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How to Pick the Perfect Antenna, part 2 April 18, 2001 All right, it's time to get back to the technical stuff. I'm not going to cover Fresnel zone issues here, because this has more to do with your mounting location for an antenna than the equipment you choose. Just remember this, Fresnel zones vary greatly depending on what frequency you're using. However, if you need a good Path Clearance Analysis Tool, Western Multiplex has put together a helpful calculator I frequently use. Water in the air What you do need to understand is how to determine what power levels are needed so that you can produce the right fade margins for your path and build a system that connects strong signals. While snow and hail are usually not a problem for fixed wireless links, rain does produce a measurable effect on signal strength. According to my BreezeCOM manual, rain falling at a rate of 0.6-inches per hour (150mm/h) can cause additional signal losses of about 0.02dB per .6 miles (1Km) between links. So in a hurricane you'll loose an extra 0.4dB on a 10 mile link if you're operating in 2.4 GHz unlicensed spectrum. Until you start trying to connect very long links or start using much higher frequencies, rain is not really much of an issue—just another factor that needs to be accounted for, when it comes to potential interference. To calculate actual signal loss in free space—a fancy term for air, contact your fixed wireless equipment vendor. I've done some testing and with normal 802.11-type gear and have found that with a 36dB maximum power setup at my WPoP I can establish stable links by using 16dB panel antennas out to 9- or 10-miles (15Km or so). Past this point, I have used high quality 24dB grid antennas. One of the best ways to learn about antennas and how they work in a fixed wireless system, is to complete a few installations for your customers. All the same, somebody like me can share some valuable insight—that is, learn from my success and a few miscalculations that occurred on the way to creating fixed wireless victories. Personal best My longest link established off from a 36dB WPoP out to an unamplified customer site is 16-miles. I've provided about a 256kbps connection speed by using 2meg 802.11 Direct Sequence Spread Spectrum (DSSS) equipment, which including a third hop created a total signal path about 24 miles long (38Km). The link has remained perfectly stable with deep snow cover or bare ground. But, the connection is relatively new, so it hasn't experienced a hot spell and I don't yet know what effect heat inversions may have on the path. But I expect I will find out what heat does to the links signal strength soon enough, as I've always feared that the antennas are not as high as they should be on either end of the link. Hit the roof I recently completed a new customer installation that needed an amplifier. I had to place the gear on the client's crank up TV antenna tower and we were not able to point the it toward the exact area that we thought the tower was—the tower was too far away to see. I installed the amp and aimed the antenna—trying to pick up the best possible signal. I was using a 24dB grid antenna with a signal of -78dB. The signal improved noticeably when I came down a couple of feet with the tower—a common indication of multipath interference when the transmitted signal arrives at the receiver from different directions. I had to build a special mounting bracket for his antenna so that it could be aimed accurately. I mounted the antenna on the tower about 2 or 3 feet (approximately 1 meter) below where it had been originally installed, and pointed the antenna in the same direction I had setup before. Next, I moved the antenna a little closer to where I thought the tower was. To my surprise—the signal went down! After cranking the tower down a few times I finally climbed to the peak of the too-steep-for-me roof and took a look-see at where the antenna was pointed. From my bird's eye view, I finally saw that the antenna was pointed at least 2-miles West of where it should be. I adjusted the antenna and pointed it more in-line with the broadcast antenna location. Basically, I ended up with a stronger signal without using the amplifier, than when I had tried to use it to strengthen the signal. I'd been aiming the antenna at one of its side lobes! That's just another reason to use high-quality antennas with low side lobes—it could have kept me from having to climb up that roof.