Andrew Spurrier & John Samin VK1EME
This page outlines the modifications made to a Conifer 2.4GHZ Grid Dish antenna for use on a Wireless LAN. We intend to set up a 1.8km link using Cisco Aironet 350 Wireless LAN cards. These antennae have become surplus in Canberra (Australia) with the demise of GALAXY Pay TV. We were able to pick up 2 for around $20 each (I'm sure you can do better!). There are other modifications around for these antenna however we have modified these slightly differently to others around. We decided to keep the existing balun setup, Conifer probably put it there for good reason, like impedance matching and beam pattern formation.... so we decided to keep it.
Oh, what's a BALUN? A balun matches a BALanced antenna (DIPOLE) to UNbalanced coaxial cable. If you used a balanced feed line to the DIPOLE antenna (like the 300ohm ladder line to a folded dipole) then you wouldn't need the BALUN. In theory if you don't use the balun then the radiation pattern of the dipole will be disrupted and the gain figures probably wont be as good. Of course this is pretty hard to measure, especially at 2.4GHZ and with around 100mw of output power.... but at these frequencies and low power levels everything counts!
Comments about antenna for 2.4Ghz.
The reason I was so keen for the Conifer Antenna is the VERTICAL dipole raditor in the nosecone. If you want to talk to an access point, chances are it will be a vertically polarized collinear antenna (omni directional), so you will want to use a vertically polarized antenna too, or suffer signal loss. Helical to vertical will strip in the order of 3db* or half your signal strength! Helical antenna are circularly polarised, and the turn direction of the helical is important too! Get this wrong and guess what? yep, 3db loss minium. Same goes for horizontal polarization to vertical polarization.
So Grasshopper, always ensure the polarization of transmit and receive antenna are the same. Something to think about.
If you are really worried about matching the feedline and radiation pattern then a Horn feed (like the Pringle can antenna http://www.oreillynet.com/cs/weblog/view/wlg/448 without the yagi) is the one favoured by most Hams. I'm sure Conifer have done some maths on their dipole already, and I'm lazy enough to trust them for a while...
* VHF/UHF Manual G.R Jessop G6JP
If you have all the tools on hand then these modifications will probably only take an afternoon to do....
Tools used:
Step 1.
The first step involved stripping the antenna to its major components. First undo the two cup head bolts that holds the dish and the nose assmebly together.
Step 2.
Next remove the locking nut and washer that hold the threaded coaxial (F type) connector in place.
Step 3.
Carefully separate (cut) the top of the nose away from the plastic case. Don't be too brutal, every millimeter counts at this frequency! I used a hacksaw, maybe a sharp knife could be used instead?
Step 4.
Push the coaxial connector up through the plastic nose cone. This took a bit of effort because of the sticky glue used to hold it in place, but a little help from a phillips head screw driver and a hammer persuaded it to come out! Remove the downconverter from the nose cone.
Step 5.
Carefully cut the BALUN assembly away from the downconverter. The BALUN is etched on the PCB, if you follow the connections it appears to be a dead short on one side. This is a half wave stub, the coax shield will be connected to this. The other side (with the surface mount components) there is a single wide 1/4 wavelength track that connects the final amplifier chip to one side of the DIPOLE antenna. Cut the PCB just above the amplifier chip - See the photos below...
Step 6.
Clean up the PCB cut with a file. Carefully solder the coaxial cable to the balun, the picture shows heliax cable connected to the balun. Here the shield has been soldered to the bottom of the balun. Make sure the coaxial cable tails are kept as short as possible. This will help to maintain efficiency.
Here is the other side of the balun. The center wire of the coaxial cable is soldered onto the single track. This track originally connected to the amplifier chip on the downconverter PCB board. Again keep the lead lengths as short as possible.
Step 7.
Here is the DIPOLE located back in the nose housing, ready to be glued. Put a small drop of glue on each locator lug to hold the dipole in place.
Step 8.
Here is the assembled modified nose cone ready for gluing back together. I've used BNC connectors, they are rated to 11GHZ and have 0.2db insertion loss at 3GHZ. You may want to use N type connectors. Don't use UHF connectors at this frequency. Be very carefull putting the connectors on the cable, a mistake (electrical short) here will be disasterous!
Step 9.
Glue the plastic nose cone and reflector back on the nose housing. Seal off the coaxial cable entrance hole with silicon glue.
Step 10.
Reassemble the Grid Dish and it's finished!
I had an idea to use one of those Microwave Leakage detectors (also operate at around 2.4GHZ) as a field strenght meter - so I can map the directivity of the antenna and do some feed comparisons. Test gear for 2.4GHZ is expensive to buy and a little difficult to home brew. But if you know better then I'd be glad to hear from you :-)
Star Log - 13 October 2001:
Of course! Both cards work as before, even without an external antenna attached. There seems to be a bug in the Cisco software. The control GUI lets you select an antenna (left, right or diversity), but this doesn't seem to work, the card status always says "diversity" regardless? Of course our idea was to use the antenna output we modified only (Is it Right or Left?). We connected the output pigtail to our modified Conifer Antenna (see this project) and the diversity seems to choose the loudest signal. We have only tested the link out over a couple of hundred meters so far and 11MBS was no problem.
Star Log - 14 October 2001:
Tested the link over a distance of 1.2km from Andrews house to a my car fitted out with the 12volt powered laptop. Both ends of the link used the modified Conifer Grid Dish Antenna. The nose cone of both Conifer Antennas came loose and we had to support the ends to keep them centered. I'm sure this isn't helping things. It seems the existing glue is breaking down and going mushy, I'm sure some Liquid Nails will solve this minor problem. Signal strength was a bit up and down at around 50%, but we later realised we were not pointing exactly at each other. Link speed was constant 11Mbs. Signal quality was in the 90's. We will try again next week end but these results were promising to start with.
Star Log - 19 October 2001:
Repaired the drooping nose cones on the Conifer Antennas. With a little heat the glue turns to treacle. I cleaned it off with some mineral turps, the applied a liberal amount of Liquid Nails. 24 hours later they are rock solid. Tested the link over 2.5 kms (measured using the street directory), line of sight and established the link no problems. With the repaired antennas the signal strength was a constant 90%, signal quality 97% at 11MBS. Easily did voice over IP (VOIP), ping times <10ms. Looks like these mods have really worked well. Seeing we have no way of measuring the efficiency of the antenna modifications, I'm very happy with the results.
Star Log - 21 October 2001:
Tested the link between our houses. We DON'T have "Line of Sight", so this was going really going to test our comms hopes. I have a 10m antenna mast already installed at my house for my HF antennas. Andrew's house is easily around 20m lower than the hill that separates us. I put one of the modified Conifer Antennas at the top of my mast. There is a 15m heliax run to my laptop. Andrew sat up on his roof with the laptop and the other Conifer Antenna on a camera tripod. We knew we could easily get the 1.8km LOS distance but were amazed to find that we could establish a link without line of sight. The signal strength reported by the cards was 30% (fair) with a link quality of 93% good enough for 11MBS link speed. With a decent mast at Andrew's I'm sure we are going to do even better. We seem to be getting some major reflections off houses. The antenna must be pointed at a corregated iron roof on the top of the hill to get the best signal....
Star Log - January 2002:
Well at last we have our masts up and we have a permanent 11Mbps link. We have installed Mandrake Linux onto laptops, and use these for the Cisco Wavelan cards. The Laptops run the security and act as gateways to the other computers on our network. Also now that we have seen some heavy rain, we have proved that the link is stable in most weather conditions (only fog to go... ). Signal strength varies from 56% to 48% during the day, and I've seen it as low as 26% during a torrential downpour. Link quality ranges from 92% to 98%. Pretty good results considering we don't have line of sight and are transmitting with 100mW of RF!
Here's the antenna installation at one end.
And the other... While this one is in with my HF/VHF antennas, both are well clear of the houses and nearest obsticals.
Here is a graph of the signal level of our link from one end, over a 4 hour period. Each sample was taken at 30 second intervals. The page is generated by a perl script, once it's completely finished I'll release the source and post a link to the page.
5 March 2002 -- As promised here is a link to the live Signal Strength page. It has been updated to display 8 hours of connection data and time of events. Source and instructions will follow soon...
http://morsecode.dyndns.org:22334/
Questions?
Email: jwsamin@mrx.com.au
13 October 2001.
:-p