Biquad antenna for 2.4 GHz for stand-alone use

Radio Laboratory Handbook of the ICTP “School On Digital Radio Communications for Research and Training in Developing Countries”

This antenna is a directive one, useful for a short distance Point-to-Point
link. It may be also used as a feeder for a Parabolic Dish or Grid. It is very
cheap and quite easy to build, requiring just a piece of wire, an N socket
and a metallic plate. The plate has a hole drilled in the middle to
accommodate an N type socket. The wire is shaped as an ‘8’, but with
squared angles and then soldered to the N socket. For this antenna the
gain will be in the order of 10 to 12 dBi, with a beamwidth of around 60
degrees.

Parts list:
- one screw-on N-type female connector
- 30 cm of copper or brass wire of 2 mm of diameter
- a square metallic plate of at least 12.3x12.3 cm (aluminum is preferable)

Tools required:
- ruler
- pliers
- file for metal
- small rat-tail file
- cutter
- saw for metal
- soldering iron
- solder
- drill with a set of bits for metal (with a 1.5 cm diameter bit)
- vice or clamp
- hammer
- spanner or monkey wrench

Construction:

1. Straighten the wire using the vice.
2. Cut the metallic plate to have a square shape of 12.3x12.3 cm. To do
   this, you may use the saw, but we suggest to use a thin (0.8 to 1.5 mm)
   aluminum plate, which can be cut with a cutter. Draw the correct
   dimensions on both sides of the plate with a marker. Carve the drawn lines
   with the cutter pressing firmly, helping yourself with the ruler. Do this a
   couple of times, on both sides of the plate. Bend the plate over the lines
   until it breaks.
3. The edges can be now very sharp. Be careful when handling the
   plate. Use the file to smooth the edges all around.
4. With a marker and a ruler, draw the diagonals on the metallic plate,
   finding its center. With a small diameter drill bit, make a hole at the
   center of the plate. Increase the diameter of the hole using bits with an
   increasing diameter.(The hole should fit exactly the N connector. Use the file if needed).
5. Now we want to shape the wire as an ‘8’, composed by two squares
   connected on one edge. The two squares have a side of 3.05 cm. Bend the wire at 90 degrees in his central point using the vice and the hammer.
6. With a marker, draw a line at 3.05 cm from the edge, and bend the
   wire at 90 degrees in this point with the vice and the hammer as before,
   but in the opposite direction (it should look like a ‘Z’).
   
7. Draw another line at 3.05 cm from this new edge, and bend the wire
   at 90 degrees in this point, in the same direction as before.
8. Draw now a line at 3.05 cm from the last edge, and a second line
   after 2.3 cm from the first one.
9. Bend the wire at 90 degrees in the second
   line, in a direction orthogonal to the plane containing the wire.
10. Shape the second half of the wire to obtain another square,
    repeating the procedure described in steps from 6 to 9
11. With the saw and the rat-tail file make two small, half-round shaped
    cuts in the connector’s nut at 60 degrees one from each other. The cuts
    should be on two contiguous edges of the nut, and should be large enough
    to fit half of the wire’s diameter. File also around the cuts (Be careful not to cut too deeply inside the nut: enough metal should remain to ensure its robustness.)
    
12. Trim the two ends of the wire at a length of 1.5 cm.
    
13. The wires should now fit in the nut’s cuts
14. Tin the ends of the wire for 0.5 cm. Tin around and inside the cuts
    of the nut with a small amount of solder, and then solder the wires to the
    nut.
15. Remove the exceeding solder with the file, so that the nut can be
    screwed correctly
16. Insert the connector into the hole of the metallic plate and screw
    the nut with the wire soldered on it.
17. We now want to solder the central pin of the connector to the
    center of the shaped wire. You may have either a short or long type of N
    connector (The central pin of the long type may be soldered directly on the shaped
    wire, while the short type of connector needs ad additional short piece of
    straight wire to be soldered into the central pin)
18. Tin the central pin of the connector and solder the center of the
    shaped wire to it
19. The antenna is now completed. If you have a Spectrum Analyzer
    with Tracking Generator and a Directional Coupler, you can check the
    curve of the reflected power of the antenna.