Biquad Antenna

13 November 2009

The biquad antenna is easy to build, and provides a reliable 11dBi gain, with a fairly wide beamwidth.

Parts Required :

123x123mm square section of blank PCB
50mm length of 1/2" copper pipe.
Short length of CNT-400 or LMR-400 low loss coax (~300mm long).
250mm of 2.5mm² copper wire (approx 1.5mm diameter).
N connector

Use some steel wool to remove any tarnish and polish it up. Cleaning the copper in this way will make it easier to solder.

Cut a 50mm section of copper pipe, and file both ends smooth. Using some sandpaper and/or some files, polish up the copper pipe (including the inside of the copper pipe, to ensure a good connection with the coax braid).

Cut a notch into one end of the copper pipe, removing approx 2mm from half the circumference.

Drill a hole in the centre of the blank PCB so that the copper pipe is a tight fit in the hole. I found a reamer to be very useful for enlarging the hole to the correct size.

Insert the copper pipe into the hole, with the notched end on the copper side of the blank PCB. The copper pipe should be protruding approx 16mm through the hole, measured on the copper side of the PCB.

Solder the copper pipe to the PCB, to ensure a good physical and electrical connection.

Quite a bit of heat is needed, due to the thickness of the copper pipe, and an electrical soldering iron probably won't be able to deliver sufficent heat. I found a small gas torch works quite well.

Antenna Construction :

Note that the length of each "side" should be as close to 30.5mm as possible (measured from the centre of the copper wire to the centre of the copper wire), which is a quarter of a wavelength at 2.4GHz.

power cable had a diameter of approx 1.2mm - 1.6mm

Remove the insulation, measure and cut a 244mm length the copper wire, and straighten it as best as you can.

Measure the mid-point of the wire, and make a 90 degree bend. The bend should be quite sharp and pronounced.

Measure the midpoints of each half, and make two more 90 degree bends in the wire, so that it looks like that shown in the photo below.

Once again, measure the midpoints of each section, and make some more 90 degree bends, resulting in what is shown below.

Do the same to the other side, resulting in the biquad shape.

Clean up all your bends, and ensure each side of the element is as straight as possible, and as close to 30.5mm as possible.

Note that you may need to trim a small amount off each end of the wire to achieve this.

Assembly

The element must now be attached to the reflector. Note that only the two "ends" of the copper wire are to be attached to the copper pipe - the centre of the copper wire must not touch the copper pipe (hence the notch which was cut into the end of the copper pipe.

The copper wire element should be approximately 15mm away from the reflector. Testing antenna performance while varying the spacing between the copper wire element and the rear reflector indicates that a spacing of approx 15mm provides the lowest SWR

12.0------- 1:1.80
13.0 -------1:1.52
14.0 -------1:1.40
15.0 -------1:1.20
16.0 -------1:1.15
17.0 -------1:1.15
18.0 -------1:1.22
20.0 -------1:1.30
25.0------- 1:2.2
29.0 -------1:3.0
30.0 -------1:3.2
31.0------- 1:3.5
32.0------- 1:3.9

Strip approx 30mm of the outer sheath from the end of the coax.

Fold the braid back over the outer sheath, and trim the centre conductor, so that about 4mm is protruding.

Insert the braid into the copper pipe, so that the end of the centre conductor lines up with the extreme end of the copper pipe, and solder the centre of the element to it, ensuring the centre of the element is not in contact with the copper pipe. Refer to some of the additional photos below for details.