The radiation pattern created by an antenna depends on the physical characteristics of that antenna, for example the size and shape as well as the materials the antenna is made of. The pattern created is 3 dimensional but the way we draw it is by looking at 2 different views, the H-Plane and E-Plane. The H-Plane is the view from directly above the antenna, so the antenna is in the middle and the radiated pattern shows in front and behind the antenna, as well as left and right. The standard doughnut shape would look like a circle on the H-Plane. This is the horizontal plane, there is no vertical information shown just forwards, back, left and right.
The other plane is the E Plane (often referred to as the vertical plane). It is looking directly at the side of the antenna. So the view you would see is the radiation pattern upwards, downwards and in front and behind, nothing from side to side.
The strength of this pattern is given in dBi. A vendor will pick a reference point where the signal strength is strongest, and assign it a value of 0dB. The other points have -xdB values to show how much the signal is decreased in a given direction.
Polarization is the direction the wave moves and there are 3 options: Vertical (up and down), Horizontal (left and right) as well as circular (the wave circles as it moves forwards). Antennas can use any type of polarisation but it should the same on both ends to prevent signal degradation.
Diversity is implemented by placing multiple antennas on a device. When an AP receives a frame from a client it uses the preamble of the frame to test both antennas signal and then switches the rest of the frame to the antenna with the best signal. This also solves the multipath problem because multipath very rarely affects both antennas equally, so if multipath is affecting one antenna the other one is probably fine or at least not as badly affected.
Antenna types:
Omnidirectional antennas send a signal of the same strength in all directions. But note this is only on the H-Plane, this is why the classic shape is a doughnut, less signal is propagated vertically compared to horizontally. A high gain omnidirectional antenna takes this concept even further, squashing the doughnut down to increase the horizontal directions and reducing the vertical.
Dual Patch antennas are essentially 2 antennas placed back to back, the idea being to radiate in two directions, a good example of this placement would be in the middle of a corridor.
Semidirectional antennas focus the signal but not completely. It is still relatively broad coverage but aimed in a direction. Types of semidirectional antennas include patch and Yagi. Patch is half of the dual patch and Yagi is more focused.
Highly Directional antennas are very focused in a single direction, an example of this is Cisco's parabolic dish.
The accessories which can affect the EIRP include:
Attenuators - These are placed between the radio and antenna to reduce the dB.
Amplifiers - Amplifiers boost the signal and is known as active gain, rather than the passive gain on the antenna. These should be placed as close to the antenna as possible for maximum gain.
Lightning arrestors - These are used to protect the system components and connection back to the wired network from a lightning strike. Another option to installing a lighting arrestor is to install a small run of fibre cable between the AP and network because fibre cable doesn't conduct electricity. Lightning arrestors don't protect from a direct strike, the fibre cable will help more but it has to be at least 1m long.
Splitters - These are used to send signal out more than one antenna, or to receive signal from more than 1 antenna. The downsides of a splitter includes up to 4dB of loss introduced, and the throughput will be halved.
Cables and connectors - As well as increasing the flexibility of antenna placement cables and connections also add loss
No comments:
Post a Comment