Choosing a Antenna for 2.4 ghz – Mesh Network
Net Control Station
April 8, 2019
Microwaves are different than UHF/VHF/HF-When you increase transmit power on a mesh node, you can’t get the same help hearing the incoming signals.
Directional antennas narrow the beamwidth to boost signals on axis while concurrently rejecting unwanted signals and noise off axis.
Behavior of Radio Waves
There are a few simple rules of thumb that can prove extremely useful.
When making first plans for a wireless network or Mesh Network:
• the longer the wavelength, the further it goes;
• the longer the wavelength, the better it travels through and around things;
• the shorter the wavelength, the more data it can transport.
Longer waves travel further
Waves with longer wavelengths tend to travel further than waves with shorter wavelengths. As an example, AM radio stations have a much greater range than FM stations, which use a frequency 100 times higher.
Lower frequency transmitters tend to reach much greater distances than high frequency transmitters at the same power.
Longer Waves Pass Around Obstacles
The distance a wave can travel depends on the relationship between the wavelength of the wave and the size of obstacles in its path of propagation. It is harder to visualize waves moving “through” solid objects, but this is the case with electromagnetic waves. Longer wavelength (and therefore lower frequency) waves tend to penetrate objects better than shorter wavelength (and therefore higher frequency) waves.
Note: Metal will stop any kind of electromagnetic wave.
Line of sight
The term line of sight, often abbreviated as LOS, is quite easy to understand when talking about visible light: if we can see a point B from point A where we are, we have line of sight. Simply draw a line from A to B, and if nothing is in the way, we have line of sight.
Things get a bit more complicated when we are dealing with microwaves.
Remember that most propagation characteristics of electromagnetic waves scale with their wavelength. This is also the case for the widening of waves as they travel. Light has a wavelength of about 0.5 micrometers, microwaves as used in wireless networking have a wavelength of a few centimeters. Consequently, their beams are a lot wider – they need more space, so to speak.
Lower frequencies (longer wavelengths) allow it to permeate objects and utilize diffraction much better than comparable 2.4 GHz or 5 GHz transmissions.
Directivity and Gain
Directivity is the ability of an antenna to focus energy in a particular direction when transmitting, or to receive energy from a particular direction when receiving.
If a wireless link uses fixed locations for both ends, it is possible to use antenna directivity to concentrate the radiation beam in the wanted direction.
When considering antennas suitable for 2.4 GHz WLAN use, another classification can be used:
Access points tend to make point-to-multipoint networks, while remote links or backbones are point-to-point. Each of these suggest different types of antennas for their purpose. Nodes that are used for multipoint access will likely use omni antennas which radiate equally in all directions, or several sectorial antennas each focusing into a small area. In the point-to-point case, antennas are used to connect two single locations together. Directive antennas are the primary choice for this application.
A brief list of common type of antennas for the 2.4 GHz frequency is presented now, with a short description and basic information about their characteristics.
A basic Yagi or more properly Yagi-Uda antenna consists of a certain number of straight elements, each measuring approximately half wavelength. The driven or active element of a Yagi is the equivalent of a center-fed, half-wave dipole antenna.
Parallel to the driven element, and approximately 0.2 to 0.5 wavelength on either side of it, are straight rods or wires called reflectors and directors, or simply passive elements.
A reflector is placed behind the driven element and is slightly longer than half wavelength; directors are placed in front of the driven element and are slightly shorter than half wavelength. A typical Yagi has one reflector and one or more directors.
The antenna propagates electromagnetic field energy in the direction running from the driven element toward the directors, and is most sensitive to incoming electromagnetic field energy in this same direction.
The more directors a Yagi has, the greater the gain. Yagi antennas are used primarily for Point-to-Point links, have a gain from 10 to 20 dBi and a horizontal beamwidth of 10 to 20 degrees.
The horn antenna derives its name from the characteristic flared appearance. The flared portion can be square, rectangular, cylindrical or conical. The direction of maximum radiation corresponds with the axis of the horn.
It is easily fed with a waveguide, but can be fed with a coaxial cable and a proper transition.
While it is cumbersome to make a horn antenna at home, a cylindrical can with proper dimensions will have similar characteristics. Horn antennas are commonly used as the active element in a dish antenna. The horn is pointed toward the center of the dish reflector.
The use of a horn, rather than a dipole antenna or any other type of antenna, at the focal point of the dish minimizes loss of energy around the edges of the dish reflector.
At 2.4 GHz, a simple horn antenna made with a tin can has a gain in the order of 10 dBi.
Antennas based on parabolic reflectors are the most common type of directive antennas when a high gain is required. The main advantage is that they can be made to have gain and directivity as large as required.
The main disadvantage is that big dishes are difficult to mount and are likely to have a large wind load. Randomes can be used to reduce the wind load or windage, as well as for weather protection.
Dishes up to one meter are usually made from solid material. Aluminum is frequently used for its weight advantage, its durability and good electrical characteristics.
Windage increases rapidly with dish size and soon becomes a severe problem.
Dishes which have a reflecting surface that uses an open mesh are frequently used. These have a poorer front-to-back ratio, but are safer to use and easier to build. Copper, aluminum, brass, galvanized steel and steel are suitable mesh materials.
The BiQuad antenna is simple to build and offers good directivity and gain for Point-to-Point communications. It consists of a two squares of the same size of 1⁄4 wavelength as a radiating element and of a metallic plate or grid as reflector. This antenna has a beamwidth of about 70 degrees and a gain in the order of 10-12 dBi.
It can be used as stand-alone antenna or as feeder for a Parabolic Dish. The polarization is such that looking at the antenna from the front, if the squares are placed side by side the polarization is vertical.
Log Periodic Antennas
Log periodic antennas have moderate gain over a wide frequency band, They are often used in spectrum analysers for testing purposes and are also popular as TV receiving antennas since they can efficiently cover from channel 2 up to channel 14. These antennas are used in White space devices that require the ability to work in widely different channels.
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