I have used several antennas of know gain as a comparison to get a estimated gain on antennas that I have built. I also was brave enough to make one of the SWR meters, which will help indicate the health of the entire RF system. That is the link, the rest of the message is a quote that summarizes my approach as well.
http://pe2er.tmfweb.nl/wifiswr/ Without the proper (multi-thousand dollar) tools, how can you estimate antenna performance? This is one method I gleaned from Mr. Rob Flickenger Using the Link Test software that comes with the Orinoco silver cards, you can see the signal and noise readings (in db) of a received signal, and your test partner's reception of your signal. As I happen to be 0.6 mile LOS from ORA headquarters, with very little noise on the channel between, we had a fairly controlled testbed to experiment with. We shot at the omni on the roof, and used the access point at ORA as our link test partner. To estimate antenna performance, we started by connecting commercial antennas of known gain, and taking readings. Then, we connected our test antennas and compared the results. We had the following at our disposal: two 10db, 180 degree sector panel antennas one 11db, 120 degree sector panel antenna one 24db parabolic dish a couple of Pringles cans and some tin foil Here were the average received signal and noise readings from each, in roughly the same position: Antenna Signal Noise 10db A: -83db -92db 10db B: -83db -92db 11db: -82db -95db 24db: -67db -102db Pringles can (shotgun): -78db -99db Pringles can (internal): -81db -98db The test partner (AP side) signal results were virtually the same. Interestingly, even at only 0.6 mile, we saw some thermal fade effect; as the evening turned into night, we saw about 3db gain across the board (it had been a particularly hot day: almost 100 degrees. I don't know what the relative humidity was, but it felt fairly dry.) Yagis and dishes are much more directional than sectors and omnis. This bore out in the numbers, as the perceived noise level was consistently lower with the more directional antennas. This can help alot on long distance shots, as not only will your perceived signal be greater, the competing noise will seem to be less. More directional antennas also help keep noise down for your neighbors trying to share the spectrum as well. Be a good neighbor and use the most directional antennas that will work for your application (yes, noise is everybody's problem.) When trying to aim a yagi (like our little can), keep in mind that they have large side lobes that extend up to 45 degrees from the center of the can. Don't point directly at where you're trying to go, aim slightly to the left or the right. We also found that elevating the antenna helped a bit as well. When aiming the antenna, hold it behind the connector, and SLOWLY sweep from left to right, with the Link Test program running. When you get the maximum signal, slowly raise the end of the can to see if it makes a difference. Go slowly, changing only one variable at a time. Remember that the can is polarized, so match the phase of the antenna you're talking to (for example, if shooting at an omni, be sure the element is on the bottom or the top of the can, or you won't be able to see it!) You can use this to your advantage to try to eliminate some noise on a long distance link: slowly turn both ends of the link from vertical through horizontal, and stop at the point that you see the most gain (and lowest noise.) -- NYCwireless - http://www.nycwireless.net/ Un/Subscribe: http://lists.nycwireless.net/mailman/listinfo/nycwireless/ Archives: http://lists.nycwireless.net/pipermail/nycwireless/
