Feb 222014

An antenna affords a WiFi network with three basic properties: direction, gain and polarization. Gain is the increase in antenna power or the quantity of energy increase that an antenna appends to an RF signal. Direction is the pattern formed by the transmitted signal. The increase in gain of a directional antenna, leads to a decrease in the angle of radiation. This allows for a greater distance to be covered while reducing the coverage angle of the transmission. We measure the radiation patterns or the area of coverage in degrees. This angle is also known as a beamwidth.

An antenna does not add any power to the transmitted signal but only redirects the received energy from the transmitter. Redirecting the energy has the consequence of supplying more energy in a particular direction than others.

Beamwidth of a directional antenna is the separation angle between half power points in a radiated pattern of an antenna. This implies that there are only two beamwidths for a directional antenna: vertical beamwidth and horizontal beamwidth.

Front to back ratio of a directional antenna

Front to back ratio is the measure of directivity of a directional antenna. It is a ratio between the energy directed in a single direction, which is dependent on the radiation pattern to the wasted energy by the antenna. When you increase the gain of a directional antenna, you shall also be increasing the front to back ratio. The best front to back ratio of a directional antenna is normally 20dB.

Indoor effects on directional antennas

Refraction, reflection or diffraction in an environment affects the propagation of any wireless signal. Radio frequency waves take multiple directions between the receiver and the transmitter. Indoor transmission of radio frequency signals differ from outdoor transmission due to the presence of floors, ceilings and other obstructions that contribute towards the attenuation of signals.

Building materials also affect Indoor transmission of signals by directional antennas. The density of building materials has an effect of reducing the propagation speeds of RF signals. RF signals with shorter wavelengths get distorted and absorbed more as compared to signals of higher wavelengths.

The actual effects of antennas installed inside a room should be tested at the actual site.

Pros and cons of directional antennas

The use of directional antennas allows you to divert radio frequency signal energy in a single direction in order to increase the transmission distances. An increase in transmission distance reduces the beamwidth of the signal. You cannot cover bigger areas around the access point as the angular coverage is small. Mounting of directional antennas is at times hard due to the fact that you have to find a particular direction where signal strength is high.

Interference on a directional antenna

Interference in directional antennas may come from cordless phones, microwave ovens and radar signals from airports. Interference causes the antenna signals to sway at particular frequencies. This variation is known as fading. Fading varies with the frequency of the signal. Interference also increases the SNR of a signal. Antenna gain helps in the improving of SNR and interference to noise ratio.

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