استفاده از دیش ماهواره ای برای ساخت آنتن وایرلس 802.11b پرقدرت
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[B]Low sidelobe 802.11b BiQuad feed for Satellite dish[/B]
The satellite 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.
[IMG]http://www.trevormarshall.com/bq-mount-thumb.jpg[/IMG]
[B]Construction of the Biquad[/B]
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
[IMG]http://www.trevormarshall.com/bq-front-thumb.jpg[/IMG] [IMG]http://www.trevormarshall.com/bq-rear-thumb.jpg[/IMG]
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
[IMG]http://www.trevormarshall.com/bq-soldering-thumb.jpg[/IMG]
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
[B]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.[/B]
[IMG]http://www.trevormarshall.com/biquad.gif[/IMG]
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made Canantenna reflector
[LEFT] This 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 sig
[IMG]http://www.wwc.edu/~frohro/Airport/Primestar/PIC00073.JPG[/IMG]
[B]Things You Will Need:[/B]
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 the United States. 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.
[B]Construction Steps:[/B]
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.
[IMG]http://www.wwc.edu/~frohro/Airport/Primestar/PIC00076.JPG[/IMG]
[B]Some Considerations:[/B]
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.
[IMG]http://www.wwc.edu/~frohro/Airport/Primestar/PIC00077.JPG[/IMG]
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.
[B]Use the Feed Can By Itself[/B]
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
[IMG]http://www.wwc.edu/~frohro/Airport/Primestar/PIC00074.JPG[/IMG] [IMG]http://www.wwc.edu/~frohro/Airport/Primestar/PIC00075.JPG[/IMG]
[B]IEEE 802.11a[/B]
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.
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