground and they have a hard centre, often of steel, etc. This gives them the rigidity to be hammered into msot types of ground.

Copper piping used for plumbing is often not strong enough to be driven into the ground as it buckles unless the ground is very soft. However it can be buried in pre-dug holes, and in this way, odd lengths of copper piping left over from plumbing jobs can be used effectively.

  •   Buried radials

The use of buried radials is another approach that is used for the RF elements of the ground connection: these enable the RF currents to spread out away from the antenna base.

These is a lot of debate about the extent of a radial system that might be needed. Various rules of thumb exist and statements including the more radials the better have served many radio amateurs and organisations using HF vertical antennas for many years.

One approach suggested by Les Moxon is to bury between 50 and 100 wires of length up to 3λ/2. It has also been said that the more radials the better, and it is better to have more short radials than a few long ones.

It has also been stated by some that where radiation is required to be optimised in a particular direction additional and longer radials can be added in that general direction.

However, it is very difficult to assess the exact differences these systems might make and how to balance the number of radials that might be needed.

Short wave and medium wave broadcast stations often use vertical antennas and as a result of this a umber of studies have been undertaken by various organisations to assess how many radials are needed.

One study entitled Ground Systems as a Factor in Antenna Efficiency was published in the Proceedings of the IRE in June 1937, and another by John Stanley in QST December 1976 was entitles Optimum Ground Systems for Vertical Antennas.

Using these references it can be seen that a radial system consisting of 120 radials, each around 0.4 wavelengths long would give an almost perfect ground system providing a feed impedance for a quart wave vertical of 35Ω and a 0dB power loss at a low angle of radiation.

16 radials of 0.1 wavelengths would give a feed impedance of 52Ω and a power loss of 3dB, while 36 radials of 0.15 wavelengths would give a feed impedance of 43Ω and a power loss of 1.5dB.

When using radials, these can be brought to a common node or point close to the base of the antenna. Wire suitable for earthing, with a good surface area to accommodate the skin effect so that a good low impedance connection can be made.

Ideal ground terrain

For a ground system to be really effective, the ground conductivity needs to be high so that the current can flow easily through the ground itself.

Sandy soil and terrain where there is a solid ground rock just below the surface, tends to give a poor level of ground conductivity, and hence poor earth or ground systems.

The ground conductivity is very much better for moist soil conditions as the moisture acts as an electrolyte enabling the current to flow.

Marshes are ideal because these retain the water and enable good conductivity to be maintained. In fact one of the best environments for a good ground connection is a salt marsh - the salt increases the conductivity.

The high conductivity soil will enable a good DC connection to be made to any metal and it will also allow the currents from buried radials to be conducted as well.

It is also found that for antennas requiring a low angle of radiation, a terrain where the land height gently falls away gives the best results. This means that a salt mash gently falling away from the antenna would be ideal . . . but most unlikely to occur as the marsh would drain!

Hints & tips

There are several practical ideas that should be kept in mind when installing an RF earthing or grounding system for an antenna.

If these are implemented, they will help ensure that the system is as effective as it can be:

• Keep lead to ground connection itself as short as possible:   There will always be soem length required to take the ground from, say, the base of a vertical antenna to where the grounding system starts to become effective. However, this should be kept as short as reasonably possible, and no additional wire length left "in case soemthing needs to be moved."

  Keeping the lead to the effective grounding system as short as possible will minimise inductance and ensure that the base of the antenna remains at earth or ground potential.

• Use thick multi-strand wire:   Even at HF, the kin effect comes into play and therefore the majority of the current is carried close to the skin of the wire. By using multi-stranded wire, the impedance int he wire can be minimised.

• Ensure screw connections do not corrode:   It is very easy for screw connections, which normally have a variety of different metals for the different elements to corrode. They should be well tightened, and possibly covered with some sealant to prevent corrosion as much as possible.

• Make solder joints where possible:   Solder joints can reduce the effect of corrosion on a joint. Where possible joints can be soldered to ensure long lasting good connections.

• Select ground area with highest conductivity if possible:   Although this is not normally easy to do, in some instances it may be worth looking at where best to locate the earthing or grounding system as occasionally there may be some choice in where one might be located, although this will govern the location of the associated antenna. Areas of higher conductivity, such as damp areas or where there is a damp salty area are idea. For most situations, there is little wriggle room, but occasionally a small change might make a difference.

These are just a few ideas and tips that may help in making any antenna RF grounding system as effective as it can be.

Grounding systems for antennas like ground mounted vertical antennas or other forms of monopole are essential to the success of the antenna.

Understanding how to make a ground system as effective as possible can greatly improve the performance of the antenna and the overall transmitting station - remember that for low frequencies where ground mounted verticals are used, the reception is unlikely to be affected very much.

Having an effective antenna system for transmitting will enable the performance to be maximised and the elast power dissipated in the ground connection.

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