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Hack 75 BiQuad Feed for Primestar Dish

Make an even higher gain antenna out of a
recycled 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 interfere with it.

Additionally, the spacing between the feed slot and the feed mounting
bar is small. It is about 55 mm, which is less than a half wavelength
at 2.4 GHz. Failure to couple efficiently to the
dish's wide aperture or to minimize radiation into
the mounting bar results in poor gain and/or significant sidelobes.

The feed is oriented for vertical polarization in Figure 5-8. To make it horizontal, merely rotate the feed
by 90 degrees. You will lose about 3 dB 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.

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 15 mm above the ground
plane, and the SWR may be adjusted by varying this distance.

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 ground plane 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, there will
be no current induced into the coax outer conductor, and a balun is
not needed.

To make the element, take a piece of 1.2-mm 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).
Next, bend the 4 remaining right angles so that the element sides are
rectangular and there is about a 1.5 mm gap for soldering to the
feed. The widths of the 2 quad elements will be approximately 30.5 mm
from wire center to wire center. Figure 5-9 shows
the completed feed.

You may use a 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 10 mm in front of the Primestar's feed
bracket.

That's all there is to it, folksyou now have
a dish with 27-31 dBi of gain and negligible sidelobe radiation
(<40 dB). The beam width is about 4 degrees. Figure 5-10 shows a model of the BiQuad's
radiation pattern.

BiQuad Antenna for PCS Cellular Radio

Need a little bit more range
for your
cell phone? You can make
a BiQuad for 1,900 MHz exactly the same as the preceding one, but you
must start with a 304-mm long pice of wire, and fold it into 8 arms
that are approximately 39.5 and 38.5 mm long. The ground plane needs
to be a little largeruse one about 160 mm (6.2 inches) square.
If you don't have a coaxial RF input jack on your
cell phone, 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 (39 mm 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 feet of coax between your phone and the antenna on your
roof to operate even in fringe areas.

The original article upon which this Hack is based is available
online at http://www.trevormarshall.com/biquad.

Trevor Marshall