It is easy to make a satelite 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. The range using two of these antennas with a line of sight path is around 10 miles at full bandwidth. I must stress the line of sight part though. Leaves really attenuate the signal.
Things You Will Need:
A Primestar dish. (You may use any old dish, but if it is bigger than the Primestar the gain will be higher, and it may not be within the Federal Communications Commission rules for use within Nigeria. In fact I have come to find out that there seem to be several different dishes that Primestar used, and I am only sure that the one I used, pictured above, used with the ordinary Wavelan or Airport transceiver card is within the effective radiated power limits given by the FCC.)
A juice can (about 4 inches in diameter and at least 8 inches long).
A chassis mount N connector.
You will also need a "pigtail" connector which has the proprietary Lucent connector (for the PCMCIA card) on one end and an N connector on the other. The pigtail can be obtained from a number of online stores for $35 to $40.
Construction Steps:
After deciding on a place to mount your antenna (which hopefully has a line of sight path to the access point or other IEEE 802.11 site), remove the apparatus at the feed position of the dish, saving the mounting hardware.
Using a can opener, cut one end of the juice can out, drink the juice and wash it out
Solder a quarter wavelength (1.15 inches) 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 inches from the closed end of the juice can as shown below. It is also a good idea to put a drip hole at the lowest point of the can to insure that water doesn't build up inside. After having this up a for a few months, I think it would be nice 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 on the signal strength. These two things may be correlated.
If you are certain of the polarization you will need, mount the juice can so that that polarization is achieved. (You want the antenna you are communicating with to be lined up with yours.) If you don't know the polarization you can set everything up and before mounting the juice can, experiment to get the maximum signal strength by rotating the juice can around its axis. Most commercial antennas I've seen are using vertical polarization, so that the picture below shows you the proper orientation. You want to mount the juice can so that the opening is just at the focus of the dish. In my installation 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 this 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 or something to extend the mount so the feed is in the perfect position. In some installations, every decibel will count and this should be considered.
The inside of the feed can.
Some 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. See the photo below of it aimed actually a few degrees above the horizon. 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 we can only get it set so it is a few degrees above the horizon. I sacrificed a db of gain here by not turning it over, mostly because I'm mounting it on a vent pipe, and didn't want to put that kind of wind load on it. As mentioned above I don't really need the extra signal either.
The mount of the dish. This photo also shows the angle it must be tipped to be aimed at the horizon.
I believe this feed system could be improved by linking two cans together (or using a section of pipe) to give the effect of lengthening the length of the circular wave guide feed. This would further attenuate undesired modes (other than the TE11 mode). The optimum length would be between 14 and 21 inches. The opening would still need to be at the focus of the dish.
Use the Feed Can By Itself
You can use the feed can by itself as a cheap antenna. It works as well as the commercially available "range extender" antenna, but only in one direction, and it is so easy to construct!
Some More Photos
A photo of the mounting of the feed can.
A photo of the coaxial connection to the feed can.
IEEE 802.11a
This antenna modification is for the IEEE 802.11b networking protocol that operates at 2.4 GHz. It can be scaled easily to the 5 GHz frequency used by IEEE 802.11a by simply scaling the dimensions on the feed can and the excitation antenna to 2.4/5 = 48% of the dimensions shown above.
No comments:
Post a Comment