Monday, September 1, 2014

Circularly-polarized Antenna Build

This summer I finally got my Quad-Copter set up with an FPV system using Hobbyking's el-cheapo 5.8GHz system (which also goes by the name 'Boscam'). Here is the Transmitter and Receiver. It was actually pretty funny, the bundle of the two costed a good but more than them separately. Anyway, after 3D Printing mounts for them both and soldering up a nice little modular system with the camera and Tx/Rx (which deserves a post of its own) I had a fully functioning FPV system.

However, I was still using the crappy little 'rubber duck' whip antennas that come with all of these things. These are the sorts of antennas you'd find in a walkie-talkie or a cordless telephone, although those manufacturers may get stingy with the space and twist the whip into a helix shape. Regardless, the antennas are linearly polarized which is a big problem for RC fliers. When the Rx and Tx antennas are oriented the same way everything actually works quite well, but when your plane or copter does something crazy, like lets say, turn, the antennas are no longer oriented the same say (barring you don't have a system to actively move them). When this happens, their ability to receive each other's signal drops drastically to the point where you are probably going to lose your video signal if you bank into a turn a good bit and you aren't in the immediate vicinity of the receiver.

The solution to this problem, well the easiest, is to use antennas that are not linearly polarized like this, but are rather circularly polarized. This way the antennas can be rotated about any axis and not lose any signal at all—as far as the system is concerned nothing has changed. After flying already pretty successfully in close range I decided it was time for an upgrade so I wouldn't have to worry about this issue once I wanted to fly further.

The one on the right is for the receiver and was after the other one. That time around having learned some things turned out much better and actually got heat shrink. The other one would benefit from some electrical tape, but I'm thinking that some of that liquid tape stuff would work quite nicely for sort-of "potting" the joints and exposed metal. I need to get some of that stuff...

Since we're working within quite tight tolerances here for the lengths of the elements, like half-millimeter precision, and doing this by hand I had to get a little inventive to get the lengths right. I figured out that I could actually use the end of my calipers meant for measuring inside diameters to actually bend the wires around if I held them in place. After accounting for the wire width in the measurements I was quite happy with my results. Almost every double-check of the lengths landed the caliper probes right in the middle of the copper wire ends. As far as I could tell visually, they were pretty close to perfect.

After some finicky yet still fairly relaxing soldering of the elements to the shield of the coax it was onto the center conductor. The bending of the elements into the signature "clover-leaf" patters went smoothly and the solder sweated the ends right onto the center conductor very nicely.

Quick test of the video while I rotated the receiver every which way gave no change in the video (which was a bit static-y as I had 5.8GHz going though a cinder block wall). Success!  

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