Antenna BiQuad

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Biquad Antenna Construction

This page details the construction of a biquad, originally intended to be used as a feed for a 24dBi Conifer dish, but I found it works very well by itself.

Contents: 

Background | Parts Required | Reflector | Making the Quad | Assembly | Testing |  References

Background
I've done quite a bit of experimentation and testing with various homemade dipoles for 24dBi Conifer dishes, and have managed to increase the signal strength.

Trevor Marshall has a webpage with information about using a biquad as a feed on a Primestar satellite dish, with very good results. I decided to try using a biquad as a feed on a 24dBi Conifer dish, to see if I could improve the performance of it.

Note that the photos on Trevor Marshall's webpage do not clearly show the construction of the biquad - particularly the way in which the quad is attached to the coax. Numerous people (including myself) have constructed biquads incorrectly, based on his photos, and found that they perform very poorly.
Use the photos of my biquad below, and refer to the websites listed in the references section at the bottom of this page for more information on the correct construction of the biquad.

Parts Required
I used the following bits and pieces:

  • 110x110mm square section of blank PCB
  • 50mm length of 1/2" copper pipe
  • short length of CNT-400 low loss coax (~300mm long)
  • 1.5mm copper wire
  • N connector

Reflector
Cut a square piece of blank printed circuit board, 110x110mm.

Note that Trevor Marshall recommends a size of 123x123mm if using the biquad as a stand-alone antenna, while 110x110 is optimal if using it as a feed for a large dish.
He also recommends attaching some lips to two sides of the reflector, to reduce radiation from the rear lobes.

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

blank printed circuit board
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).

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

a short secion of copper pipe, notched at one end
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.

making a hole in the centre
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.

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

solder the copper pipe to the PCB 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.


Making the Quad
The quad is made from a length of copper wire, bent into the appropriate shape.

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

the shape and dimensions of the biquad
I had some offcuts of power cable lying around, and found that 2.5mm^2 power cable had a diameter of approx 1.6mm - a little bigger than the 1.2mm that Trevor Marshall specifies, but didn't think it would make a significant difference to the performance of the biquad.

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

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

90 degree bend
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.

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

bend it some more...
Do the same to the other side, resulting in the biquad shape.

make it symetrical...
Clean up all your bends, and ensure each side of the quad 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 quad 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 should be approximately 15mm away from the reflector.

the quad soldered onto the copper pipe
Strip approx 30mm of the outer sheath from the end of the coax.

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

fold the braid back, trim the centre conductor
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 quad to it, ensuring the centre of the quad is not contacting with the copper pipe. Refer to some of the additional photos below for details.

solder the centre conductor to the quad

another view
I used a coax crimper to crimp the end of the copper pipe onto the coax. This ensures that the coax would not move inside the copper pipe.

the copper pipe crimped onto the coax

the completed biquad
Now it's time to terminate the coax with an N connector, and do some testing. If you intend to mount the biquad outside, I'd recommend you place it into a weather proof enclosure. Numerous people have used small tuppaware containers successfully.

another view of the completed biquad

Testing
Some very rough initial testing using the biquad as a feed on a 24dBi Conifer dish looks very promising, with the signal strength being at least as as good as my homemade Conifer dipole (I was holding the biquad at approximately the focal point of the dish, and hadn't even removed the Conifer dipole).

I also managed to get a marginal link to an access point 10km distant, using only the biquad by itself!

Update: some more testing has been undertaken, with the biquads recording a gain of approx 10-12dBi

References
Trevor Marshall's BiQuad 802.11b Antenna
Simple Double-Quad Antenna
Reseau Citoyen: BiQuad
Lincomatic's Homebrew WiFi Antennae

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