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What is the best type of Antenna ?

Posted by John Barker on January 06, 2015 0 Comments

A lot of customers ask us that question regarding an outdoor hot spot installation they are planning. Well, the answer depends on the characteristics of the site, and where the antenna will be located. The best type of antenna will certainly not be an omni-directional antenna although omnis are very popular. We stock one high gain omni for customers who insist, but we stock a lot more patch and sector antennas. A sector array is appropriate for many outdoor hot spot sites.

Lets see first of all why an omni-directional antenna has limitations for many outdoor hot spot applications. The diagram below sums up the main problem

 

 

The omni-directional antenna radiates electromagnetic energy at a right angle to the antenna. The field strength is 360 degrees around the antenna. If the antenna is mounted on a mast then the field strength is way above users. If the antenna is mounted at ground level then obstacles between the antenna and the user will attenuate the signal strength.

The ideal antenna for the outdoor application shown in the diagram is a down-tilt omni directional: there are no obstructions between the user and the antenna, and the user is in the path of maximum field strength. You can buy a down-tilt omni-directional antenna: it is constructed using sectorized antennas. Another name for the down-tilt omni-directional antenna is a sector array. All cellphone towers are down-tilt omni-directional antennas: usually built using three sector antennas, each with 120 degrees of arc. Three antennas give 360 degrees of arc.
The unique characteristic of the sector antenna is the electromagnetic radiation pattern. The diagram below attempts to show the radiation pattern.

The horizontal radiation pattern is determined by the antenna design. Sector antennas are constructed for 90 degrees, 120 degrees and 180 degrees; therefore 4, 3 and 2 antennas respectively are needed to provide a 360 degree radiation pattern.

The vertical radiation pattern is determined by the gain of the antenna: a 14dBi antenna will have approximately 10 degrees vertical angle between the –3dB points, and a 20dBi antenna will have approximately 7 degrees vertical angle.

If the hot spot antenna is going to be placed in the center of the area of coverage like a cell phone tower, then it makes sense to have 360 degrees of coverage. Most hot spot applications however require a much smaller arc of coverage.

Lets take one example: we have customers who install hot spots in marinas. The sector antenna needs to be mounted as high as possible, this ensures that the boats closer to the antenna don’t block the access of the boats at the furthest point of the marina. In many cases the highest point in a marina is a dry stack storage building, where boats are taken out of the water for the winter and stored. A dry stack building can be 50 or more feet high and is usually located in one corner of the marina. Marinas usually approximate to a rectangular shape. Take a look at the diagram below.

The sector antenna can be mounted on the corner of the dry stack building shown. The correct arc of coverage will be 120 degrees for this marina.  The antenna should be mounted as high as possible and then tilted down to provide a good signal strength for the farthest boat. The sector antenna installation is shown in the diagram below.

The access point that powers the antenna should be selected for power output and receiver gain depending on the size of the marina. An outdoor access point should be should be mounted close to the antenna to avoid losses in the RF cable. A short piece of LMR-400 cable with two N connectors can attenuate the signal 3 or 4 dB (i.e. reduce the signal strength below half!!)

For the marina shown in the diagram, the access point can have +26dBm or +28dBm output and provide good coverage. For a larger marina it will be necessary to have an amplified access point with an output of +30dBm and a receiver amplifier that provides an additional 20dB of gain.

We know of a number of marinas that have used this type of installation very successfully. Customers are happy with the area of coverage and the cost for the marina owner is reasonable.

The principles described above can be applied to many other situations such as inside a large airport terminal building, or coverage of a large public park.

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