Re: Antenna - 19dbi homemade?
- From: Jeff Liebermann <jeffl@xxxxxxxxxx>
- Date: Wed, 14 Nov 2007 08:06:49 -0800
David Fairbrother <schoolsarge@xxxxxxxxxxxxxxx> hath wroth:
Jeff Liebermann wrote:
David Fairbrother <schoolsarge@xxxxxxxxxxxxxxx> hath wroth:
I'm looking to become part of a community "mesh", mostly as a matter of
interest and partly to open a free AP in my neighbourhood. I've
contacted the guy who runs the mesh and have discovered I'm 1.7km from
one of his nodes.
Ummm.... that's kinda far.
It's not that far..similar point-to-point links of 6km have been
established in this area. I am also able to pickup a signal from his
node without any kind of amplifier (via a builtin laptop wireless of all
things) though it doesn't stick around for long.
Picking up the signal and maintaining a full time connection are quite
different. For example, if your friend with the node is using an
amplifier, his transmit range will be much farther than his receive
range. You could hear him, but may not be able to talk to him. If
you can talk to him, you may not be able to stay connected.
The laptop test is a good one as it indicates that you have a chance.
The internal antenna laptop antennas usually have fairly low gain (0
to 2dBi), so a 19dBi antenna or better will certainly be a major
improvement. It's also possible that you won't need a 19dBi antenna
and can get away less gain.
The unknown here is interference. A high gain antenna has a big
advantage, which could turn into a big disadvantage. The narrow
horizontal beamwidth means that any interference to the side and back
of your antenna will be greatly reduced. The bad part is that any
interference along the line of sight whil be greatly increased.
Sigh. First, the way I understand it, mesh networks kinda look like a
spider web, where connections arrive from all angles of the compass.
What you're building is a point to point link. That's fine, but what
happens when someone wants to connect to *YOUR* node, and finds that
your antenna is pointed at a distant connection. Here's an example of
one mesh network layout:
<http://pdos.csail.mit.edu/roofnet/doku.php?id=map>
Where would you fit in with a highly directional antenna?
A second AP with omni antennae?
Maybe. You can't just plug in a 2nd different antenna into the 2nd
port on a typical routers diversity switch. That doesn't work. The
explanation is a bit messy. See:
<http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008019f646.shtml>
especially the Golf Course case study, where they have an
omnidirectional antenna in one port, and a highly directional yagi in
the other. The switching problem is for real.
A 2nd AP might be possible, and would certainly not have the diversity
switch problem, but might be an issue with whatever mesh networking
protocol and firmware you're using.
You could use a Wilkinson splitter/combiner. That will divide your
transmit power equally between the two antennas. However, the receive
power will not be split making this a fairly useful solution. The
only catch is about 0.5dB additional loss through the splitter.
<http://www.hyperlinktech.com/web/signal_splitters_2400_2way.php>
You can easily build one of these yourself:
<http://yu1aw.ba-karlsruhe.de/spliter-n.gif>
May I suggest a compromise. Build a sector antenna. These have the
advantage that they are very easy to build (if you can solder well).
I've built a few and they are quite good. See:
http://pe2er.nl/wifisector/
http://yu1aw.ba-karlsruhe.de/vhf_ant.htm
http://www.brest-wireless.net/gallery/AntenneAmos
http://www.brest-wireless.net/wiki/materiel:amos
These antennas have a reasonably high gain, very narrow vertical
beamwidth, but also have a very wide horizontal beamwidth. Two or
three of these combined can make an excellent omni antenna, with the
added bonus of facilitating down-tilt. You can start with one, and
add sectors as required.
Cheers, will look into it. Yeah, a professional antenna is a damn sight
easier to setup than a homebrew, and as you say (in the long run) cheaper.
I like building my own antenna, but I have some of the (antique) test
equipment necessary to do the job. Building antennas is easy.
Testing, tuning, and optimizing them, is not.
<http://802.11junk.com/jeffl/pics/home/slides/lab.html>
Low gain antennas (<12dBi) can be built without much testing and
tuning. Mistakes and creativity are easily tolerated. That's not the
case with higher gain antennas, where the bandwidth of the antenna is
roughly the same as the 2.4GHz wireless band, and a small mistake can
send the tuning out of the desired frequency range. If you must have
high gain, they buy the antenna. If you can live with lower gain,
then make your own.
--
Jeff Liebermann jeffl@xxxxxxxxxx
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
.
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