Antenna Magus

The Bi-Quad antenna is one of the handiest, cheapest and easiest to build WiFi antennas.

This blog entry will explain how to easily build one like in the picture below using cheap materials and a TNC-RP (reverse polarity tiny-N) low cost Wi-Fi router connector.

The antenna in the above image was designed using Antenna Magus for the following design objectives:

• Center Frequency: 2.45 GHz.
• Gain: 10.5 dBi
• Bandwidth: 20% (500 MHz)
• Feed: Coaxial feed with TNC-RP connector (typically used in Wi-Fi routers.)

Here is a diagram of the antenna with physical parameters:

(Note that if you click on the above diagrams they will open as 1:1 scaled versions. I decided to make them available if you want to manufacture this antenna yourself. The easiest is to print them to 100% of the original size and use these as footprints/templates to construct the antenna.)

Manufacturing process explained in 6 steps:

Step1: Cut 2 pieces of 2 mm copper brazing wire and bend to follow the profile of the printed diagram. Solder the two pieces of copper wire to form the Bi-quad?s radiating structure as illustrated in the image below.

Step 2: Measure and cut the ground plane to the same size as shown on the printed footprint and drill a 6mm hole in the center – I used a piece of off-cut metal that I got from a local canned fruit factory for my ground plane.

Step 3: Cut a copper spacer with a length of 30.6mm and inner diameter of > 5.5 mm. This is only to support the Bi-quad copper wire and feed. The most important requirement is that the coaxial cable needs to fit through the middle – varying the thickness by a couple of millimetres should not greatly affect the antenna?s performance. I used a copper pipe with 7.8mm outer diameter and 5.7mm inner diameter which I got from a refrigerator repair shop for R 30/meter (<5 US$/meter).

By now you should have the following components:

Step 4: Solder the copper pipe to the ground plane. Note that both pieces should be sanded to have rough surfaces and preheated in order for the solder to take easily. This can be quite tricky as the copper is a good heat conductor and cools down very quickly. (We used two soldering irons ? one standing upright through the middle of the ground plane with the piece of copper on top over the solder point. This ensured constant heat to both objects and the other solder was used on the outside to apply the soldering agent.) Here is the result:

Step 5: Solder the coaxial cable?s braid to the top of the copper pipe. The easiest is to fold it slightly over and to solder it to the outside of the copper pipe as shown in the following images.

Step 6 (final step): The final step is to connect the copper wire to the feed section. The following image shows the complete antenna with the male TNC-RP (reverse polarity tiny-N) connector. These connectors are very popular and used with wireless Wi-Fi routers. They are easy to obtain and cost only a few dollars.

Measurement vs simulation

The following graph shows the measured |S11| compared with the simulation.

The measurement shows that the antenna is well matched over the desired band (- 20 dBi) but has an unwanted ripple on the S11 measurement. I only realised the cable and connector that I got from an old Wi-Fi antenna (also designed for 50 ohm) actually uses a 75 ohm cable which caused unwanted reflections between the connector and the radiating copper wire. I ordered a new cable and will update the post as soon as I replaced the old with the new cable.

I hope this blog entry will be useful for those of you wanting to improve your Wi-Fi reception at home!

Author: Robert Kellerman

This entry was posted on Tuesday, December 8th, 2009 at 10:21 am and is filed under Antenna related. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.