This page details the construction of an easy-to-make collinear 360 degrees omni-directional, vertically polarised, antenna for 802.11b g wireless networking. The antenna is very robust and compact, and has a gain of approximately 5 to 6dBi.
This collinear simply consists of a length of copper wire with some loops located at specific locations.
The length of the bottom section is 1/2 wavelength, the centre section is 3/4 wavelength, and whip section on the top is slightly less than 3/4 wavelength, apparently to reduce the capacitance effect.
The 802.11b standard uses 2.412MHz to 2.484MHz frequency range, so at the centre of that frequency range, 1/2 wavelength is 61mm, and 3/4 wavelength is 91.5mm.
These dimensions appear to be consistent with similar commercial antennas. When building my first collinear using this design, I made the loops in a similar way to those seen on many commercial antennas which have multi-loop coils, with the wire above and below the coil being centrally located with regards to the loop.
However, after doing some further research, I realised this was incorrect, and made another collinear using the loops as per the approach described above. Some quick comparison testing indicates the collinear with the offset loops performs much better than the collinear with the centered loops.
Visit Here for more.