Bob,
Forgive me being pedantic but a cantenna is nothing like a Yagi - the
former is a form of circular waveguide horn antenna and the latter is a
travelling wave antenna. Specifically, the difference is in the way in
which the E-plane and H-plane fields are formed and this affects the
beamwidth in the respective planes. Symmetrical looking horns, for
example, seldom have the same elevation and azimuth beamwidths. The
other key thing about a cantenna that is worth noting is that is should
be a minimum length, set by the need to get the modes organised in the
circular waveguide, and beyond that there is little to be gained by
increasing the length, just by increasing the aperture - which is where
you might add a conical section at the end (but the design of this
needs care or it will have little effect). With a Yagi on the other
hand, the minimum you need is a dipole and then the gain progressively
increases as directors and a reflector are added. With a Yagi,
however, the gain goes up by a fraction under 3dB every time you double
the number of elements - so a 20 element Yagi needs to double in length
to effect a 3dB improvement, whereas the bandwidth goes down as
elements are added. So there is a practical limit to the length of a
Yagi antenna. There are plenty of ham-radio sites and free software to
help you to design either type of antenna but if you want to feed a
reflector I would suggest that you fabricate some rectangular waveguide
out of copper plated pcb and use a rectangular waveguide horn, where
the E and H plane fields will be better matched and your resultant
beamwidth will be similar in Az and El.
In many administrations, there is a fixed maximum effective radiated
power for 802.11 systems, and this is calculated by taking the power
output of the transmitter, subtracting the feeder and connector loss
and adding the antenna gain. So if you have a manufactured system with
20dBm eirp (effective isotropic radiated power) and replace the 7dB
antenna with a 20dB one you'll strictly need to reduce the power output
of the transmitter by 13dB - by a software setting or by adding more
feeder loss. Note here though that if you choose the latter you'll
also increase the NF of the receiver and this will unbalance the link
budget. So to stay street-legal in many administrations there is a
maximum size of antenna that is useful before the receiver has to be
degraded (unless Tx and Rx can be separated) as the software settings
for Tx power are generally only able to adjust for a few dB (my
experience is that they only cover around 6-9dB). In a street legal
system there is still an advantage for putting a higher gain antenna in
place and that is that the higher gain antenna (such as a Yagi or a big
horn) will have a narrower beamwidth and this will help to reduce
interference from unwanted sources (such as another network a few
blocks away).
Sorry that this was a bit off topic.
Regards
Ian
On 17 May 2004, at 07:19, Bob Cunningham wrote:
--Because of the distance, you will need a high-gain antenna such as a yagi (a cantenna is a type of yagi) -- or a parabolic -- at one end, at least.
Depending upon the distance (and what kind of obstructions are in between), you may need high-gain antennas at each end. Of course, you'll have to carefully point the antennas at each other. (You may need to have someone at each end -- in contact with each other via walkie-talkies or cell phones -- to make small adjustments until you get maximum signal when you set everything up.)
Clear "line of sight" is best. Completely clear of trees or whatever
if possible. Any obstructions (windows, walls, or even just trees)
will significantly reduce the strength of the signal. You'll definitely
need high-gain antennas at both ends if you have any obstructions ...
and even then, the signal strength may end up being too weak.
If you can manage to hook up a good antenna to your computer in your new house and point it directly to your mom's house, that should work. Otherwise you may need a 2nd wireless access point at your new house, which has a "wireless bridge" capability compatible with the AP at your mom's house. Put their antennas on the roof of each house (pointing directly at each other), with as short a cable as possible between each AP and its antenna. Depending upon the make/model of the 2nd AP, you may be able to also be able to use the 2nd AP as a local AP in your new house in addition to its "wireless bridge" function.
[I've a couple of setups similar to this, including one at a large outdoor soccer complex where LOS distances ran up to 2km. We used yagi's out on the fields, all pointing to a central (fairly tall) omni antenna in a "point-to-multipoint" setup. Clear line of site was essential at that range with those kinds of antennas in order to get really good signals.]
If you end up placing antennas outside (on roofs or whatever) be sure to bolt your antennas onto something sturdy. If you're using poles, use as shortest, strongest poles you can. Otherwise you'll lose your antenna aiming whenever the wind picks up.
On May 16, 2004, at 4:30 PM, Zac White wrote:
Hello, I might be moving out of my mom's house and into a house 2 blocks away. I would like to "borrow" the wifi from my mom's house from the other house. Does anyone have any suggestions on the cheapest way to go about doing this? I was thinking a cantenna pointed right at my house from the roof of the other house and some kind of cheap linux box set up as a repeater in the attic or something. I love kismac btw :).
Tel: +44 1629 733800 Mob: +44 7717 220707 Fax: +44 1629 733807 E-mail: ibeeby@xxxxxxx