Offset Parabolic Wifi Antenna

Part 1: A Helical Antenna for Wifi Networking

The linear focus parabolic project, shown on this website, provides a nice boost in performance, but a more effective reflector and feedhorn combination is possible. Presented here is an offset fed parabolic requiring less effort to assemble than the linear focus antenna, yet outperforming it by a very large margin. My design is more simple and uses commonly available parts. Four turns does a good job of illuminating a reflector; ten or more turns make a stand-alone antenna suitable for window mounting. It is easier to build than the linear focus design and is MUCH more effective. Continue reading for details!

First, find a parabolic reflector designed for Ku band direct-to-home satellite service. These are now inexpensive and abundant - the one pictured above was discarded by a local resident. After washing off the dirt, spiders, and millipedes, I found the dish to be in excellent condition. Similar reflectors can be found in thrift stores, flea markets, and other habitats of electronic scroungers. Seek large reflectors (at least 76cm or 30" diameter), since these will provide greater gain and directivity.

Consider how to feed the antenna. I have seen web pages featuring tin can feedhorns, biquads, patch antennas, and helical radiators. A helical feed seemed suitable for this project as it was simple to build, inexpensive, and provided good gain and directivity. Being circularly polarized, a helical eliminates sensitivity to antenna orientation, and resists attenuation in rain. Though there is a 3 dB loss working linearly polarized stations, designing for more turns brings the gain to parity with the tin-cans and biquads. Also, when circular polarization is to be used on both ends of the link, senses need to match. Right hand with right hand or left with left. Bear in mind that polarization reverses in a parabilic reflector. To Calculate dimensions for your antenna, use this excellent calculator, plugging in a frequency of 2450 MHz.

PARTS REQUIRED:

1. one square piece of single sided copper clad PC board for a ground plane
2. one PVC kitchen drain tailpiece (3.8 cm / 1.5"diameter) to hold the helical windings
3. six 1/8" plastic cable ties
4. a length of copper circuit tape (adhesive backed, width 3mm or 1/8") or #14 copper wire
5. one suitable chassis connector (I used a reverse sma type matching the connector on my adaptor)
6. one 90 degree angle bracket with screws and bolts to fit

CONSTRUCTION:

1. Center the tailpiece on the PC board, copper side, and mark the circumference in ink.
2. Mark four locations on the circumference, spaced 90 degrees, where the cable ties will hold down the PVC tube.
3. Mark one location on the circumference, exactly between two 90 degree markings, where the coaxial connector will be mounted.

At this point you should have a PC board with a circle in the center, four tick marks on the circle at 90 deg intervals, and one tick mark exactly between two others.

4. Drill 1/8" holes on the inside and outside of the circumference at the cable tie locations.
5. Drill a hole directly on the circumference suitable for the chassis connector. Carefully measure and drill other holes for this connector if necessary.
6. Drill four holes, spaced 90 deg apart near the bottom end of the PVC tailpiece.
7. Drill holes to accomodate a small 90 degree corner bracket.
8. Drill holes on opposite side of board to accomodate USB wifi adapter that will be affixed with cable ties.
9. Tin the copper around the connector mounting hole, then mount the connector. Clip the center pin to keep it only long enough for connection to the helix windings.
10. Cut out a notch to accomodate the connector; it should clear center conductor, but avoud cutting out excess PVC material.
11. Feed cable ties through from the back side of the board, through holes in the tube, and back through the board. Tighten the cable ties, making sure the tube is firmly held to the copper ground plane.
12. Use a ruler and the edge of a sheet of paper to create a template for positioning the windings on the PVC tube. Distance zero represents the ground plane, then add the feedpoint distance, then ticks matching the turns spacing. Use the template to mark your tube on both the feedpoint side and the opposite side. The objective here is to create a guide to use while precisely winding the helical wifi antenna...

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