Antenna Mounting
If an antenna is not correctly installed, it will not provide optimum performance, reducing RF power radiation.
Whip antennas
Whip antennas (1/4 wavelength) are normally connected directly to the wireless device. These antennas are “ground-dependant”; that is, they need to be installed onto a metallic plane to “ground” the antenna. Without a ground plane, the antenna will have reduced radiation. For 900MHz and higher frequencies, the required ground plane is small and the wireless unit itself is normally sufficient.
If the whip antenna is connected via a coaxial connector (as opposed to a fixed connection), some weatherproofing technique is required if the installation is outside. The wireless unit and antenna can be installed inside a weatherproof enclosure.
If the enclosure is metallic, then most of the RF energy will be absorbed by the enclosure, although this is still an acceptable installation if the radio path is short. A better installation is to use a non-metallic enclosure which will have little blocking effect on the RF signal. Note: make sure the mounting plate in the enclosure is also non-metallic.
This mounting method is a good installation for distance application – the antenna is not vulnerable to mechanical damage as it does not protrude outside the enclosure.
Dipole and collinear antennas
These antennas are connected to the wireless unit via a length of coaxial cable. If the cable is larger than 6mm diameter (1/4 inch), do not connect the cable directly to the wireless unit. Thick cables have large bending radii, and the side-ways force on the unit connector can cause a poor connection. Use a short “tail” of RG58 between the thick cable and the wireless unit.
The polarity of these antennas is the same as the main axis, and they are normally installed vertically. They can be mounted horizontally (horizontal polarity), however the antenna at the other end of the wireless link would need to be mounted perfectly parallel for optimum performance. This is very difficult to achieve over distance. If the antenna is mounted vertically, it is only necessary to mount the other antennas vertically for optimum “coupling” – this is easy to achieve.
Dipole and collinear antennas provide best performance when installed with at least 1 to 2 “wavelengths” clearance of walls or steelwork. The wavelength is based on the frequency:
Wavelength in metres = 300 / frequency in MHz
Wavelength in feet = 1000 / frequency in MHz
Hence, antennas at 450MHz need a clearance approx 1 metre (3 feet), 900 MHZ requires ½ metre (2 feet) and 2.4GHz 15 cm (6 inches).
Antennas can be mounted with less clearance, however the radiation from the antenna will be reduced. If the radio path is short, this won’t matter.
It is important that the antenna mounting bracket to well connected to “earth” or “ground” for good lightning surge protection.
Yagi antennas
Yagi antennas are directional along the central beam of the antenna. The folded element is towards the back of the antenna, and the antenna should be “pointed” in the direction of the transmission.
Yagis should also be mounted with at least 1 to 2 wavelengths of clearance from other objects. The polarity of the antenna is the same as the direction of the orthogonal elements. For example, if the elements are vertical, then the Yagi will transmit with vertical polarity.In networks spread over wide areas, it is common for a central unit to have a omni-directional antenna and the remote units to have Yagi antennas. In this case, as the omni-directional antenna will be mounted with vertical polarity, then the Yagi’s need to also have vertical polarity.
Care needs to be taken to ensure that the Yagi is aligned correctly to achieve optimum performance.
Two Yagis can be used for a point-to-to link. In this case, they can be mounted with the elements horizontally to give horizontal polarity. There is a large degree of RF isolation between horizontal and vertical polarity (approx –30dB), so this installation method is a good idea if there is a large amount of interference from another system close by transmitting vertical polarity.
An important mounting tip – if a Yagi has drainage holes in the dipole element, do not mount the antenna with the drainage.
Mounting near other antennas
Avoid mounting an antenna near any other antenna. Even if the other antenna is transmitting on a different radio band, the high RF energy of the transmission from a close antenna can “deafen” a receiver. This is a common cause of problems with wireless systems.
Because antennas are designed to transmit parallel to the ground rather than up or down, vertical separation between antennas is a lot more effective than horizontal separation. If mounting near another antenna cannot be avoided, then mounting it beneath or above the other antenna is better than mounting it beside.
Using different polarity to the other antenna (if possible) will also help to isolate the RF coupling.
Coaxial cable
If a coaxial cable connects to the antenna via connectors, it is very important to weatherproof the connection using sealing tape. Moisture ingress into a coaxial cable connection is the most common cause of problems with antenna installations. A three layer sealing process is recommended – an initial layer of adhesive PVC tape, followed by a second layer of self-vulcanising weatherproofing tape (such as “3M 23 tape”), with a final layer of adhesive PVC tape.
Allowing a “loop” of cable before the connection is also a good idea. The loop takes any installation strain off of the connection, and also provides spare cable length in case of moisture ingress into the connectors later; in which case the original connectors need to be removed, the cable cut back and new connectors fitted.
Avoid installing coaxial cables together in long parallel paths. Leakage from one cable to another has a similar effect as mounting an antenna near another antenna.
Lightning Surge Protection
Power surges (also known as “voltage surges”) can enter wireless equipment several ways – via the antenna connection, via the power supply, via connections to equipment connected to the device, and even via the “ground” connection.Surges are electrical energy following a path to “ground” or “earth”. The best way to protect the wireless unit is to remove or minimize the amount of energy entering the unit. This is achieved by “draining” the surge energy to ground via an alternate path.
Wireless devices need to have a solid connection to a ground point, normally a earth-electrode, or several earth-electrodes if the soil has poor conductivity.
A “solid connection” means a large capacity conductor, not a small wire. All other devices connected to the wireless unit need to be grounded to the same ground point. There can be significant resistance between different ground points, leading to large voltage differences during surge conditions. Just as many wireless units are damaged by earth potentials as direct surge voltage.
It is very difficult (put not impossible) to protect against direct lightning strikes, but fortunately, the probability of a direct strike at any one location is very small, even In high lightning activity areas. Unfortunately, surges in power lines can occur from lightning strikes up to 5 km away, and electromagnetic energy in the air from lightning activity long distances away can induce high voltage surges.
Antenna Connection
Electromagnetic energy in the air will be drained to ground via any and every earth path. An earth path exists via an antenna and the wireless unit. To protect against damage, this earth-path current must be kept as small as possible. This is achieved by providing better alternate earth paths.
Where an external antenna is used, it is important to ground the antenna to the same ground point as the wireless unit. Antennas are normally mounted to a metal bracket – connecting this bracket to ground is satisfactory. Surge energy induced in antennas will be drained via the grounding connection to the antenna, via the outside shield of the coaxial cable to the ground connection on the wireless unit, and via the internal conductor of the coaxial cable via the radio electronics.
It is the third earth path which causes damage – if the other two paths provide a better earth path, then damage can be avoided.
When an antenna is located outside of a building and outside of an industrial plant environment, external coaxial sure diverters are recommended to further minimize the effect of surge current in the inner conductor of the coaxial cable.
Coaxial surge diverters have gas-discharge element which breaks down in the presence of high surge voltage and diverts any current directly to a ground connection. A surge diverter is not normally required when the antenna is within a plant or factory environment, as the plant steelwork provides multiple parallel ground paths and good earthing will provide adequate protection without a surge diverter.
Connections to other equipment
Surges can enter the wireless unit from connected devices, via 1/O, serial or Ethernet connections. Other data devices connected to the wireless unit should be well grounded to the same ground point as the wireless unit.
Special care needs to be taken where the connected data device is remote from the wireless unit requiring a long data cable. As the data device and the wireless unit cannot be connected to the same ground point, different earth potentials can exist during surge conditions.
There is also the possibility of surge voltages being induced on long lengths of wire from nearby power cables. Surge diverters can be fitted to the data cable to protect against surges entering the wireless unit.
The same principle applies to 1/O device is not close to the wireless unit, the risk of surge increases. Surge diverters for 1/O wiring are available to protect the wireless unit.