Not trying to be pedantic here, but I think some clarification is needed regarding reflection, refraction, and diffraction.
Reflection is the phenomenon that occurs when an EM wave encounters a surface (larger than approximately one wavelength), hits it, and leaves the surface with reduced intensity. Same thing as light hitting a mirror. It's the big one for multipath. Refraction is the effect that "bends" an EM wave when it transitions between two different media with different permittivities. We see this effect when you put a stick in a pool of water, and the stick looks bent at the surface of the water. Diffraction (knife-edge diffraction) is the phenomenon that allows EM waves to travel around objects. It's the reason you can get an AM radio signal in a valley obstructed by mountains on all sides, the mountaintops diffract the signal and "fill" the valley with RF. It's my understanding that this phenomenon is not really worth considering at the frequencies we're using. Hope that helps! Patrick Shoemaker Vector Data Systems LLC [email protected] office: (301) 358-1690 x36 http://www.vectordatasystems.com Marlon K. Schafer wrote: > Glad to be of help Forbes! Once in a while I make a lucky guess. grin > > OK, design note time. > > I work to keep all of my links in the -65 to -75 range. If you look up the > specs for your radios you will likely find that this is well above the > lowest receive signal level, but not too far above it. > > Why is this important? As I've been taught over the years (Thanks Jaime, > Bob and others) most knife edged refraction (a bounced signal off of an > object) will be about 30 dB down below the original signal level. > > So if your radio has a max sensitivity of -94 and you have a -65 signal > you'll be almost out of the possibility of the receive radio even picking up > your refracted signal. Think of the refraction as a reflection or echo. An > echo makes it really hard to understand someone. Multipath is the same > thing. But if you can make the echo so quiet that you can't hear is it > won't hurt anything, even if it's there. > > MOST of my towers put out LESS than 1 WATT!!!!! A few are still close to 4 > watts, but changes in AP's allow me to run ever lower tx powers. Believe it > or not I have customers getting over 2 megs of service from systems that are > less than 2 watts and at ranges of 18 miles! Yes I can prove this if anyone > wants to come visit.... > > If you are picking up your towers at levels above -60 to -65 you'll have a > LOT more trouble on your network. Especially tower to tower. Turn the > danged things down. If you need more power at a customer's place install a > bigger antenna! > > Over the years I've run into many companies that try to use a bigger stick. > They systems never work well for long. The more customers go on it the > worse that problem gets too. I fact I have started pulling customers from a > competitor in the area. He's running a system near me that looks to be > running about 42 watts (remember we're only allowed 4!!!!). Why am I > getting his customers? Because his system doesn't work right. Much of that > is due to his design flaws. > > OK, next question is, how do I know what he's running? All you need to know > is what gain the antenna is at your end, what the receive signal is and how > far the transmitter is away from you. From there it's easy with a formula. > At 42dB I have him at about a 15dB antenna and 1 or so watt TX power. A > VERY common config sold by some distributors. It's too bad, these never > work well long term and rotten wisp networks give us all a bad name. > > Anyway, Forbes, try turning that power down. WAY down. I'll bet you can go > back to g mode and have even more stability than you have now. > > laters, > marlon > > ----- Original Message ----- > From: "Forbes Mercy" <[email protected]> > To: "WISPA General List" <[email protected]> > Sent: Wednesday, May 20, 2009 1:47 PM > Subject: [WISPA] Thanking Marlon and multipath experience > > > I admit that most of you can dance around me in the field of RF. I'm more > of an installer and management type. Today I had a Nano CPE with a -48 dbi > signal which, as you know, is amazing; it should be it was a block away from > the tower. The problem is the customer had long outages and erratic > service. Aiming away from the tower kept the great connection but still > crappy pings. > > Marlon suggested Multipath as a possibility and advised to turn the radio to > "B" only and reduce the power on the radio. I had heard of Multipath but > never experienced it. We changed both settings and while the signal stayed > at -48dbi the time-outs and erratic pings went away. 90% of our system is > two or more miles from towers, so we never had this problem before. This > particular town is the only place where the towers are on buildings all > within ½ mile of nearly everyone > > > It answered a lot of problems for others in town we have been battling > supposed interference with and I just wanted to publically thank Marlon for > exposing me to a side of RF I hadn't even thought of. This is what makes > WISPA worth it to me. > > Forbes Mercy > President - Washington Broadband, Inc. > > No virus found in this incoming message. > Checked by AVG - www.avg.com > Version: 8.0.237 / Virus Database: 270.10.25/1956 - Release Date: 02/16/09 > 18:31:00 > > > > > -------------------------------------------------------------------------------- > WISPA Wants You! 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