Video Downlink for KAP
Posted by Tom Benedict on 04/12/2013
This is the third and final installment of KAP articles. In Then and Now I described the evolution of my KAP rig from my first flight to the rig I use regularly today. In A Progression of Kites I described the additions I made to my kite bag over the years, and what those kites provided for in terms of KAP. This article describes my video downlink system.
The previous two articles were written almost as a history: “First this happened, then this happened.” But over the years as I solved the various problems I ran into with my video downlink system, I wrote articles describing what I did. So the history version of this has already been written. Rather than repeat what I wrote, this article is more of a how-to for adding a video downlink system to a KAP rig.
In its very simplest design a video link is a wire that connects the video output of a camera with the video input of a display device. It’s possible to set up a KAP rig this way, but the wire would have to be long, heavy, and would be a nasty thing to have in your hand when lightning strikes. For obvious reasons, it’s preferable to send the video signal to the ground some other way: radio.
It bears mentioning at this stage that unlike a wire, leaping into the world of radio places the operator in a new situation. At this point you’re transmitting your signal in such a way that it can potentially interfere with other wireless devices. In most countries radio transmitters are regulated, and typically fall into two categories: Either the device has been declared fit for use by an untrained person or it has not. In the US this declaration comes in the form of FCC Part 15 approval. If the device has an FCC Part 15 sticker on it, you can use it without a license. If it doesn’t, you can’t. The laws in other countries will vary, but that’s how it is here.
As was pointed out to me when I started down this road a couple of years ago, if you wind up using equipment that doesn’t carry your country’s seal of approval, it’s up to you to get whatever licenses are necessary for you to stay on the right side of the law. Unfortunately most video transmitter gear requires a license. (In case you’re wondering, that’s a picture of a 2.4GHz transmitter module from an RC radio. Most RC gear is approved for unlicensed use. The manufacturers are well motivated to make sure their stuff is certified. Video manufacturers? Not so much.)
There’s another benefit to studying for and getting your radio license. The whole point of the exercise is to give you the information you need to successfully and safely experiment in the world of radio. And that’s precisely what you’re doing! You’re exploring the world of radio by setting up an amateur TV station and a receiver. Questions that you’ll need to answer will include how much power to use, what kind of antenna to use, how much radiation exposure you’ll get, etc. By virtue of studying for and receiving your amateur radio operator’s license, you’ll find the answers to these and other questions. It’s well worth it.
Back to the video link!
The next question you’re probably asking is, “But what do I have to get to make it work?” Unfortunately there’s no right answer because there’s more than one way to do it. Here are some things to consider:
Most video hardware is built to handle either NTSC, PAL, or both of these formats. When looking at your gear – camera, video transmitter, video receiver, and monitor – keep in mind which system you’re using and make sure all your hardware will handle it. I got hardware that will handle both, so I can switch the entire setup from NTSC to PAL and back without penalty.
All of this hardware will have to be powered. In the world of FPV RC aircraft, there’s an advantage to running your video system on a completely different power supply than your radio system. That way when your video system drains your batteries dry, your aircraft doesn’t suddenly fall out of the sky in an uncontrolled heap. This isn’t as big a consideration in the world of KAP since the kite will fly regardless of the state of the batteries. (A single line kite with a dead battery flies exactly the same as a single line kite with a fully charged battery. The kite really doesn’t care.) So there’s an advantage to choosing gear that can all be powered off of a single source.
Between the transmitter and the receiver you will need a pair of antennas. At their simplest an antenna is a piece of wire trimmed to a particular length (1/4 the wavelength of the transmitted signal). At their most complex they can be fairly complicated pieces of equipment that preferentially transmit and receive in a particular direction, polarize the signal in a particular way, etc. Keep in mind that the size of the antenna is always dependent on the wavelength of the transmitted signal.
And while keeping that in mind, consider that radio gear for sending and receiving video signals can be built to operate on a wide range of wavelengths. At the longer end of the spectrum (literally) you can get 900MHz hardware. At the shorter end you can get 5.8GHz hardware. In between you can find hardware built to 1.2MHz, 1.3MHz, 2.4MHz, etc. The longer your wavelength, the better the signal will penetrate solid objects and bend around corners, and the less power it will take to get a particular range. This favors longer wavelengths. Conversely, the shorter your wavelength, the smaller your antenna. This favors shorter wavelengths. And no matter what, you don’t want your video system to interfere with your RC transmitter, either at the primary frequency or at a harmonic (n * freq). So if you’re using 2.4GHz RC gear, don’t use 2.4GHz or 1.2GHz video gear.
Finally, you’ll need some form of display on which to see the image from your camera. Options for this range from fully enclosed headsets like the ones from FatShark to standalone monitors. The display you choose will depend greatly on what you’re doing.
Regardless of whether you’re planning to build a video downlink for KAP or an FPV setup for RC aircraft, it’s really not safe to use a self-enclosed headset if you plan on operating your gear by yourself. The AMA’s safety guide for FPV specifically says you should use a spotter when flying FPV, and that they should maintain line-of-sight on the aircraft at all times. It’s really no different for KAP. If you’ve spent any amount of time flying single line kites, chances are you’ve seen them do something unexpected. If you’re watching the kite you can typically do something to recover. If you’re staring into a self-enclosed headset, you won’t notice until it’s way way too late. If you plan to use a headset for KAP, also plan to bring someone along to operate the kite. I’m a solo KAPer, so I built my system to use a monitor.
Something to look for when getting a monitor is what it does when it loses signal or encounters a glitch. Many of them switch to a blue screen. Because radio signals are almost always glitchy, this means that the monitor will spend most of its time being blue. Look for a monitor that will show static when it loses signal or encounters a glitch. Most of the ones that do will proudly tell you of this on the packaging or in the advertisement, and may even say, “Made for FPV use!” My first two monitors did the blue screen thing. Skip the pain and get a good monitor from the get-go.
Now for the real question: How do you wire all this stuff together? Fortunately the world of FPV has made these systems a lot more turnkey than they used to be. Unfortunately they’re not as turnkey as, say, plugging servos into a receiver and moving them with joysticks. Some soldering is typically required. Here’s the diagram for the system I built using Boscam 5.8GHz hardware:
I used a 3-cell LiPoly battery to power the video system, so the video transmitter is getting between 12.6V and 11.6V. The UBEC steps that down to 5V for the RC receiver and servos. I don’t use the Vcamera output on the transmitter to power the camera, and rely on the camera’s own internal battery for its power.
Keep in mind this is for a KAP system. I built out a similar system for FPV. In that case I omitted the UBEC and didn’t provide any connection between the video system and the RC system. This was in order to avoid RF interference from the airplane’s ESC, motor, and servos, and to keep the video system from draining the flight battery, as I said earlier.
When setting up your radio gear, carefully check the channels the gear can operate on to make sure the frequencies are legal in your country. Not all countries allow amateur use on all bands, and not all countries define the bands the same way. You should be able to find the frequencies used for each channel in the manual for your hardware.
When setting up the channels on mine I ran into another problem: The Boscam 5.8GHz Rx and Tx use banks of DIP switches to set the channel. Unfortunately on one of them 1 is up and 0 is down, and on the other 1 is down and 0 is up. Even worse, the setting for channel 1 on the transmitter and the setting for channel 1 on the receiver are completely different! So for those using the Boscam 5.8GHz system, this diagram appears to be correct if you are looking at the numbers on the DIP switch banks (original source):
Because I knew I wanted to be able to swap cameras on my KAP rig, I wired the three video/audio/ground wires to a male servo connector and built individual camera cables that ended in female servo connectors. To swap cameras, all I have to do is swap cables.
Quick aside: If you plan to do anything with any sort of servo gear – KAP, RC airplanes, robotics, whatever – get a bag of male connectors, a bag of female connectors, a spool of servo wire, and a crimp tool. Places like Servo City carry these as stock items. I have never regretted getting mine. The only regret I’ve had is that I didn’t get bigger bags of connectors! You’ll find these come in handy for more than just servos.
Ground stations vary from person to person. I built mine for KAP, so it’s small, self-contained, and can be attached to my RC transmitter.
I use the same ground station for FPV. It consists of the video receiver and antenna, a monitor, and a battery. The wiring is typically a little more straightforward than on the transmitter gear. Both video receiver and monitor usually come with RCA connectors, so it’s just a matter of plugging everything in. In my case, though, the RCA cables were all male! Rather than use a bunch of female/female gender changers to connect the cables together, I made all new cables. But it’s not necessary to go this far with your gear.
Once you have all your bits and have wired them together, test it thoroughly on the ground. Having a KAP video downlink fail in the field is a bummer, but it’s not the end of the world. Most of us start off either using autoKAP or aiming by looking up at the camera, so not having a video downlink isn’t a show-stopper. It’s just not fun. If you’re building out an FPV link for an RC aircraft, losing your video feed mid-flight can be a lot more disorienting and may lead to a crash. So test first. Then test again. Then, just for grins, test it again. I spent way too many KAP sessions chasing video problems to chance it these days.
Whatever route you go, remember to fly safely whether you’re flying a kite, a plane, a helicopter, or a multi-rotor. Using a video downlink of any sort means you’re taking your eyes off of your aerial platform. In the case of a kite, finding it again is just a matter of looking up the line. In the case of an airplane or helicopter, it may take you a while to pick it out of the sky. Use a spotter. And expect things to go wrong. They always do.