antenna-theory.com

[merelda]

Hi,

If I want to try make a simple omnidirectional wifi antenna using co-ax, where should I start?
I'd imagine it being a half or quarter wave dipole, but I'm entirely unsure about that..
I'm hoping for something more technical, like the antenna-theory website, not using pringle cans as suggested from other website

Thanks in advance!

[bigSteve]

Yes - for an omnidirectional simple wifi antenna a dipole will work well. A pringles can will be different to get right and will definitely not be omnidirectional.

For a 2.4 GHz dipole antenna, the total length will be about 2.4 inches (6.25 cm). So there should be two arms, each of length 1.2 inches.

So you have a coax feed? You will have to tie one of the dipole arms to the center conductor and the other dipole arm to the outer conductor. If you use two generic pieces of wire of the above length, this should get you most of the way there. Then just try it out and see what happens. To fine tune it you would need a (very expensive) network analyzer, and that's not really worth it.

See here:
http://www.antenna-theory.com/antennas/halfwave.php

[merelda]

Hi,

Thanks for the reply!

I should be able to scavenge an RG58 somewhere, or just tear something apart if it works.

What is the best material to use for the dipole arms though? I have a basic open source network analyzer, hope that works.

But you also mentioned that 0.48 wavelength are quite common for halfwave dipole? And I'm also concerned about what is the actual central band for 2.4GHz wifi? I got really confused about the 802.11 standards.

p.s. the website is an excellent source to learn from!

Thanks!!

[bigSteve]

The central frequency is 2.44 GHz. The band is from 2.4-2.48 GHz.

The material for the antenna can be anything conductive - copper, aluminum, iron, wire, aluminum foil, whatever.

Dipoles typically need to be made slightly shorter than a half wavelength - 0.48 roughly to get them exactly in tune. If you have a network analyzer, then yeah you can just look at return loss - if the dip in s11 (the resonance) is too high in frequency, then make the antenna longer. If the resonance occurs at a frequency lower than 2.4GHz, you can trim the dipole and it will shift up in frequency.

If you have trouble, try to take some pictures of your setup and draw what you did and I can probably help.

[KV124]

I assume one of you is talking about a antenna network analyzer, a very expensive piece of equipment. The other is talking about a LAN packet sniffer, which is just a piece of free software these days (ironically it too was an expensive piece of equipment back in the day...)

Keith

[merelda]

Hi,

Thanks for the reply, I post the setup and results soon.

And yes, I was talking about the sniffer, thought they were the same thing...