Selecting the right antenna will save a lot of heartaches down the road
Dual-polarity parabolic antenna
In broadband wireless, there are many types of antennas available to suit the requirements of the application. The primary aspect of antenna selection is usually gain. But equally important criteria include such things as beamwidth, side and rear lobe rejection, cross-polarity isolation, and VSWR (voltage standing wave ratio dud). Antennas also come in single and dual-polarity models. The newer dual-polarity antennas are used in MIMO (multiple-input–multiple output) technology where there are actually two transceivers in each radio–one connected to the vertical polarity of the antenna, the other to the horizontal polarity. Click here for a short primer on antennas courtesy of KBT Communications, Inc. By transmitting and receiving using two radios on the same frequency–just on opposite polarities, MIMO wireless devices can actually achieve twice the throughput compared to a single radio of the same channel bandwidth.
Dual polarity antennas as it turns out are not all created equal. We tested a number of dual-polarity parabolic antennas that showed very good specifications. But some of the test candidates flunked our tests miserably. The throughput performance and error-rate are highly dependent on the amount of cross-polarity isolation of the antenna or antennas in the case of two physical antennas with their feeds polorized opposite one another. The minimum isolation should be at least 22 db, perferably 28 db or even better. High quality antennas are generally going to cost more, but the performance payoff is worth the difference.
Antennas are available in several different physical shapes, types, and sizes. Flat panel antennas have become very popular in broadband wireless due to their relatively small “faceprint”. Flat panel antennas are rectangular or square and generally have gains in the 21dbi(approx. 12″x12″) to 28 dbi (24″x24″) range. They can be an attractive choice since they don’t require radomes for harsh environments unlike parabolic antennas and “blend” more easily with surrounding architecture. Beamwidths of flat panel antennas in the 21 to 28 dbi range are generally from 9 degrees to 4.5 degrees respectively. Beamwidths (patterns) and passive gain are mutual. Simply put, high-gain antennas achieve their gain by concentrating RF energy in a narrow pattern or beam. The narrower the pattern, the higher the gain.
Grid antennas usually consist of a curved or parabolic reflector made of light-weight tubing or wire mesh. Grid antennas are tuned to reflect and focus RF energy to the feed assembly whereas parabolic antennas are usually solid metal. While grid antennas present a much lower wind-loading factor, they generally offer less gain in a given size compared to solid parabolic reflectors.
Some new generation broadband wireless bridges are incorporating a small flat-panel antenna ( 12″x12″) with the active transceiver components making for a very compact unit. The feed from the unit is usually fast Ethernet with power coming via a POE injector located inside. The gain of the “integrated” antenna units is in the 18 to 21 dbi range with beamwidths of 9 degrees.
If you don’t know which antenna is best suited for your project, contact a specialist or drop us a comment. We’ll be happy to respond to any questions or comments you may have regarding this or any other post on this site. We’ve tested a number of different types for our own clients and will be happy to share the results.