There is a lot of good information available on the internet for homebuilt 2.4 GHz WLAN antennas for 802.11. Here is one that will outperform most of those antennas and most commercial ones as well. It’s called a Curtain Quad. It is cheap and simple and has a gain of about 17 dBi.
The following describes a version of this antenna that I built and tested.
. The material list is as follows:
1. Cut the antenna out of the fencing wire to the dimensions shown.
2. Remove about 1 3/8 inch from the center of the center wire. Squeeze the wires near the center wire until the gap is about 1/8 inch. Scrape and tin the ends of the wire stubs. The speaker wire transformer will be attached here later.
3. Solder a 1+ inch piece of speaker wire to the N connector. One wire goes to the center pin; the other wire to the outside part of the connector. I drilled a small hole in the connector flange for the wire to be inserted into and soldered in place. Leave about 3/4 inch of insulation on the speaker wire (as measured where the two wires are together). This is the transformer, transforming the antenna impedance to about 50 ohms.
4. Mount the N connector onto the piece of aluminum sheeting.
5. Cut out a 14 by 12 inch section of the foam sheet and glue aluminum foil to the backside.
6. Cut a hole through the center of the foam sheet.
Make sure the hole is big enough to allow the transformer to bow slightly.
7. Mount the antenna to the other side of the foam sheet. Secure with tape, thread, or glue.
8. Pass the speaker wire transformer through the foam and solder to the center wire stubs. Make the final total length of the transformer to be 1 inch, the same as the foam thickness, with 3/4 inch of insulation still remaining in the middle of the transformer. If you are using a newer type of N connector with about 1/8 inch of connector sticking out beyond the flange (as shown in step 3 above), then the transformer will be bowed slightly in the hole in the foam.
9. Secure the mounted connector to the back of the antenna with duct tape or aluminum backed tape.
As shown in these pictures, the polarization is vertical. For horizontal polarization, rotate the antenna 90 degrees.
The hardest thing to control is the transformer because it depends on the design of the connector, the speaker wire, and the length and spacing of the sections of the speaker wire where the insulation has been removed. Try to keep the total length of the transformer about 1 inch between the face of the connector and the antenna. If you have access to SWR measuring equipment, the transformer and the 0.8 inch dimension on the antenna can be adjusted for best SWR. But even an “imperfect” transformer should work ok. See the following link for a discussion of SWR and other topics http://www.oreillynet.com/cs/weblog/view/wlg/448 .
For use out-of-doors you will need to figure out how to weatherize this antenna. One way to make an outdoor version of this antenna would be to replace the aluminum foil with 1/2 inch by 1/2 inch or smaller hardware cloth and remove the foam.
It’s up to you to make sure this antenna is used in a system that obeys all FCC regulations.
For more information on the Curtain Quad, see my web page http://home.comcast.net/~ross_anderson/CurtainQuad.htm .
My homepage “Ross’s Antennas”, with links to my other pages, is http://home.comcast.net/~ross_anderson
Photography by Joane Anderson.
Ross Anderson firstname.lastname@example.org June 10, 2007