Planet Fox > Microwaves > Sectoral Antenna for the 2 and 5 GHz ISM Bands

Sectoral Antenna for the 2 and 5 GHz ISM Bands

Discone diagramI got the idea for this from the antennas used on cell phone towers. These antennas, called sectoral antennas in the industry, are usually made from an array of dipoles or crossed dipoles with an electrically adjustable corner reflector for shaping the beam. A typical cell tower will have 9 or more around the perimeter, each serving a separate, wedge shaped area.

While the typical cell sectoral uses crossed dipoles ('bowties'), I had some trouble getting these to work at such a high frequency. The typical dipole is ½λ long, which is only about 25 mm at 5 GHz, and multiple dipoles would be needed to work well with both bands. This is on top of the problem of matching the antennas to the transmission line. Too much work. What I eventually decided on is the disc/cone (discone) antenna, like the ones popularly used with police scanners. The best part is that there's no matching network of any sort required, since the discone is a good match for 50Ω coaxial cable, such as RG8.

Building the Antenna

The formula given in the is ARRL antenna book is L=246/fMHz for the lowest frequency, although they don't really specify what measurement that's given in. I'm going to assume it's feet, since that's the only way that works out to a reasonable size. Using that equation we get a result of 31 mm for L. B, the base of the cone, should be equal to L, and D, the diameter of the disc, is equal to L×0.7=22 mm.

I built the disc and cone out of a small sheet of copper, like the ones people make jewelry out of, soldered together with an ordinary soldering iron. The cable that I used inside the antenna is regular cheap RG-58, which you can get pretty much anywhere. The center conductor needs to be soldered to the disc, and the braid needs to be soldered to the cone. The cone should have just a millimeter or two of spacing from the disc. When you're done, measure the resistance between the center conductor and shield, it should be infinite or near infinite.

Building the Weather Cover

Mounting bracketsNow, here's what I built the weather cover out of:

  • Small sheet of black plastic
  • Aluminum flashing
  • Drywall mud pan
  • Rivets
  • Milk jug
  • Two angle brackets and two U-clamps

I used the black plastic as the backing, everything gets attached to it. I used pop rivets to attach the aluminum flashing (in the shape of a corner reflector) to the black plastic, then a few more rivets to connect a couple of pieces of milk jug plastic to the corner reflector. Then I used hot glue to attach the discone to this whole contraption.

Four screws and two angle brackets are bolted to the back of the black plastic, this is what will hold it to the mount once everything's put together. The lead from the antenna is soldered to a TNC connector. I chose the TNC connector for this for several reasons, the main one being is that pretty much all access points, including the one I'll be using this with, have RP-TNC connectors, and I already had some laying around. Most of the nerds people who build these things use N connectors, but those are bulky, expensive, and offer little if any performance gain over the very similar TNC connector at these frequencies.

TNC Connector
Soldering the connector

Putting It All Together

RG8 CoaxThe last step was to mate the weather cover and backing. I used this weird industrial strength epoxy resin that smell like the most toxic thing ever, but it seems to be holding up well. I painted the drywall pan white so that it doesn't stick out as much (mine was red when I started). It's also probably a good idea to drill a very small weep hole in the bottom of that pan, so that if water does make its way in it has a way out.

I've put mine up on the corner of my garage, connected to an old Cisco Aironet access point via a custom jumper I made out of a 4 foot section of Belden 8214 RG-8 (BIG HONKIN) coaxial cable, terminated with a TNC connector on one end and an RP-TNC connector on the other. It works pretty well, I get good even coverage of my front yard and I didn't have to buy an outdoor access point. I think overall this project cost me about $10, since I had most of this stuff laying around already. My only costs were about $4 for the copper sheet and $6 for an RP-TNC connector.

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