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.)
--
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