[Cu-wireless] Summary of antenna test results
pfolk at gargtech.com
Mon Nov 12 15:00:38 CST 2001
We had two each of four types of antennae in addition to the built-
in ones: pringles-can-based, 90cm helical (made by myself and Dave
Young), 55cm helical (made by NCSA), and parabolic grid antennae
(manufactured commercially). The pringles-can took about $10 and
1/2 hour to make, the helical cost about $15 and took 4 hours to
make. The parabolic grid cost about $59.
The way strength is measured is signal-to-noise ratio, in decibels
(for signal strength S and noise level N, SNR = 10*log(S/N)). That
means a change of ~3 (actually 10*log(2)) indicates a 100% change
in actual signal strength. Anything above about 2db will provide a
connection, but below about 12db it will be slower, and below 15db
it seems pretty flaky. Someone walking in the path of the signal
near the antenna causes about a 3db drop.
The best, by far, was the parabolic grid (looks like a rectangular
2'x3' sattelite dish made of wire). At a distance of 600m, we got
slightly less than 50db---a very strong signal. With the normal
antenna, you get that at 10-15'. But the pringles can was amazing!
At 1300m, we got about 25db, putting it quite a better than the hel-
icals, and definitely good for a multi-mile link. The helicals put
in a rather pitiful showing; NCSA's provided 15-18db and ours pro-
vided 17-20db at 600 meters. This may be due to our rather crude
aiming methods---we should give them at least one more chance.
The grids were impressive, but by dollar and minute spent the
pringles cans were spectacular. Our next goal is to try to find
the maximum distance we can get, which requires finding the long-
est line of sight we can.
I bought the parabolic grids from <http://www.electro-comm.com/>
(download the PDF catalog). If they want to charge you the MSRP, let
me know and I'll sell it to you at dealer cost. Our pringles cans
were based on <http://www.oreillynet.com/cs/weblog/view/wlg/448>. You
can get most of non-antenna-specific the hardware from DoIt in down-
town Champaign (w/ a 15% discount if you're a student), but I have
plenty of extra copper tubing, a few extra all-thread pieces, and a
tubing cutter I can lend, so don't get those =)
p.s. Dave has more details wrt the performance numbers if people want
More information about the CU-Wireless