I'd argue none of the 3 above. For 900Mhz and 2.4Ghz (because of wider Verticle beamwidths) I'd advise.... Downtilt until your mainlobe (3db mark) edge is no longer going beyond your maximum intended coverage range, or a bit more, if you can afford further isolation from the potential adjacent cell interference source.
The primary goal is to keep harmfull interference from shooting off to the horizon (ultimately protecting your network since receive gain/beamwidth is reciprocal). How much tilt is required to do that, depends on the antenna beamwidth, and the height that you decide to install at, for what ever reason. In 5.8G its less critical because you can make up for it on CPE side antenna side, and ther eare much fewer noise sources at 5.8Ghz. That is until your 5.8G area gets congested :-( But because 5.8G may have smaller verticle beamwidths, the nearfield potentially could be more of a concern, that the other freqs. Tom DeReggi RapidDSL & Wireless, Inc IntAirNet- Fixed Wireless Broadband ----- Original Message ----- From: "Michael Baird" <m...@tc3net.com> To: "WISPA General List" <wireless@wispa.org> Sent: Tuesday, March 10, 2009 8:53 AM Subject: Re: [WISPA] 900 Downtilt at 300' > I'm following this thread trying to pick up a general rule of thumb to > start out with for down tilt (I have 2.4 systems though). > > I've seen three different methods of calculating it specified. > > 1). Downtilt enough so all your customers fall within the main lobe's -3 > db line. > 2). Downtilt so that your beam is focused half-way to your recommended > coverage. > 3). Downtilt so that you reach the furthest customer in your projected > coverage area. > > These all seem mutually exclusive, unless I'm missing something. > > Regards > Michael Baird >> I disagree, Precise Tilt does matter quite a bit with Tiltek 900Mhz >> sectors. >> >> Let me give an example of mounted at 400ft with Tiltek sectors having 17 >> deg >> vert beamwidth. >> >> 8 deg downtilt, min .25 miles, max horizon. >> >> 9 degree downtilt, min .24 miles, max 8.6 miles. >> >> 10 degree downtilt, Min distance .22 miles, Max distance 2.8 miles. >> >> 11 deg downtilt, min .21 miles, Max 1.7 miles >> >> Near field coverage is rarely a problem with 900Mhz, regardless of the >> tilt. >> But what people forget is how much the far field is effected by just a >> single degree. >> >> The difference between 9 versus10 degrees is the difference of "5 miles >> !" >> coverage at optimal signal strength. >> The difference between 8 versus 10 degrees is the difference of whether >> you >> interfere with your other towers 30 miles away versus 3 miles away. >> >> With 900Mhz, EVERY DB counts. The reason is two fold. 1) The noise floor >> is >> ften high. 2) Its very easy to get colocated AP antenna self >> interference, >> when foliage can degrade the signal of a single link severally. For >> example, >> the Front-to-back isolation loss could be equivellent to the loss of >> foliage >> in a path. >> >> The goal is to get the highest signal uniformally to the largest area >> within >> your desired coverage area. Then you can always lower CPE transmit power >> as >> needed on links without foliage loss. In my 900 deployments, I have >> found >> that 3db lost or gained can be the difference between a typically good >> versus bad link. >> >> Now, its true the above beamwidths are only the distances that show "3 >> db" >> loss, so a 10 degree downtilt, sector will still have a significant >> amount >> of signal going out to and heard from the the horizon. But every DB >> counts. >> >> The critical question becomes do you mount high or not? Higher avoids >> more >> trees. HIgher hears more interference. We found what was best for us was >> to >> go higher, but add more downtilt. We shoot for 10 degree downtilt. But it >> can be a delicate balance, dependent on your environment and noise levels >> and locations. We will usally put a larger focus on reducing noise to our >> adjacent cell sites, even if at the cost of gain to our intended coverage >> area. >> >> Tom DeReggi >> RapidDSL & Wireless, Inc >> IntAirNet- Fixed Wireless Broadband >> >> >> ----- Original Message ----- >> From: "Cliff Olle" <w...@eccentrixtechnologies.com> >> To: "'WISPA General List'" <wireless@wispa.org> >> Sent: Monday, March 09, 2009 9:25 PM >> Subject: [WISPA] 900 Downtilt at 300' >> >> >> >>> For the 900 Mhz connectorized AP (by cyclone) with the 120 tiltek >>> antenna, >>> if I am mounted at 300', what amount of down tilt is normal? >>> >>> >>> >>> -------------------------------------------------------------------------------- >>> WISPA Wants You! 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