Biquad Antenna Construction
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
antennaI'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.
I used the
following bits and pieces:
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).
- 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
- 250mm of 2.5mm2 copper wire (approx 1.5mm
- N connector
I've also heard of people using CDROM as
the reflector, as the foil on it will certainly reflect
Cut a square piece of
blank printed circuit board, 123x123mm.
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
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
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
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
Solder the copper pipe to
the PCB, to ensure a good physical and electrical connection.
solder the copper pipe to
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
is made from a length of copper wire, bent into the
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.
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
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
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
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 .
the element soldered onto
the copper pipe
Strip approx 30mm of the
outer sheath from the end of the coax.
strip the outer
Fold the braid back over the outer
sheath, and trim the centre conductor, so that about 4mm is
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
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
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
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
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
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
another view of
the completed biquad
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.
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
For information on
connecting a biquad antenna to a wireless radio, have a look
at the page on using
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
Failing to match the polarisation will
result in significant signal loss.
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.
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
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.
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
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
Need More Gain?
higher-gain variation of the biquad that's virtually just as
easy to build, have a look at the contruction details for a double
biquad antenna. The double biquad has approx 13 dBi gain.
Remember that the dBi scale is logarithmic, where 3
dBi is a doubling in gain. An increase in gain of 2 dBi is
equivalent to an increase of 60%.
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