"Build Your Own" High Gain Omnidirectional Antenna

G'day people,

I am interested in setting up a wireless link between a friend's place and mine (3-4kms) which will also be open to the public if anyone feels the urge to use it.

At this stage I'm not really all that keen on spending much (or any!) money on the project as I don't know how far I want to go with it. I currently have an internal wireless network setup in my house so I do own wireless cards etc.

I was hoping on starting off by building my own high gain omni directional antenna. I have looked around a fair bit on Google and found several different theories but don't really know where to begin. I was wondering if anyone on the forum has had any luck such a project? I'd be interested to hear how other people have got on.

How much gain would one expect from a home made antenna? I see that there are some 15dB gain omnidirectional antennas available to purchase, is it at all possible to get this sort of gain with a home made one?

Many thanks in advance for any advice.

3-4kms say about say about 2 miles, I am not too sure on this without some kind of repeater.

How much output is your run of the mill card or wireless point got, about 500mw is it not so that's about 1/2 a watt! not a lot of power.

I don't know much about wireless networking, but I know a bit about radio communication and radio wave propagation from using radio communications years ago.

Unfortunately I would say it would be against the FCC laws to use any kind of microwave amplifier to boost that 500mw is it is 500mw for a very good reason, so forget that but you can to what you like when it comes to antenna.

What you do have is directional beam antennas like the yagi, what this antenna does is put all its output in one direction, hence the name beam.

It differs from a omnidirectional antenna as omnidirectional antennas put there power out in all directions, so if your access point is north then a omnidirectional will put its output north south west and east waisting a lot of power in the process.

With directional it will only go north south west or east depending on where you point it. So a directional will always have more power than a omnidirectional, for point to point communications its always good to go directional.

You got to make the most of the 1/2 watt of power, location of antenna and propagation method are most important. I am not a expert on the subject myself.

__________________
The Cat Arena

Its not illegal, but total EIRP, ie total output, radio + Antenna mustn't be over 4 Watts.

Most SOHO wireless gear puts out about 30mW, although there are specialised brands that will do up to 200mW. This is plenty of power for a 3-4km link provided you have a good line of sight.

Homemade antennas seem to vary in performance between designs and how well they are built. You will need to assess whether you want a point to point link, or give wider coverage for others to join on. I use a waveguide antenna I built from a design by Trevor Marshal that claims a 15dBi gain over 360 degree spread. Testing showed this to be pretty accurate. It was connected to a 200mW access point (cable loss kept this within legal EIRP limits) and we pulled up to 20km out of it when testing.

Most client devices are on rooftops within 4km and it works brilliantly as there are not too many tall trees and buildings. They usually operate in the order of 60mW and have a variety of antennas ranging from 15-24dBi.

I have sucessfully built a couple of 15dBi Cisco yagi clones and we found they did the trick for clients and lived up to expectations. Unfortunately I can't find the plan for these on the internet any more.

The best thing to do is experiment. I have spent countless amounts of time playing with wireless gear (homemade and commecial) in different locations for exactly the same reason as yours and it is quite fun to see what can be achieved.

Try to stay away from booster amps as they increase noise and latency, and keep your antenna cables short!

See http://www.trevormarshall.com/waveguides.htm for the waveguide details.
My project is http://www.marlwifi.org.nz

Here is a cheap solution that I have seen working: http://www.usbwifi.orcon.net.nz/ (Boosts Bluetooth too - useless fact of the day.)