The View Up Here

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Video Downlink for KAP

Posted by Tom Benedict on 18/11/2011

Over a year ago I picked up a 900MHz video link system for doing elevated photography using a pole. I’ve been using the pole for years, but I’ve done my pointing blind, similar to how I’ve been doing KAP. I figured the video downlink would make pole work easier. To make a long and painful story short, I destroyed the board camera that came with the system, almost destroyed my Canon A650 IS, and let some of the magic smoke out of my gentLED-CHDK cable. In disgust, I boxed the whole @#$! thing up and put it on the shelf in my closet.

A recent discussion on the KAP forum provided a much-needed kick in the seat of the pants. Other people got theirs to work, so what was my problem?!

More specifically, the thread answered two of my outstanding questions: First, one of the members of the forum figured out you could power the video transmitter with a 9V battery. This is significantly lighter than the 8xAAA power supply I tested initially. Second, one of the members of the forum figured out you could power the receiver and video screen using the 12V power source for the RC transmitter that’s used to control the KAP rig. With this newfound knowledge in hand, I hacked the radio I use for doing pole elevated photography.
RC TX with Video DownlinkI’d been tossing around how to mount the monitor for ages, so the design didn’t come out of nowhere. I wanted it simple, easy to use, and easy to install. The Hobby King and Turborix 2.4GHz radios have a big empty spot in the case just below the antenna, so I knew it was safe to pop two holes there to attach the monitor mount. The mount itself was bandsaw and file work, finished with bead blasting. Not shown is a rubber pad, glued to the mount with E6000 adhesive. This keeps the monitor from skidding around. The monitor is one I picked up off of Ebay back when I purchased the 900MHz video gear. It has a 1/4″-20 hole in the back, which makes it a snap to mount.

Power taps off of the switched side of the power for the radio come out the back to power the video receiver and the monitor. When the radio is powered off, everything is off. When it’s powered on, RC Tx, video Rx, and monitor all come up at once. This worked out GREAT.

Unfortunately the range on the little 100mW video link just isn’t there. It’s fine for pole work, but it peters out past about a hundred feet. Bummers!

I went back to Hobby King and found this little gem. It’s a 200mW 5.8GHz video link for about the same price as the 900MHz link. I got one on order, and will test it as soon as it arrives. People use this setup for first person view (FPV) on RC airplanes, and claim a range of 1km (yes, that’s over 3000′!) unmodified. Others have replaced the antennas with base loaded cloverleaf helical antennas, and have bumped that range even further. 3000′ should be fine for KAP, so I don’t plan to make cloverleaf antennas for this.

Since the ground unit doesn’t have additional batteries on it, it’s really not that cumbersome to use. I still need to add a hood so I can use it in direct sunlight, but that’s a matter of bending some corrugated plastic (another idea from that same forum thread.)

The thread had one more gem to offer up: The video jack on the Canon Rebel T2i is a 9-pin mini USB. The wiring diagram for the video jack showed up in the thread, as well as a link for where to buy the mating plugs. I found one on Spark Fun that also includes a breakout board, making the wiring a real snap. I already needed one of these for another project I’m working on for someone else, and figured this was just what I needed for making a wiring harness for my KAP rig. Yahoo! I ordered three so I’d have a spare.

I don’t know if video is something I’ll want to do with KAP long term. But between the upgrade to the T2i and concerns about rig stability, I’m more than a little stuck. I haven’t done much KAP, and have been increasingly disappointed with the KAP I’ve done. Maybe giving myself a viewfinder is the way to go. Maybe pulling the T2i out of the rig and going full aut0-KAP with the A650 is the way to go. I honestly can’t say. But without trying all the options, I’ll never know.

– Tom

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