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Modifying Conifer Antennas for Wireless Networking
part 1
(first published May 2002, revised June 2002)
This page details a method for constructing a new dipole for a Conifer dish, resulting in improved performance over the more common dipole modification.
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.Antenna Disassembly
Start by disassembling your Conifer antenna.
Remove the 2 or 4 bolts which attach the mounting bracket and the feedhorn to the dish.
Remove the reflector from end of the feedhorn by removing the small screw in the centre of it.
remove the screw holding the reflector onto the feedhorn
Remove the nose cone from the feedhorn. Some people have reported being able to remove the nose cone after cracking the glue with a hammer and screwdriver.
cutting the nose cone
I normally use a hacksaw to cut along the join to remove the nose cone from the feedhorn. Other people have reported carefully squeezing the end of the feedhorn in a vice will crack the glue, allowing the nose cone to be removed.
the feedhorn with the nose cone removed
Remove the nut and washer from the base of the feedhorn, and remove the down-convertor from the feedhorn.
remove the nut & washer at the base of the feedhorn
the down-convertor
A hammer may be necessary to persuade the down-convertor to separate from the feedhorn.
Separate the metal feedhorn base from the plastic body of the feedhorn, and remove the sticky glue residue using mineral turps.
remove the sticky glue residue
You'll have to drill out the feedhorn base to approx 10-11mm in order to be able to fit the coax through it.
Secure the feedhorn base in a bench vice, and carefully enlarge the hole until you can fit your chosen coax through it.
drilling out the hole in the base
I found a 10mm masonary drill bit at very slow speed works quite well.
Dipole Construction
Start by cutting off a 50mm length of copper pipe, and cut some slots in one end, making the length of the slots as close as possible to 30.5mm.
cutting the slots
Clean up the slots with a small needle file (for the car buffs, a points file works quite well too).
Clean up both ends of the pipe with a file, and use some sandpaper to clean up the external surface of the copper pipe. Also cleanup the inside of the copper pipe (the cut end, as you'll need to solder it, and the other end to ensure a good connection to the coax braid.
cleaning up the copper pipe
I clean up the pipe by holding it in the chuck of my drill (holding the drill on the workbench), and then using sandpaper and a file on the rotating pipe.
the completed copper pipe with slots
Cut off 30.5mm of the small brass tube to make the balun, and clean up the ends with a file.
Using a small drill bit, drill a hole near one end of the brass tube. This hole will make it easier to solder the coax core into brass tube.
the hole in the balun
For the dipole, we used some brass plate, approx 12mm wide by 61mm long. The length of the brass plate isn't too critical just yet, as long as it's at least 61mm long. It'll be trimmed to the correct length once the parts have been soldered together.
Mark the centre of the brass plate, as this is where you'll have to solder the small brass pipe.
mark the centre
Then hold the copper pipe against the brass plate (with the slotted end against the brass plate), and mark it's location.
mark the location of the copper pipe
Then hold the copper pipe against the brass plate, and mark it's location
mark the line to cut
Now cut the brass plate along the blue line, and clean up the cut ends with a file. I find a junior hacksaw works quite well for this.
the brass plate after being cut
Clean up the cut edges, and remove the tarnish with some sandpaper.
the polished brass plate
We've used both RG-213 and CNT-400 coax for these modifications, and they require slightly different approaches to the coax core.
Note that CNT-400 or LMR-400 is recommended, rather than RG-213, due to the lower impedance.
Strip approx 30mm of the black outer sheath off the coax.
RG-213 coax with the outer sheath stripped off
Fold the braid back over the remaining outer sheath.
the braid folded back
Strip off the central insulation, and if using coax with a stranded core (ie, RG-213), double each strand of the core over, and tighten up the bends with a pair of pliers.
the core folded over
Fit the brass tube over the coax core, with the hole previously drilled being located closest to the coax. Solder the brass tube to the coax core, using the hole to supply solder onto the join.
the brass tube soldered onto the coax core
Push the copper pipe over the folded-back braid on the coax, until the brass tube protrudes past the end of the copper pipe by at least a few millimetres.
Note that you may need to un-braid the coax braid, to as it is a pretty tight fit.
the copper pushed onto the coax
Tin the two pieces of the brass plate where they need to be soldered to the brass tube and the copper pipe.
the tinned brass plate
Tin the end of the brass tube and the end of the copper pipe with some solder.
Solder the brass tube onto the previously marked centre point on the larger of the two brass plate halves.
the brass tube soldered to one half of the dipole
Now slide the copper pipe down against the brass plate, and solder it to the brass plate, ensuring the two slots are aligned against the long sides of the brass plate.
the copper pipe soldered to one half of the dipole
Solder the other half of the brass plate to the copper pipe, ensuring there's an air gap of approximately 1mm between the two brass plate sections.
the assembled dipole
Measure the overall length of the brass plate, and trim the length to make it 61mm long. This is the dipole, and its length should be as close to 1/2 wavelength as possible.
Using the original dipole as a template, measure, mark and drill the the holes in each end of the dipole. These holes are used to locate and hold the dipole in the feedhorn.
If you've got access to a coax crimper, use it to crimp the copper pipe onto the coax braid, to ensure a very firm connection, and trim the excess braid which is still protruding past the end of the copper pipe.
two holes drilled in the dipole,
and the copper crimped onto the coax Reassembly |
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发表于 2007-2-5 15:25:06
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