Modifying Conifer Antennas for Wireless Networking part 1

This page details a method for constructing a new dipole for a Conifer dish, resulting in improved performance over the more common dipole modification.

Contents:

part 1: Conifer Antennas | Background | Parts Required part 2: Antenna Disassembly | Dipole Construction part 3: Reassembly | Testing | References

Conifer (ex Galaxy) Antennas
The antennas we're using are made by Conifer (now known by the name of their parent company, Andrew Corporation), and were used in Australia by a pay-tv company called Galaxy.
Galaxy went out of business several years ago, so there are a lot of un-used Conifer antennas on people's roofs in Australia.

The most common Conifer antenna used by Galaxy is the 18dBi grid, while the 24dBi grid is a little less common. Note that both the 18dBi and 24dBi grids use an identical feedhorn, so this page is applicable for both.


18dBi and 24dBi ex-Galaxy antennas made by Conifer,
with a 30cm ruler (bottom right) for scale
However, 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.


an 18dBi Conifer (as installed by Galaxy)
Background
Numerous people have posted guides on modifying Conifer antennas (ex-Galaxy) for use with wireless networking. Most of these guides show how to disasemble the feedhorn, cut off the end of the down-converter PCB, and solder coax onto the PCB dipole.


the most common mod - coax soldered to the cut pcb
Of all the sites out there, ChrisK's page on his Galaxy modification was the most interesting, as he rebuilt the dipole from scratch, ensuring the measurements of the dipole and balun were as accurate as possible for operation at 2.4GHz.

ChrisK based his dipole on a design shown on this page, and Marcus and myself believed we could construct similar or better dipoles, and decided to use a brass plate for the dipole (instead of the thin brass tube which ChrisK has used).

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.

Below is a cut-away diagram showing the parts used in the construction of the dipole.


diagram showing components fitted together
Parts Required
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)
• female n-connector

the raw materials: copper pipe, brass tube, brass plate
Most Bunnings and Mitre10 hardware stores should stock these materials - ask at the trade counter if you can't find them.
Alternatively, hobby stores should certainly stock these materials.

The brass plate I used is 12mm wide, and 0.6mm thick, while the copper pipe has an internal diameter of 10.8mm, and the brass tube is labelled as "3/16 round brass - stock no 129" with an external diameter of 4.5mm.
This means the ratio of the inner diameter of the copper to the outer diameter of the brass is 10.8/4.5=2.4, which is close enough to the required ratio of 2.3.


navigation: part 1 | part 2 | part 3

Posted By Sameera Chathuranga

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