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Wireless LAN Antennas FAQ'S

Point to Multi-Point

 
How do I know which wireless access point antenna to select for my outdoor WLAN / WISP?

This depends on how your subscribers or clients are located with respect to the access point and what type of terrain is in between. You can place an omni-directional antenna such as our Telex Wireless model 2439 (10 dBi gain) near the middle of your group of clients at a hub (Access Point) location. This works best if your facilities/customers are no more than 6 miles (9.5 km) from the hub and unobstructed by hills, trees or buildings. You may also select to use several sector antennas at an Access Point location. Our model 2443 (12 dBi 120 degree panel) or model 2444 (14 dBi 90 degree panel) wireless antennas work great for distances up to 12 miles (19.4 km) with clear LOS or up to 6 miles with some trees and buildings in the path. Greater distances may be obtained by using tower-mounted amplifiers with antenna heights above 100 feet HAAT. Whichever wireless antenna you choose, please make sure that it is Industry Canada or FCC certified with your radio!


How high should I place my outdoor wireless Access Point antennas?

This depends upon a lot of factors. If you have a building with roof access, this is usually the best option, since the feedline losses may be minimized if the equipment can be placed near the antennas. A minimum height is usually around 75 feet. This places the antennas above most trees. This height will also give a radio horizon of approximately 12 miles, assuming flat terrain. If you have taller trees, or tall buildings nearby, you may wish to use an antenna height of 200 feet or more. This gives a radio horizon of 14 miles. As towers may fall under local zoning ordinances, you may also wish to consider water towers, grain elevators or utility poles as other options. Placing Aceess Point antennas higher than 100 feet exposes them to greater amounts of interference, more feedline losses, zoning restrictions, FAA lighting requirements, and larger cell areas. Existing towers may be located using these sites - TelecomSiteSource, FCC Antenna Structure Registration and Wireless Radio Tower Locator. Grain elevators may be located using this site - Grain Elevator Locator.


What are the advantages of using sector antennas instead of an omni?

There are several good reasons to use sector antennas:
More capacity - By using 3 sector antennas on DSSS channels 1, 6 and 11 with 3 AP's, you can triple the number of clients in a given area.
Better signal levels - Sector antennas usually have more gain than omni's and can be mechanically downtilted to focus where the users are. This results in fewer retries and less packets lost. A WIPOP sector antenna will pay for itself if just one customer did not need an amplifier.
Channel Re-Use - Because the sector antenna can be downtilted, the signals are not thrown out to the horizon. This allows that channel to be re-used several miles away at a different cell site.
Eliminate interference - Because a sector antenna is directive and usually has good front-to-back (F/B), it can reduce or eliminate interference from sources that are behind the sector antenna.


Example of channel reuse

How do I hook up four 90 degree sector antennas on one tower?

Conventional thought says that there aren't enough non-overlapping 2.4 GHz DSSS channels to put 4 channels on one tower. Usually, panel antennas with high F/B are selected, and channel 1 antennas are placed on opposite sides (e.g. North & South) and channel 11 antennas are also placed on opposite sides (e.g. East & West). If separate access-points are used for all 4 antennas, the isolation may need to be increased between antennas on the same channel by spacing them farther from the tower face or on opposite corners of a building. FHSS systems may use separate frequency sets on each panel without problems.

However, there is new evidence that supports the use of DSSS channels 1,4,8 and 11 on the same tower. A white paper from Cirond Networks discusses this possibility. Also, check out this article from ExtremeTech. Isolation will need to be increased between antennas in this case by spacing them farther from the tower face, or by vertical separation of 10 feet or more.


What wireless antenna should I use to cover a small campus area of a few buildings?

If your coverage area is small with distance to the hub of less than a mile (1.6 km), a small omnidirectional antenna such as our 2426AA (5 dBi gain) or 2437AA (7.5 dBi gain) may be used. If the AP will be located on the edge of the campus, a 120 degree sector antenna such as our 2443AA 12 dBi panel antenna may be used.

What wireless antennas should I use for my clients (CPE)?

This depends upon the hub antenna, cable type and length, distance, data rate and terrain. You should test your system first before a final wireless antenna selection. For WISP systems using +36 dBm EIRP at the AP and clear LOS, use the following table as a guide:

Distance (miles)
CPE antenna gain
0.5 - 2
7 - 9 dBi
2 - 5
9 - 15 dBi
5 - 7
15 - 20 dBi
7 +
20 - 24 dBi