[kismac] Re: Wireless Link

  • From: Bob Cunningham <bob@xxxxxxxxxx>
  • To: kismac@xxxxxxxxxxxxx
  • Date: Mon, 17 May 2004 08:59:14 -1000

I stand corrected.

There are real 802.11b/g Yagi antennas (e.g., the ones supplied by
Cisco and Enterasys for use with their APs). Some (e.g., the
Enterasys antennas) have cylindrical plastic coverings which make
them look like vaguely like cantennas, although longer. Having opened up
one of those housings a while ago, I can verify that the Enterasys
antenna is indeed a Yagi, exactly as Ian describes (a rather fragile one,
hence the need for the plastic covering).

And while there are some instructions for building Yagi's on
the Web, instructions for building cantennas are much
more common.  And although you end up with something that may
outwardly look vaguely similar, what you get is an entirely
different (and notably less efficient) kind of antenna.

Use a Yagi if you can get (or make) one. Or a reflector-type antenna ...

On May 16, 2004, at 9:52 PM, Ian Beeby wrote:


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.



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 :).


Ian Beeby

Tel:      +44 1629 733800
Mob:       +44 7717 220707
Fax:     +44 1629 733807
E-mail: ibeeby@xxxxxxx

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