Quarter wave omnidirectional antenna for 2.4 GHz

Radio Laboratory Handbook
of the ICTP “School On Digital Radio Communications for Research and Training in Developing Countries”
( Marco Zennaro & Carlo Fonda )

This is a simple and cheap omnidirectional antenna good for surveying signal strength using wlan tools. It is built using an N-type female chassis mount connector and five short lengths of copper or brass wire. The driven element is the wire soldered into the central conductor of the connector, with a length of one quarter wavelength. The four radials are also one quarter wavelength long, and are soldered into each corner hole of the connector. Each groundplane is cut to length and then bent over at 30 degrees from the horizontal to attempt to match the impedance to 50 Ohms. The gain of this antenna will be between 3 dBi and 4 dBi depending on final tuning.

Parts list:

• one N-type chassis mount female connector with four-holes flange (good quality one with teflon insulator is recommended)
• 20 cm of copper or brass wire of 2 mm of diameter

Tools required:

• ruler and goniometer
• pliers
• a file for metal
• a small rat-tail file for metal
• one powerful soldering iron of about 80 -100 W
• one soldering iron
• solder
• vice
• hammer

Construction

1. With the pliers, cut the wire in 5 pieces of 4 cm each.
2. File the flange of the connector near the holes in order to remove the plated surface. Use the rat-tail file to do the same on the internal part of the holes. This is done to prepare the surface of the connector for tinning.
3. Power on the high power soldering iron and let it heat for a couple of minutes. Apply the soldering iron to the connector until it gets hot (really hot! Be very careful!), but avoid melting the dielectric. Then tin the area around and inside the holes by applying solder until it flows. Avoid filling the holes with solder. Tinning is required to facilitate the process of soldering the wires to the connector.
4. Smooth with the file one side of each of the wires. Tin the wires for around 1 cm at the smoothed end, using the high power soldering iron.
5. Bend at 90 degrees 0.5 cm of the tinned side of the wires with the pliers. Do it for four of the wires, leaving one straight. Help yourself with the vice and the hammer.
6. Place firmly the connector in the vice, avoiding damaging the screw. Place the tinned bend side of one wire in a hole of the flange. Keeping it with the pliers, position the wire horizontally and along the direction of the diagonal. Apply the high power soldering iron shortly to the connector and with a small amount of solder, solder the wire to the connector. Avoid melting the dielectric of the connector. You may find useful getting somebody else keeping the wires in place with the pliers while you are soldering.
7. With the low power soldering iron, tin the central pin of the connector. Keeping the straight wire vertical with the pliers, solder its tinned side in the hole of the central pin.
8. Trim the exceeding part of the wires under the flange.
9. With the pliers, bent over the four radials at 30 degrees from the horizontal plane. This is done to match the impedance to 50 Ohms. To facilitate this operation, you may draw the 30 degrees angle on paper, and compare the antenna with it as shown.
10. Trim the radial at a length of 3,05 cm measured from the corner of the flange. Smooth the end of the wires with the file.
11. Trim the central wire at 3.05 cm measured from the flange surface. If you have a Spectrum Analyzer with Tracking Generator and a Directional Coupler, you can leave the central wire 0.5 cm longer and check the curve of the reflected power of the antenna. Trimming the wire at steps of 0.1 cm or less, you can tune the antenna to have the minimum reflected power at a frequency of 2.44 GHz. The pictures below show the display of the Spectrum Analyzer at the beginning and at the end of the tuning procedure. A difference of a few millimeters changes the frequency of resonance of the antenna of some hundred MHz. You are done!