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Hack 74 Primestar Dish with Waveguide Feed

Use a cantenna waveguide in conjunction with a
recycled satellite dish.

Primestar was
recently purchased by Direct TV, who is now phasing out all the
Primestar
equipment. This means that the dishes are being trashed, and are
available for other uses such as the one I describe here. It is easy
to turn a surplus Primestar dish into a highly directional antenna
for the very popular IEEE 802.11 wireless networking. The resulting
antenna has about 22 dB of
gain, and is fed with 50 ohm
coaxial cable. Usually LMR400 or 9913 low-loss cable is used
if the source is more than a few feet from the antenna. (See [Hack #62] for more details on cabling
options.)

Figure 5-6 shows the Primestar dish in action.

To build your own, you'll need the following parts
and a couple of hours:

A Primestar dish (you may use any old dish, but if it is bigger than
the Primestar, the gain will be higher)

A juice can (about 4 inches in diameter and at least 8 inches long)

A chassis mount N connector

After deciding on a place to mount your antenna, remove the apparatus
at the feed position of the dish. Be sure to save the mounting
hardware. Using a can opener, cut one end of the juice can out, drink
the juice, and wash the can out. Solder a quarter wavelength (1.15")
of wire onto the center conductor of the chassis mount N connector.

Using a punch or whatever other tools you deem necessary, mount the N
connector so that it is about 1.2" from the closed end of the juice
can. It is also a good idea to put a drip hole at the lowest point of
the can to ensure that water doesn't build up
inside. After having one of these on my roof for a few months, I
learned it would be a good idea to put a plastic lid on the open end
of the can so that the inside doesn't rust. During
the time mine has been up, it has rusted and I have lost a couple of
dB of signal strength. These two details may be correlated.

Mount the juice can so that the opening is just at the focus of the
dish. In my installation (see Figure 5-7), I
didn't quite achieve this, but I only lost a dB or
two by taking the easy route. I still have about 25 dB
signal-to-noise ratio, so the loss wasn't important
to me. The easy route is to mount the can as far back as you can
along the mount, by punching two holes through the can and bolting it
in. The perfectionist's method would be to find the
best feed place (which I found to be just a little farther back) and
use some PVC tubing to extend the mount so the feed is in the perfect
position. In some installations, every decibel will count, which
should be taken into consideration.

Other Considerations

This
antenna is very directional. You must have it aligned
very carefully or you will lose a lot of signal. It also needs to be
mounted securely, so the wind won't be able to rotate
it even a few degrees.

This antenna is an offset fed dish, which means that the

feed horn (our juice can) is not
positioned as much in the way of an incoming signal, so it
doesn't shadow the dish. This makes the aiming a bit
tricky, because it actually looks like it is aimed down when it is
aimed for the horizon (as you can see in Figure 5-6). You can use the scale on the dish mount to
determine the elevation it is aimed at. The dish
isn't as directional in the up/down directions as it
is side to side. This is fortunate, because without turning the
mounting upside down a standard mount will only get it a few degrees
above the horizon. I sacrificed a dB of gain here by not turning it
over, mostly because I mounted it on a vent pipe, and
didn't want to put that kind of wind load on it. As
already mentioned, I don't really need the extra
signal either.

IEEE 802.11a

As if this hack
weren't already hackish enough, this antenna is
easily adapted for use with 802.11a
gear at 5.8 GHz. Simply scale the dimensions on the feed can and the
excitation antenna to 2.4 / 5 = 48% of the dimensions just mentioned.
Remember that
wavelength goes down as frequency
increases, so antennas of equivalent gain at higher frequencies are
actually smaller.

The original article that this Hack is based on is available online
at http://www.wwc.edu/~frohro/Airport/Primestar/Primestarl .

Rob Frohne