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Parabolic Antenna

This antenna is based completely around a template designed by Michael Erskine, the original template can be found at:-

http://www.freeantennas.com/projects/template/index.html

You should also check out the updated template available at:-

http://www.freeantennas.com

Michael's template is aimed at being placed next to the standard antenna of an access point so that no modifications to the access point are required.  This wasn't going to work for me because I needed to get the antenna outside the building in order to obtain line of sight to the target access point.  My intention was to add an antenna to a Linksys WAP11 v2.2 and use it as a repeater.

I could have used a cantenna such as the baby food tin and that would have worked just as well for my scenario, it also would have taken up much less of a Saturday afternoon as the cantennas only take 10 minutes to make but I was curious to see how well the parabolic template implementation worked in comparison.

My design goals were to be lightweight, durable, reasonably easy to make and of course work effectively.  I decided to over-engineer a solution and this is what I came up with.

First, I built a frame out of mild steel welding rods, these were gas welded to produce the basic antenna shape

The middle pair of frame rods then were used to profide support for the dipole assembly.  This is just sheet tin folded over and spot soldered

I needed to provide my own dipole assembly and had two options in mind.  Either arrange for the original Linksys antenna to be mounted in the focal point of the parabolic dish or make my own dipole.  I didn't have a RP-TNC connector to hand so I opted for the "make my own".  I wanted to be able to tune the focal point so soldered a piece of brass tube into the dipole support plate (technical term for a bit of tin plate!)  It doesn't matter what size tubing you use, I happened to have some K&S tubing from a modelling suppliers and by combining this with some slightly smaller tubing arranged a "trombone" fitting so that the inner tube could be moved in and out to find the focal point.  I happened to use 11/32nd and 5/16th tubing.

Here you can see the precise fit of the inner tubing which can slide in and out and is then used to make the dipole which comes later

The reflector was made out of aluminium mesh, the type you repair car bodywork with as a support for body filler.  It's soft and easy to bend around the frame.  I just fixed it to the frame with small bits of wire which I twisted to keep it secure.

Now for the dipole.  I applied very little skill here, radio-hams might cringe but again, this was a quickie solution rather than an overly technical one.  I just needed to meet the design goal, I didn't need to achieve anything more.  The dipole is simply made from two pieces of 30.5mm brass tubing soldered to the coax which comes through the dipole assembly tube.  Strip the coax a short amount and attach one tube to the centre core and one to the braid.

Hot glue it all in place to the small dipole assembly tubing and you now have a dipole that can slide in and out and can be rotated if necessary for fine tuning!

Once the optimum location is found (Use something like Netstumbler or any other signal strength determination tool) and then solder the 11/32nd and 5/16th tubing together to hold everything in place.

I soldered another piece of tin plate to the bottom of the frame and mounted one of those flexible stalk suction cups that I like so much

Finished antenna painted and ready to go!

Performance

The question is how did it work in comparison to a cantenna?

Initially things didn't look too good, the repeater wasn't working too well with lots of dropped packets.  I connected my laptop up directly to the antenna and was getting about -83dB, my Orinoco card works just fine at that but of course this was feeding a Linksys WAP11 so a whole different ball game.

I held the antenna at arms length away from the window (the line of sight is tight already, pretty much the angle shown in the picture directly above where the antenna is mounted on my fridge) and as soon as I got just an arms length away, the signal shot up and all was well, so the only problem was that my mounting was a bit short and needed  to project a bit more for my application.

To compare the antenna against a cantenna design, it's much much more directional.  Cantenna's can be aimed anywhere roughly in the right direction and will immediately start to work, the parabolic dish snaps in at the right point but is pretty weak otherwise.

From my limited testing the gain seems to be about 2dB greater than the cantenna although in order to connect my parabolic antenna to the laptop, I had to use a couple of adaptors and a pigtail to the Orinoco so it's quite possible that the signal might be a total of 3dB greater, which of course, is double the signal!

In terms of difficulty to build, as I said, I over-engineered a solution, Michaels template can be made of out foil covered card like I used for the cardboard horn antenna and thus can be made in about 10 minutes too.