How to Pick The Perfect Antenna—continued

There are a lot of different types of antennas out there. For the most part, the antennas pictured are the ones you are most likely to use. But please note that the radiation pattern samples are examples only and should not be thought of as the actual patterns for the different antennas. If you have a long link you will want a high gain—narrow focus—antenna. These antennas will usually be grid or solid dish antennas.

For short links you'll want to use just enough gain to give you the operating margin—extra power—that you will need in relation to typical weather patterns in your service area. You may need to modify that if you have to go through a tight spot. Use some smaller cable or a longer cable run to drop the signal down to where you need it.

For your broadcast site—commonly called a Wireless Point of Presence (WPOP) or Point to Multi-Point (PtMP)—use the type of antenna that covers the area your customers will be in. If you'll be putting your WPOP in or near the middle of your customer base and you're only going to have one Access Point (AP) or broadcast radio at your WPOP, an omni antenna should work fine. However, there are a few points to consider:

• Take care not to use an antenna that has a vertical beam width, which will shoot right over the top of your customers.
• If your WPOP is on a 500-foot structure and the closest customer will only be a few hundred feet away, a 15dBi omni with a 10deg vertical pattern (5deg above and 5deg below the centerline) would not be a good choice of antenna.
• Make sure that you don't go too high on your power levels. A 500mw amplifier is 27dB and if you add that to the 15dB of the antenna you'll be at 42db. Just a shade over the 36dB allowed by the FCC! So, please note—every 3dB is 2 times or 1/2 the power in terms of watts.

When using multiple APs on a tower or if your customers will all be in one direction, you should use sectored antennas. Think of them as a panel of antennas that have a very wide pattern in one plane and very narrow pattern in another. As an example look at this 15dB 90deg panel antenna. The H-Plane is the pattern as you look down on the coverage area, E-Plane would be looking at the pattern from the side. You can tilt down these antennas so that none of the coverage is wasted up in space where there are no customers.

If you have the budget for it these are a very nice way to go as they don't waste signal or put it where you don't want it to go. They can be had with just about any dispersion pattern, both E and H planes, and and there are even adjustable units available.

At the customer site Customer Premises Equipment (CPE) is a fixed wireless system that is thought of as a Point to Point link. There is a special formula to figure out exactly how much power is allowed. But for all intentsive purposes, 48dBm ERP (Effective Radiated Power) will work a-okay. This allows you to more easily use many different antennas at the CPE site.

Your customers won't be concerned about power levels, they worry about the aesthetics. Many customers don't want big grid antennas on their houses, so flat panel antennas are often used. Panel antennas also have smaller side lobes and wider main lobes—a lobe is a high power area, or the peaks on the charts above, making them better at eliminating multi-path interference and easier to aim.

Some panel antennas come as a device that looks like a plastic box. These can be painted to match the structure—helping to keep Mama happier with Dad's new high speed Internet service. Of course, you could put your WISPs logo and phone number on them and do some advertising! But whatever you do, do not cover them with a metallic paint, for rather obvisou reasons.

—End

Part 1: How to Pick the Perfect Antenna	Part 2: Big Fun With The Technical Stuff
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Related articles:
	[Feb. 5, 2001]	Wireless LAN Primer
	[Dec. 15, 2000]	FW: A Method For Broadband Internet Access
	[Oct. 11, 2000]	Lightning and the Wireless Antenna