Construction of the Version 3 Cylindrical Patch Antennas (CPAs)
Here's how to make our super-duper linearly polarized cylindrical patch antennas. Depending on what you can get donated, each patch antennas should cost you around $200: $100 or so for the artwork, and $100 or so for the PCB itself. Note that because the PCB material is so thin, and this is so specialized, it may make sense to find a small, speciallized PCB manufacturer in your area and use them (that's what we did).
Finally, see the CPA design page for a bit more information.
- 0.005" pcb with 1oz-copper patch antenna pattern etched in it, along with a 0.050" hole for the eyelets (~$30 material + artwork + processing = $50?)
- 0.35" I.D. brass body, nickle plated eyelet
- 1/16" (0.062 inch) Ultra High Molecular Weight (UHMW) polyethylene (PE) sheet with adhesive on one side ($5) All our antennas are less than 5.5" high, and about 16.5" long.
- 0.005" soft copper sheet ($3)
- SMA to RG316/U connectors - RG316/U coaxial cable: WiFi = 20 in, ATV = 15 in, GPS = 14 in.
- can of 3m "Super 77" acrylic spray-on adhesive
- CLEAN, FLAT, CLUTTER FREE work surface. Don't forget about the flat part, that matters.
- A stainless steel ruler >= 18 in. long. SS is important so that the exacto knife doesn't catch on the soft aluminum of a cheaper ruler.
- Lots of exacto blades.
- A exacto blade cutting matt is nice to have, or maybe some cardboard so the work surface doesn't get cut up.
Consider wearing gloves!
- Cut the antenna PCBs to the correct height with an exact-o knife using the cutting guides: cut on the inside of the traces so that there's no copper from the cutting guides left on the antenna PCB. If a few slivers remain, use a hot soldering iron and an exact-o blade to gently pry up the remaining copper.
- Cut the eyelet down to 1/32" (0.031") if it's not already that length.
- Mask the microstrip feed point with kapton tape within about 1/16" of the eyelet hole and tin the feed point. Clean the tinning flat with solder braid or similar. Remove any residual flux.
- Place the eyelet in its hole with the flared end on the copper side.
- Turn the antenna over and VERY VERY carefully tap the flaring tool into the straight side. Do this very slowly and carefully! (Specifically, use the "Lasting Impression" punch on its broken side as the 1st flaring tool, followed by the defective auto-center punch, finishing with a small flat-faced hammer.)
- Solder the eyelet to the copper. Use enough solder to make a decent fillet against the eyelet, but don't go over the top of the eyelet's rim.
- Cut the copper down to the correct size. You can use the face down PCB as a width guide, BUT NOT a length guide since the PCB is longer than the copper. We have been cutting the length to 16.469" (16 1/2 - 1/32)"
- Wrap the copper around the bottom of the airframe mandrel.
- Use 1/4" kapton tape vertically on the copper seam: Use three pieces, one short one in the middle (about 1/2" long) and two long ones at the top and bottom of the seam. The bottom piece can wrap around the bottom of the mandrel and go onto the inside surface. The top piece simply runs off the copper onto the mandrel. Overlap the seam with about 3/16" of tape on the bottom and top. Use small, clean (no fingerprints) pieces because these pieces will end up IN the final antenna.
- Using masking tape and paper, wrap the exposed mandrel so it won't get spray adhesive on it. If it does get spray adhesive on it, acetone will take the adhesive off.
- Clean the copper using alcohol and gentle, circular motions with a fine abrasive pad. Dry off the alcohol. (For the pad we have been using one of those non-metallic green kitchen pads.) Don't lift up the kapton tape while you're cleaning the copper, but do be sure to clean up the entire surface area.
- Cut the UHMW PE sheet down to the correct height using the face down PCB as a guide. Score one side of the PE at approximately the correct length of the patch but leave at LEAST 2" of extra length on each side -- this will allow the end pieces, which can't be curled, to be removed later. The PE sheet can be cut with good, sharp, scissors, or with repeated light strokes from an exact-o knife.
- Very carefully, roll up the PE with the adhesive/kraft paper on the outside. Try not to break the kraft paper. Roll it into a diameter slightly less than the airframe, say ~ 4 inches diameter. Use rubber bands to keep it rolled up for a while. (We have been pre-heating the PE to ~170degF for 3 minutes, and re-baking the curled sheet for an additional 3 minutes.
- Slightly unroll the PE and trim off the pre-scored side with an exact-o knife so that there is no "straight" section at the end.
- Starting at the copper seam, wrap the PE around the mandrel.
- Mark and cut the PE so that it makes a complete cylinder around the copper with as little gap as possible. We have been using two sheets of 8.5"x11" paper (~3/1000" thick) evenly overlapped around the copper to help size the PE. This is a good idea because the adhesive is on order 5/1000" thick. (Actually the PE has been coming out a little long, consider going to one sheet of paper.) Once the PE is marked (we score it lightly with an exact-o knife against the previously trimmed edge) remove it from the mandrel and cut through with repeated light strokes from the exact-o knife. Check the fit around the mandrel again and remove additional material if needed. (Typically the PE has to be exact-o-trimmed twice, but only twice.) To remove small amounts of PE we use a very small (~3" long) wood plane.
- Hold the PE sheet in place around the mandrel and wrap the PCB around the outside, centering all three seams. Hold the whole affair in place with a rubber band. Poke a #64 or so carbide drill through the hole in the eyelet and drill part way through the PE.
- Remove the PCB and PE and use the #64 to finish the hole through the PE. Follow up with a drill that is the size of the coax's inner dielectric, (about 1/16" or #52) drill out a hole this size in the PE at the location of the previous hole. Both these holes can be drilled by hand, simply twirling the drill between your fingers.
- Use a center punch on the PE at the drill hole. This makes a depression which allows the eyelet to sink into the PE a bit, so it doesn't stick up when the PCB is wrapped around the PE.
- Clean the inside of the PE with alcohol. Use tape to hold the PE in a circle that can be rolled along a flat surface.
- Electrocute the inner surface of the PE by using a corona discharge against a CLEAN, grounded surface. Don't kill yourself in the process. Spark it all over the inner surface, take your time. We do 3 repetitions of 3 passes, one pass moving circumferentially, one transversely, and one transverse with a swirly pattern.
- Put small strips of tape on the very bottom of the mandrel - make sure it does NOT cover the copper ground plane. The tape will keep the adhesive spray from making the bottom of the mandrel sticky and thus hard to maneuver on the working surface.
- Go outside.
- Using 3M's "super 77" acrylic spray adhesive and the "wide" nozzle (3" circular pattern, color coded white), spray the copper on the mandrel and the inner surface of the PE. Clear the nozzle first until you get a nice, fine spray. Then from about 6 - 12 inches, spray the adhesive. Make a nice, thin coat. Note that it may look bubbly; that's fine. Just don't overdo it. Don't forget to clear the nozzle of adhesive by spraying it upside down.
- Go back inside.
- Clean the adhesive from the bottom edge of the PE with acetone. This is optional, but it makes the PE slide on the table so much better.
- On a flat, level, CLUTTER FREE surface, put a piece of notebook paper down and put the mandrel on top of it. With the hole down, wrap the now sticky PE around the Cu. Starting at the copper seam, work your way around the cylinder burnishing or rubbing the PE onto the Cu. To keep the PE straight with the mandrel it's a good idea to almost wrap the PE around the mandrel before touching the PE to the seam. That way the bottom of the PE is guaranteed to be in the same plane as the bottom of the mandrel. The working time of the spray adhesive is about 15 minutes, so starting the bond within about 5 minutes is prudent.
- Get a 1/16" (0.062") piece of the coaxial cable's inner dielectric. Put in a stiff wire, like a steel spring wire. It should fit snuggly to be retained by the dielectric.
- Carefully peel off the kraft paper on the PE.
- Put the coax dielectric w/wire into the hole drilled into the PE.
- Put the mandrel on top of a piece of notebook paper on a flat, level, CLUTTER FREE surface.
- Clean the PCB with 70% alcohol, removing corrosion with a fine abrasive pad if required.
- VERY, VERY SLOWLY AND VERY, VERY CAREFULLY align the PCB hole with the wire sticking out of the PE. DO NOT LET THE PCB TOUCH THE ADHESIVE ON THE PE! As with the PE, wrapping the PCB around the mandrel almost but not touching the adhesive will assure that the bottom of the PCB aligns with the bottom of the mandrel. This is a slightly dangerous maneuver, and three people are not too many.
- Once the wire is through the hole, let the PCB material touch the adhesive and starting from the hole work outward burnishing/rubbing down on the PCB to get it to stick to the PE. Slowly work around the mandrel, making sure there are no air bubbles and no wrinkles in the PCB.
- The PCB may be a bit long - that's fine
- Remove the masking tape and paper from the mandrel.
- Using an exact-o blade, cut off the kapton from the top and bottom of the seam.
- Slide the antenna off the mandrel. (Note, to open the antenna up carefully cut along the seam in the patch to sever the kapton in the middle of the seam. However, this doesn't need to be done until mounting the antennas on the avionics module.)
- Using a needle nose pliers, push the stiff wire through the coax inner dielectric (which is now trapped between the PCB and copper) into the copper. Push through the copper if you can. Try to make a dielectric-sized round bump in the copper. It may be necessary to press down directly on the copper to avoid de-lamination or to more exactly shape the bump.
- Remove the stiff wire
- Use a Dremmel tool with a sanding cylinder to sand off the bump. A nice, round hole in the copper should result. The hole should be big enough to pass the inner dielectric trapped inside the PE.
- Remove the previously trapped inner dielectric.
- Strip the coax so that there is 1/16" of wire, 1/16" of inner insulation, and then a perpendicular "fan" of ground conductors that is no more than about 1/8th (0.125" wide), which will make a circle of no more than 1/4" in diameter.
- Mask the ground plane with kapton tape, leaving a 1/4" square around the feed hole. Using an exact-o, enlarge the resulting square patch into a circular area slightly under 3/8" diameter and centered on the hole.
- Clean off the inner copper with no-residue flux. (Do we actually do this?)
- Solder the inner conductor of the coax into the eyelet and clip it flush. This will hold the braid tight against the inner copper.
- Using a >= 200W iron, solder the coax outer braid onto the copper ground plane. Clean up any solder outside of the 1/4" braid disk. (Use less solder than we typically do.)
... and that's it!