Building a 2.4GHz Vertical Collinear Omnidirectional Antenna


Update (June 4, 2002): It looks like the original links to the pages of the excellent design for this antenna are broken. To that end, here are links to compressed archives of the original design images (which are all you really need to build the antenna) in .zip and .tgz format. I certainly hope that the original designers and authors will forgive me.

Background

I have had a wireless LAN in the house for at least two years now. My first wireless subnet consisted of the now-defunct WebGear's Aviator 2.4 wireless cards. These cards are supported under Linux using Corey Thomas' ray_cs PCMCIA modules and work well, albeit at 2Mbps.  I have since upgraded to using D-Link DWL-650 and Linksys WPC11 cards. These cards are supported by several drivers, such as the orinoco_cs and hermes PCMCIA modules , along with a proprietary Samsung driver, and the latest wvlan_cs module .

The external antenna for my wireless LAN is connected to a hardware-hacked D-Link DWL-650 PCMCIA wireless LAN card . This diatribe details my efforts and results of building an antenna that  is very easy to build and that provides much better coverage than just using wireless cards. The coverage is so good that I could, if desired, provide Internet access to six or seven neighbors.

The antenna is used for a home-based wireless LAN... there have been a number of on-line articles about such efforts, particularly by folks in Seattle and Linux-based homeboys in Australia. Necessity must be the mother of invention, and their results are encouraging.  But there were few, if any, *inexpensive* 2.4Ghz-based external antenna solutions -- until now!

Note that the antenna I built is *not* a yagi-type, which is used to focus a tight beam to another point for point-to-point wireless connectivity. I wanted an 'omni-directional' type for a bigger radius of coverage. The yagi antennas are easier, cheaper, and faster to build. Here's a link with directions:

http://www.netscum.com/~clapp/wireless.html

Here is a *much* easier to build yagi design:

http://www.guerrilla.net/reference/antennas/2ghz_helical/index.html

Connector

IMPORTANT: You'll need to attach an external connector to your wireless LAN card or access point. Fortunately, the D-Link DWL-650 cards (currently about $80) readily accept modification.To see how to solder an external antenna connection to a D-Link DWL-650, go to:

http://kevlar.burdell.org/~will/antenna/

Don't use the directions there on opening the card; instead, see:

http://users.skynet.be/chricat/DWL-650.html

Basically, turn the card over. See the four little thingies that look like screws? They're not. Place the card face down, then take a toothpick and press in from the side at each of the holes to release 'em. You can then pull the plastic cover off (little wings hold the cover under the metal part of the card; I ripped 'em off after soldering on the antenna connector, dremel'd a notch for the attached wires, then slid in and snapped the cover back on - look Ma, no superglue!

Here are the connectors I used - a 50-ohm BNC female with an male adapter, which connects to the female adapter on the end of the finished antenna. Note that an 'N' connector would be better instead of the Rat Shack UHF connector, but hey, i'm using what's convenient.

The Design

My first attempt at building an antenna was according to specifications detailed here:

http://www.guerrilla.net/reference/antennas/2ghz_collinear_omni/

My newer antenna, the one described on this page, was built according to specifications here:

http://www.guerrilla.net/reference/antennas/2ghz_collinear_omni_lowpwr/

The Parts

(2) 12" length of 3/64 K&S brass rod ($.50)
(2) 12" length of 3/32 K&S brass tubing ($1.20)
(1) 12" length of 11/32 K&S brass tubing ($1.40)
(1) Rat Shack chassis-mount UHF SO-239 coax connector, #278-201($2)
19" 1" PVC pipe from Homeless Despot ($1.99 for 10'; enough for two 'clean' (no printing) tubes)
(2) 1" PVC end caps from Homeless Despot ($.64)
some solder
some quick-set epoxy
some nuts and bolts
a piece of wire
Total cost: $5.74

Building

Follow the directions on the above link. Everything can be assembled using a pair of pliers, a Dremel w/cutting wheel, a drill, a soldering pen, and a screw driver. I ended up setting the decoupler piece of brass (3cm) using epoxy, then soldering a piece of wire to the outside connector. Total build time was about an hour. A small ruler with millimeter and centimeter marks will help (or you can convert the measurements to inches).

After completing the antenna, I fit the female N-connector into the 1" PVC end cap like this . The connector is bolted on and lock-tite is used over the nuts. Note that the original design doesn't say anything about insulating the antenna, but I went ahead and used some shrink tubing before epoxying the "Decoupling tube." The little wire is an ugly hack because the coupler tube must connect with the outer part of the antenna connector, but can't because of epoxy and the diameter of the Rat Shack connector (if i used the correct, less "lossy" N connector, this wouldn't be a problem).

When assembled and before sliding into the 1" PVC tube, I cut two thick cardboard circles w/slits, sized for the inside diameter of the PVC tube. This keeps the antenna solidly inside the tube without the need for tape or glue. Don't judge me on my wire-bending or soldering skills - I'm half blind and have carpal tunnel after working on computers for more than 25 years.

The Result

The original antenna I built was touted, according the directions, as yielding a 3dBi gain (as I built a 4-element, not an 8-element antenna). The new antenna is touted as having a 5dBi gain. I used my own favorite method of measuring the effectiveness of the new antenna by pacing off away from the source (after mounting the antenna in a vertical position) while pinging a remote server w/my laptop.

The results? A decidedly improved range.

With no antenna, my wireless LAN area extends from the front door to the back door. I live in a three-level brick townhouse end unit. The antenna is mounted at chest height above my laptop in an upstairs bedroom (i don't feel like climbing into the attic, getting on the roof, or drilling holes in the ceiling - my wife would have a sh*t fit).

With the previous antenna, the LAN's range extended to the edge of the back deck (about 10 feet from the back door) and out the sidewalk in front about 100 feet (this is through walls and brick mind you).

The new antenna seems to have about doubled the range, with no ping loss, even after walking across the street. All in all, i'm pretty happy. And i'd be interested in hearing about any other successes out there.

My home page.

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