This page details the construction of a biquad antenna. The
biquad antenna is easy to build, and provides a reliable 11dBi
gain, with a fairly wide beamwidth.Various sets of instructions are:
Background
I've done quite a bit of experimentation and testing with various home made dipoles for 24dBi Conifer dishes, and have managed to increase the performance of the dish.
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 of the dish.
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:
* 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
Note that you don't have to use blank PCB for the reflector. You can use any material that's electrically conductive, can be electrically connected to the coax braid, and will reflect microwaves (ie, any metal plate will do fine).
I've also heard of people using CDROM as the reflector, as the foil on it will certainly reflect microwaves.
Reflector
Cut a square piece of blank printed circuit board, 123x123mm.
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 Element
The element 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 element
I had some offcuts of electrical power cable lying around, and found that 2.5mm2 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 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 (test results available here).
the element 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 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.
solder the centre conductor to the element
another view
Note that the feed between the rear reflector and the biquad element needs to be shielded. Using coax to feed the biquad element directly, and positioning the coax inside the copper tube achieves this.
Use of bare conductors as a feed between the reflector and biquad element results in a radiating feed (such as this one), which will have a detrimental effect on the biquad's performance.
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 terminate the other end of the coax with an N connector.
If desired, you can add spacers at each end of the element, to ensure the element doesn't move in relation to the reflector. Refer to my double biquad page for more details on making spacers to support the element.
If you intend to mount the biquad outside, I'd recommend you place it into a weather-proof enclosure, to prevent corrosion, and to prevent water ingress into the coax.
Numerous people have used small tuppaware containers successfully.
This can be achieved by drilling a hole in one side of the container, and pass the coax tail through the hole, leaving the biquad itself inside the container. Seal up the hole for the coax with some silicone, and your biquad should be protected against the elements.
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 home made 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 a 180 degree waveguide on an access point 10km away, using only the biquad by itself, connected to a 30mW RoamAbout wireless card.
Some more detailed testing with multiple antennas, including the biquad shown above, indicates the biquad has a gain of approx 11-12dBi.
A friend has access to some antenna test equipment, and performed some tests on the biquad featured on this page.
The azimuth plot (ie, radiation pattern) of the biquad is shown below, and shows a 3dB beamwidth of about 50 degrees.
azimuth plot of the biquad
Variations
A number of people have suggested the spacing between the element and the rear reflector should be a 1/4 wavelength (ie, 30.5mm) instead of 15mm. However, test results (such as these) indicate the SWR of the biquad is minimised when the spacing is about 15-17mm. Increasing the spacing to 30.5mm increases the SWR significantly, thus reducing the efficiency of the biquad.
For a higher-gain variation of the biquad that's virtually just as easy to build, have a look at the double biquad.
Usage
When using a biquad to establish a link to another wireless device, you should ensure the polarisation of the biquad is the same as the antenna you are connecting to. Similarily, if establishing a link with two biquads, ensure they are both oriented for the same polarisation.
Failing to match the polarisation will result in significant signal loss.
vertically polarised vs horizontally polarised
Changing the polarisation is just a matter of rotating the entire biquad antenna by 90 degrees.
The biquad antenna is not particularly directional, but has a fairly wide beamwidth.
The 3dB beamwidth for a biquad (without side lips) is typically about 40-50 degrees, thus making it ideal for any applications where you want fairly wide coverage.
The relatively wide beamwidth also makes a biquad very suitable for war-driving and stumbling, allowing you to pick up signals without having to align the antenna directly with the signal source.
While a directional antenna, such as a Conifer dish (3dB beamwidth of a 24dBi Conifer dish is approx 7 degrees), is better suited for point-to-point links, the narrow beamwidth of a Conifer dish requires more precision when aligning the antennas (the narrower the beamwidth, the less susceptible it will be to interferance from other sources). An antenna with a wider beamwidth, such as a biquad, doesn't require the same precision for alignment, thus making it easier to get a link working.
Kits
If you're one of those people who may not have all the tools required for building a biquad antenna from scratch, or you don't want to shop around for all the parts required, you can buy a DIY kit containing all components from WarDrivingWorld.
In November 2006, WarDrivingWorld sent me one of their DIY biquad kits to review. The kit contains all the pre-cut and pre-drilled parts required to build a biquad antenna.
For more infomation on this kit, including antenna comparision test results, read my Review of the WarDrivingWorld DIY Biquad Kit.
References
Trevor Marshall's BiQuad 802.11b Antenna
Simple Double-Quad Antenna
Reseau Citoyen: BiQuad
Lincomatic's Homebrew WiFi Antennae
BiQuad 802.11b Antenna
11dBi, wide band
Some of Trevors test equipment
Low sidelobe 802.11b BiQuad feed for Primestar dish
The Primestar dishes are high gain, low cost, parabolic reflectors with an offset feed. They have superior sidelobe performance when compared with a wire grid antenna, reducing the chance that somebody off of the axis of your link will be able to interefere with it. But they are hard to feed because the f/d ratio varies from about 0.5 in the vertical axis to 0.8 on the horizontal axis.
Additionally the spacing between the feed 'slot' and the feed mounting bar is small (about 55 mm), which is less than a half wavelength at 2.4GHz
Failure to couple efficiently to the dish's wide aperture, or to minimize radiation into the mounting bar, will result in poor gain and/or significant sidelobes.
The feed is oriented for vertical polarization in this photo. To make it horizontal merely rotate the feed by 90 degrees. You will lose about 3dB of gain when using the horizontal mode, as the biquad's radiation pattern is a better match for the dish's oblong shape when vertical polarization is used.
Primestar Dish with Biquad Feed in vertical polarization mode
Construction of the Biquad
I used Printed Circuit board scraps for the 110 x 110 mm reflector, but it will be just as effective if made out of sheet brass or copper. Aluminum can be used if soldering of the rigid coax is not required at the feed point.
The reflector's 'lips' are 30 mm high, and serve to reduce coupling into the mounting bar. Note that they are only required along the main edge axis of the reflector. The lips cut down radiation from the rear lobes of the biquad by about 6 dB
The best SWR is obtained when the biquad loop is about 15mm above the ground plane, and the SWR may be adjusted by varying this distance.
If you are making a stand-alone antenna, rather than a feed, you will get better gain from a reflector 123 x 123 mm
The front view of the feed
A piece of 3/4 inch copper piping makes a tight fit with the mount supplied on the Primestar dish
The rigid 0.141 diameter coax is soldered to the groundplane to provide physical support for the structure. If the biquad element is constructed carefully there will be no component of radiation along the axis of the coax, no current is induced into the coax outer conductor, and a balun is not needed.
An SMA connector can be seen on the end of the rigid coax used to support the biquad element
Rear, showing mounting tube
To make the element take a piece of 1.2mm bare or enamelled copper wire exactly 244 mm long. Bend it in half, and then make the bends at the halfway point on each leg (where the solder joints will be). Then bend the 4 remaining right angles so that the element sides are rectangular, and there is about a 1.5mm gap for soldering to the feed. The widths of the two quad elements will be approximately 30.5mm, from wire center to wire center.
You may use standard coax cable to connect at this point, if you do not have rigid cable available, but you will have to figure out how to support the loop physically.
The best SWR is obtained when the loop is about 15 mm above the ground plane and when the reflector is mounted about 10mm in front of the Primestar's feed bracket.
Detail of the rigid coaxial cable soldering
That's all there is to it, folks -- you now have a dish with 27-31 dBi of gain and negligable sidelobe radiation (<40dB). The beamwidth is about 4 degrees.
Biquad visualized with 4NEC2
Look at NEC2.org for information on simulating the performance of the stand-alone Biquad
BiQuad Antenna for PCS CELLULAR Radio
Need a little bit more range for your cellphone? You can make a Biquad for 1900 MHz exactly the same as the one above, but start with a 304 mm long pice of wire, fold it into 8 arms approximately 39.5 and 38.5 mm long. The ground plane needs to be a little larger, use one about 160 mm (6.2 inches) square. If you don't have a coaxial RF input jack on your cellphone you can couple the signal into its existing antenna using a single quad as a matching stub. It's not perfect, but in practice it works well. Solder an alligator clip to either of the high voltage apex (39mm from the feed) of a single 152 mm loop, and clip that to the antenna stub you are currently using. Now you can put 100 ft of coax between your phone and use a roof antenna (the BiQuad) to operate even in fringe areas.
