Bruce wrote in
:
On Wed, 19 May 2010 09:32:26 -0700, John Navas
wrote:
On Mon, 19 Apr 2010 22:28:25 +0200, "Steve Lusardi"
wrote in :
As I stated Bruce, the TV antenna is a good plan. It will be a true
parabola and the gain over what you have will be significant. Don't
forget about the 30 degree included angle that is built in. You will
have to move the detector up to the center for your application.
The shape of the antenna is not critical -- in practice a typical
curved bowl will work pretty much the same as a carefully constructed
parabola.
Since I wrote the original I have done more research and it appears
that the current favorite is the offset feed dishes. One article has a
method of calculating the reflection angle and then cutting a string
for the far side of the dish and a second for the near side and mount
the feed at the point that the two strings meet.
Also I found several articles abut the construction of double quad
antennas and one of the articles showed the graphs of test with 12 db
gain.
I may change to the quad if I can get that much gain from a smaller
antenna as none of the tests I saw indicated that the dish produced
really astonishing gain.
Cheers,
Bruce
(bruceinbangkokatgmaildotcom)
All of this discussion is MOST commendable, however it is also most
moot. All the gain in the world isn't going to overcome the natural
signal to noise ratio problem of inverse square law propagation from the
router's antenna against the intense solar and thermal noise of a sunny
day and a hot parking lot. The routers were specifically designed to
limit range to approximately 100 meters by reducing their power output
to a pittance, like your sellphone. Some routers only run 10-20mw into
horrible antennas made of a piece of pc board. The best ones only run
200mw tops into a 3db whip with space diversity receivers to hear your
20mw powerful beast coming back to them.
Highly directional antennas, just as with UHF analog TV and its
"ghosting" problem, do help reduce multipath propagation IF the antenna
is very tight patterned with very little back pattern, such as the
Pringle's Cantenna we've been building for years.
http://www.turnpoint.net/wireless/has.html
http://www.turnpoint.net/wireless/cantennahowto.html
These waveguide antennas have no rear radiation as the back of them is
solid metal. Antennas designed around HF, VHF even UHF are of little
use on microwaves, however cute. Waveguide antennas are used for radar
for a reason.
http://www.oreillynet.com/lpt/wlg/448
As you can see, their signal to noise ratio is quite impressive, much
more impressive than an open HF/VHF/UHF design.
"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.)"
Our measurements between my hotspot 20 meters up an oak tree under an
inverted plastic bucket and the USAF enlisted barracks (I support the
troops) are very similar. Some days the combination of high humidity
and high temperature obscure my 200mw into a 6db co-linear quite badly
over the 1.2km path length to the roof of the 4 story barracks building
where the Pingle's Cantennas are mounted on various pipes to hide them
from paranoid schitzophrenic inspections.
Sky News in London gave the Pringle's Cantenna a boost, recently:
http://www.youtube.com/watch?v=xO-KO3McAOY
of course, blaming it for hacking, to infer it should be outlawed by the
UK nanny state.
http://www.youtube.com/results?searc...n+antenna&aq=f
There's lots of great videos from the "hackers". Oddly, most of them
look like anyone in your neighborhood, not some dangerous cyber
criminals.
I'm sure the news paranoids will win, at some point, and we'll all be
arrested by Homeland Security in the USA.
--
Creationism is to science what storks are to obstetrics.
Larry