The Conifer antennas used by Galaxy were designed to operate at a different frequency than wireless networking, and have a down-convertor integrated in the feedhorn. They need to be modified before they can be used for 802.11b wireless networking, and this page describes one way to modify them, achieving very good results.
To ensure the correct balun impedance of 50 ohms, the ratio of the inner diameter of the copper tube to the outer diameter of the brass rod should be approx 2.3.
The important dimensions are:
length of the dipole is 1/2 wavelength
length of the balun is 1/4 wavelength
ratio of inner diameter of copper tube to outer diameter of brass rod
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 1/4 wavelength is 30.5mm.
Here is a cut-away diagram showing the parts used in the construction of the dipole.
The materials we used to perform this modification:
Conifer (ex Galaxy) antenna
low-loss coax (such as LMR-400 or CNT-400)
50mm of copper pipe (~10mm internal diameter)
61mm of flat brass bar (~12mm wide by ~0.5mm thick)
30.5mm of brass pipe (~4-4.5mm outer diameter)
We’ve modified a number of Conifer feedhorns, and have done some testing in a controlled environment, to determine which dipole modification achieves the best results.
The testing indicates that this dipole provides at least 3dB better signal strength than the more common Galaxy modification (ie, soldering coax onto the cut-off down-convertor PCB).
These test results have been verified by other people’s test results too. Recently, a fellow WAFreenet member replaced the pcb dipole in his 18dBi Galaxy dish with a dipole I constructed using the method described above, and saw an increase in SNR of approx 4-5 dB.
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