The View Up Here

Random scribblings about kites, photography, machining, and anything else

Changes to the Video Down-link Hardware

Posted by Tom Benedict on 05/03/2012

From 2007 until a couple of months ago, I’ve done kite aerial photography using an RC radio to control pan, tilt, and sometimes yaw on a camera rig suspended from a kite line. Aiming has been done by looking at the camera rather than by looking through it. And for the most part this has worked well. I did some good photography, made some good panoramas, and had a truckload of fun flying kites.

Kiholo Bay

A number of events over the past year or so have made me want to try using a video down-link. I ordered the hardware, found out I needed a ham radio license in order to use it, studied for my license exam, passed it, and finally built a video down-link for my rig. It wasn’t completely smooth sailing, but it worked. More to the point, it worked well enough to convince me this is a direction I’d like to go. So I set out to fix the problems I’d run into and re-design the system so it fit in with how I like to do KAP.

My style of kite aerial photography is a little rough and ready. Everything goes in a backpack so I can hike in to wherever I need to be. When I’m flying I can tolerate holding a remote and a winder, but that’s it. Most of the time I’ll clip the winder off to a waist strap so I can have both hands free to operate the remote. I like to keep an eye on the kite. If things get rough, I want to be in a position to drop my transmitter on the ground so I can devote 100% of my attention to the kite. This has saved me from losing both kite and camera in the past, and will continue to save me in the future. I have stayed with this philosophy since I started doing KAP in 2007, and I see no reason to change.

When I added the video down-link to my setup, I wanted it to follow this same model: All of the ground-side video hardware had to fit on my RC transmitter. No extra stuff. No batteries in the backpack. No stray cables to prevent me from dropping some or all of my gear so I can deal with an unruly kite. All of the air-side video hardware had to fit on the KAP rig as discretely as possible. Once installed, I had to be able to wrap both KAP rig and RC transmitter up in cloth and shove them into my backpack for transport. If things went wrong in the field, I had to be able to drop the RC transmitter and ground-side video gear and not have it break. Everything had to be robust enough to handle this kind of treatment or it wouldn’t survive my style of KAP.

I came up with a good arrangement.

KAP Rig as of 5 March, 2012

The monitor is hard-mounted to the RC transmitter. It has a 1/4″-20 socket on the back, which made this easy. The hood is formed from some ABS plastic I had in the scrap bin. The 5.8GHz video receiver is attached to the back of the transmitter with industrial strength Velcro. I’m not 100% happy with the wiring between the monitor and the receiver, but it’s at least neat and tucked out of the way. All of the ground-side electronics are powered off the same battery supply. At the moment this is 8xAA Eneloop rechargeables located in the RC transmitter, but a future plan is to replace this with a LiPoly transmitter flat pack battery. For now it works, and appears to give hours of service. When the time comes to upgrade, I already have a battery picked out. (Thanks again to Bill Blake for pointing me in the right direction on batteries!)

The KAP rig has the video transmitter mounted above the RC receiver, on the opposite side of the rig’s aluminum frame. I am not convinced this offers sufficient RF shielding, but for now it works. A single cable runs between the video transmitter and the camera. A second cable made by James Gentles runs between the RC receiver and the camera’s remote shutter jack to remotely trigger the shutter. When installing the camera, these are the only two cables that need to be installed. Keep it simple.

5.8GHz Video Antennas

I replaced the stock antennas on the video transmitter and receiver with a cloverleaf and skew planar wheel, respectively. There were custom made by PNPRC.com. These are right-hand circularly polarized antennas. They offer a reasonable amount of gain over a simple half-dipole, and also offer some amount of rejection for linearly polarized signals. As an added bonus, PNPRC tests their antennas for good SWR match at 5.8GHz. Put it all together, and they’re a nice upgrade for a video link system. I haven’t had a chance to test these in the air, but already on the ground I can see a benefit.

These antennas are somewhat fragile. One was slightly bent during shipping. Easy enough to bend back, but I can see that some form of protection will be needed when I’m using them in the field. For the moment I’m removing them in order to pack my gear. As a longer term solution I’m leaning toward some radomes. Considering the size of the antennas, ping pong balls might be the perfect size.

For early tests I powered the air-side unit using a separate 9V battery. This was bulky, it added weight, and rechargeable 9V batteries have nowhere near the current capacity of an alkaline 9V so I was forced to use alkalines. Not my favorite solution. But it worked well enough to convince me the video down-link was a good idea.

More recently I completely re-vamped the video power system on the ground and in the air:

KAP Video Switch

Here’s how the new setup works:

The ground-side unit has a single master switch. When the RC transmitter is powered off, everything on the ground is powered off including the video hardware. When the RC transmitter is powered on, power is also available to the ground-side video hardware (monitor and receiver). Power to the ground-side video hardware is controlled using the quad-pole double-throw switch shown above. One of the poles is used to switch the power to the ground-side video hardware.

The other three poles are wired to a pair of 5k potentiometers. This lets the switch toggle between two fixed set-points for an RC channel at the same time it is controlling power to the ground-side video hardware. When the ground-side video hardware is powered on, that RC channel goes to 2.0us timing. When the ground-side video hardware is powered off, that RC channel goes to 1.0us.

On the KAP rig, the RC channel associated with the switch is wired up to an RC MOSFET switch from Pololu Robotics. Above 1.7us timing, the MOSFET passes the rig’s Vbatt on its outputs. Below 1.5us, the MOSFET passes 0.0V on its output.

The output of the MOSFET is connected to a 2.5-9.5V variable boost regulator, also from Pololu Robotics. Powered by anything above 1.5V, it’ll output a fixed voltage that can be set using an on-board potentiometer. I set mine to 9.3V and plugged it into the video transmitter on the KAP rig.

The net effect of all this is that the RC transmitter has a single switch that can be used to control power to the video hardware, both on the ground and in the air. This opens up a method of doing KAP that saves on batteries, and very closely follows how I do photography from a tripod:

When I’m using a tripod, I like to scout a scene handheld first. Once I find one or more vantage points from which I’d like to do photography, I’ll attach my camera to my tripod, rough-position the camera, then look through the viewfinder to see where I need to move it to get the composition I’m after. Once I’ve composed the photograph, I’ll do a final focus and finally trip the shutter. Then it’s on to the next vantage point.

When I’m doing KAP, I can power off all of the video hardware and set down the radio so I have both hands on the winder while getting the camera to altitude. Once it’s at altitude I can walk the camera into position, flying it by eye the same way I have done since 2007. Once I think the camera is in position, I can power up all the video hardware and take a look at the viewfinder. If I need to adjust my position I can. Then I compose, focus, and trip the shutter. Then the video hardware goes back off while I walk to the next camera location. With the exception of the kite, it’s almost the same procedure.

From past experience I know I can use my KAP rig without video hardware for many hours in the field without running out of battery on the ground or in the air. Video hardware places an additional drain on the batteries of both the KAP rig and the RC transmitter. But by having the power switchable in this way, it’s minimized.

– Tom

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7 Responses to “Changes to the Video Down-link Hardware”

  1. The system is looking good Tom, shame PNPRC don’t do 2.4Ghz antennae! I guess I’ll have to the swap to digital on my RC eventually.

    While you are at the tweaking phase you might want to consider a mod to reduce damage to the controller during transit. I cut the control sticks down so they don’t stick proud of the case. I have had to un-jam the gimbals on a couple of occasions and the last time I opened the case I thought I’d better do something to prevent permanent damage. 10 minute job with a hack saw and file and I can chuck the thing in a bag now without worrying if the sticks will get crushed into the gimbals when I pack it for the walk in /out. The cut down sticks work fine, even with gloves on.

    B

  2. Tom Benedict said

    That is an excellent idea!

    I’ve had good luck for the past five years or so just wrapping the transmitter in a cloth diaper. (Don’t ask… cloth diapers are one of the universal tools, right up there with bailing wire and duct tape.) But I know they get force put on them every time I do this.

    It’s funny, both my radios have adjustable sticks that let you make them longer. No one ever seems to make them shorter. It didn’t even occur to me until you posted this. But what an excellent idea. I don’t need the fine control out of the radio that someone flying a plane or helicopter would need. Sounds like a good mod to me.

    Thanks!

    Tom

  3. Tom Benedict said

    Oh! And I think there are other companies doing the 2.4GHz cloverleaf and skew planar antennas. Here’s one:

    http://videoaerialsystems.com/products/transmitter-antennas/cloverleaf-circular-polarized-antenna/

    I’m certain I’ve seen others.

    Tom

    • Thanks for the link! The 2.4Ghz version isn’t quite as dinky as the 5.6GHz and you are right these things look fragile but there must be a weight saving over the ‘duck’ antenna I’m using at the mo. Keep us posted on performance and I’ll fork out the cash if you think its a big reception gain once its in the air.

      These post are great Tom: practical help on this application of RC is in short supply!

      B

      • Tom Benedict said

        Oh good! I was beginning to be concerned I was writing stuff that wasn’t useful. I’ll keep writing.

        Next post is probably going to be about antenna placement. Ralf Beutnagel sent me a couple of really good emails about his experiments with antennas. Food for thought the next time I get my gear out in the field.

        If you haven’t seen Ralf’s antenna pictures on Flickr, they’re worth the look. He’s also experimenting with cloverleafs and skew planar wheels, but is also trying out Yagi-Uda antennas, quirls (a quad with twisted elements to cause circular polarization), and all sorts of other good stuff.

        Tom

      • It’s the practical that gets my attention- every time! Don’t worry Tom you are not shouting in to a bucket- a lot of shy retiring types read this stuff and keep quiet about it, this isn’t really a declamatory field is it?

        I’m 4 years into this now and I would not be anywhere if it wasn’t for the online postings of folks like you!

        Thanks again…and keep it up!

        B

  4. gamekeys said

    gamekeys…

    […]Changes to the Video Down-link Hardware « The View Up Here[…]…

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