Antenna Characteristics
Different antennas are needed for different applications. An open office space is very different to to a strip of closed offices. How do you cover a large open area such as a lobby? Covering an outdoor area is different than an indoor one and so on.
Radiation Patterns
Antenna gain is normally measured against an isotropic antenna, measured in dBi. This antenna only exists in theory though. It’s shaped like a tiny round point that when alternating current is applied, radiates a signal equally in all directions, in the form of an ever-expanding sphere.
The relative signal strength around an antenna, showed on a plot, is known as the radiation pattern.
The radiation pattern can be shown in a three-dimensional plot in form of a sphere where XY plane lies flat along the horizon and the XZ plane lies vertically along the elevation of the sphere. The first plane is referred to as H plane, horizontal, or also as the azimuth plane. The second one is known as the E plane, elevation.
Polar plots can also be used where concentric circles represent relative changes in signal strength as measured at a constant distance from the antenna. Normally the outermost circle represents the strongest signal strength, and the inner circles represent weaker signal strength.
Gain
Antennas don’t amplify the transmitter’s signal, they are passive devices. So what is gain? Gain comes from the antenna focusing the RF energy as it’s propagated into free space. The gain is then how effective the antenna is in focusing the energy in a certain direction.
Isotropic antennas radiate the energy in all directions equally, so it can’t have any gain.
Based on the above it’s logical that an omnidirectional antenna will have lower gain than a directional antenna. Think of the RF energy from an isotropic antenna as a perfect sphere, the energy from an omnidirectional antenna as a donut and the energy from a directional antenna as a carrot.
Beamwidth
Antenna gain is mostly useful for link budget calculations. The beamdwidth is more useful to show the antenna’s focus. Beamwidth is normally listed in degrees for the H and E plane.
The beamdwidth is determined by finding the strongest point on a polar plot. All the points on the plot where the signal strength is half of the strongest (-3 dB) are identified and lines are drawn towards these points. The angle is then measured and the angle could be 30 degrees in the H plane and 55 degrees in the E plane, for example.
Polarization
Remember that the signal leaving an antenna consists of two parts, an electrical field wave and an magnetic field wave. The electrical portion of the wave will always leave the antenna in a certain orientation.
Polarization is the electrical field wave’s orientation, with respect to the horizon. Vertically polarized antennas produce vertical oscillation and horizontally polarized antennas produce horizontal oscillation. There is also a magnetic field wave that is oriented at 90 degrees from the electrical field wave. The polarization in itself is not very important more than that it should match between the transmitter and receiver. If they don’t the signal may be poorly received. Cisco antennas use vertical polarization but the polarization may be affected if someone mounts the AP in a non standard way.
Antenna Types
Antennas are available in a variety of styles, shapes and radiation patterns. Some are intended for indoor use and some for outdoor, depending on weather resistance and mounting options. They are designed for a specific frequency range and need to be approved by a local regulatory body such as the FCC in the US. The two basic types of antennas are omnidirectional and directional.
Omnidirectional Antennas
The omnidirectional antenna is generally made in the shape of a thin cylinder. The signal is propagated equally in all directions away from the cylinder, but not along the cylinder’s length. This produces a donut-shaped pattern where the signal extends further in the H plane than the E plane. This antenna is suitable when a large area needs to be covered such as a large room or floor area. The antenna would be located in the center. The omnidirectional antenna has a low gain, due to spreading the RF energy in a broad area.
The dipole is a common type of omnidirectional antenna. Some dipole antennas can be folded up and down and some are rigid and fixed. There are two separate wires that radiate an RF signal when alternating current is applied across them. The gain is normally around +2 dBi to +5 dBi.
Dipoles are often used on APs mounted from ceilings in rooms or in hallways.
Monopole antennas are very short which make them a bit more aesthetically pleasing than dipoles. They contain only a short length of wire. The monopole has a metal ground plane where only part of the antenna is visible away from the wireless device. The radiation pattern is similar to the dipole but not as symmetrical. The gain is typically around 2.2 dBi in both the 2.4 GHz and 5 GHz bands.
To further reduce the size of the antenna and make the AP more aesthetically pleasing, many APs have hidden antennas inside the device’s smooth case. Gain is typically around 2 dBi in the 2.4 GHz and 5 dBi in the 5 GHz band for integrated omnidirectional antennas.
Antennas in mobile devices such as laptops and phones are tiny and generally have zero gain, or even negative gain!
Directional antennas
Gain is higher with directional antennas due to having the RF energy more focused in a specific direction. They are typically used in elongated indoor areas, such as the rooms along a long hallway or the aisles in a warehouse. It’s also common to use them offer coverage outside a building or long distances between buildings.
Patch antennas have a flat rectangular shape so that they can be mounted on walls.
The patch antenna produces a broad egg-shaped pattern that extends away from the flat patch surface. Gain is typically around 6-8 dBi in the 2.4 GHz band and 7-10 dBi in the 5 GHz band.
The Yagi antenna looks like a thick cylinder on the outside but the inside is made up of several parallell elements of increasing length. It produces a more focused egg-shaped pattern than the patch antenna. Since the energy is more focused, the gain is higher, typically around 10-14 dBi in the 2.4 GHz band.
Dish antennas use a parabolic dish to focus received signals onto the antenna mounted at the center. The parabolic shape helps waves arriving from the line of sight reflect onto the center antenna that faces the dish. Transmitted signals are aimed at the dish and travel away from the dish along the line of sight. The dish antenna has the most focused and narrow beam and hence the highest gain, between 20-30 dBi.
| Type | Style | Beamwidth H Plane | Beamwidth E Plane | Gain (dBi) 2.4 GHz | Gain (dBi) 5 GHz |
|---|---|---|---|---|---|
| Omnidirectional | Dipole | 360 degrees | 65 degrees | 2.2 | 3.5 |
| Omnidirectional | Monopole | 360 degrees | 50 degrees | 2.2 | 2.2 |
| Omnidirectional | Integrated | 360 degrees | 150 degrees | 2 | 5 |
| Directional | Patch | 50 degrees | 50 degrees | 6-8 | 7-10 |
| Directional | Yagi | 30 degrees | 25 degrees | 10-11 | |
| Directional | Parabolic dish | 5 degrees | 5 degrees | 20-30 | 20-30 |
Antenna Accessories
Sometimes the path link budget can’t be met between a transmitter and receiver. This could be due to the distance and the FSPL being too great. The transmitter may not offer enough power or the cable connecting the transmitter to the antenna is too long or introduces too much loss. The amplifier, an active powered device can provide additional gain as long as the EIRP does not exceed the maximum value allowed.
If the scenario is the opposite where the signal is too strong for nearby receivers, even with the lowest possible transmit power level, an attenuator can be used to absorb part of the energy between a transmitter and an antenna. The attenuator is a passive device.
Transmitters or receivers connected to outdoor antennas risk being struck by lightning which can introduce a tremendous amount of energy through the antenna. This can damage the WLAN equipment and portions of the network. A lightning arrestor can protect against lightning and sits inline between an outdoor antenna and a WLAN device. The lightning arrestor has two connectors that attach to the two ends of coaxial cable as well as a grounding lug, connecting to the nearest building or or electrical ground. The RF signal will pass through but sudden spikes of of electricity will be bypassed to ground. It will not prevent a direct lightning strike though but protects against static electricity discharges or transient voltage spikes during thunderstorms.