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Video Downlink – Questions and Answers

Posted by Tom Benedict on 23/12/2012

Ever since installing it, I’ve had some issues with my video downlink. For anyone doing FPV on an RC aircraft, this will all be old hat. But for me doing KAP, some of it came as a surprise. What kind of issues? You guessed it – crummy signal.

Lots of things can contribute to bad video: RF interference, noise on the power supply, antenna feed line losses, a poorly tuned antenna, the list goes on and on. Over the past several months I’ve checked most of these, but didn’t find any real answers. Yesterday I rebuilt the entire ground-side system, from power to receiver to monitor. As far as I can tell it’s now solid as a rock. Which leaves the air side.

My video transmitter is currently powered off of the same battery that drives my RC receiver and servos. (If you’ve been doing FPV, this will already come across as a bad sign.) I’d seen some warnings against this, but all of them tied back to electrical interference with an electronic speed controller in the “aircraft”. KAP rigs typically don’t have an ESC since there’s no prop to turn. No ESC, no interference, right? Well… Possibly. But there are other ways to kill a video signal than interference.

The problem I ran into didn’t have to do with noise so much as running into a limit of how much current the battery could source. The battery pack on my rig was powering a radio receiver, two full-sized servos (including a high torque metal gear servo), a GentFocus cable (which draws milliamps, at most), a 12V boost-buck power supply for the video system, and the video transmitter itself (a 200mW transmitter… no small beast.) Add it all up, and when everything was up and running it was drawing my batteries down pretty hard. After trying the same setup using a bench power supply, I came to the conclusion that the single battery pack just couldn’t handle the draw.

So I’m giving up on the single battery idea. I’m going to rework the transmitter to use its own battery – a three-cell LiPoly – and to be almost completely independent of the RC electronics. The only connection will be an RC power switch for the video hardware. (It’s nice to be able to power down your video remotely when you’re not actively using it.) If the bad signal persists, I’ll yank that connection, too. I’m planning to package the battery, transmitter, and RC switch into a nice, neat enclosure that I can install when I want video, and remove when I don’t. Easy peasy.

Meanwhile I’ve gutted the video from my KAP rig. It’s amazing how simple and clean KAP gear can be when you’re not constantly pushing it to do something more. Maybe having the video packaged as an option rather than as an integral part of the rig is the right way to do it. I’m curious to see how much I’ll use video once it’s no longer a built-in feature of the rig.

– Tom


5 Responses to “Video Downlink – Questions and Answers”

  1. Jasja said

    Also been having bad video, so I am hoping to do the same these holidays, but will power it from a AA battery, like Kappix is doing… Transmitter requires less power than yours I think.

    • Tom Benedict said

      I really really like Ramon’s setup. So yeah, if you can power it from a single AA, by all means do. His setup is so clean, it’s where I think video hardware for KAP should go.

      Once the new batteries get here, I’ll give them a try and write a post about how they work out. No real ingenuity on my part for the battery approach. I looked on the RC FPV forums to see what batteries people were using for the transmitter I’m using. It should just work. But we’ll see.

  2. I use a 5.8 Ghz Immersion RC video Tx powered off a 3s LiPo rated at 11.1v, 1450mAh this is plenty of power for the relay for around 5 hours work. I use a Ubec
    (I binned the ferrite ring) from the same battery to power everything else at 5v. I have a good signal up to 200m+ down range at 60m elevarion I discovered it was a mistake to use the 5v supply out of the videoTx, hence the Ubec. I get some loss when I pan the rig and null the Tx antenna but nothing serious.

    The Ubec has the same effect as seperating the power supply with a 2nd battery for a lot less weight.


    • Tom Benedict said

      Bill, I’m forehead-slapping right now. DOH!

      That’s brilliant! And the ferrite ring wasn’t necessary? That’s really encouraging. If the separate video battery works out, I’ll pick up a UBEC and give this a try with a single battery. The video battery I have on order is an 850mAh 3S LiPoly, so it’s a little small. But if this works I wouldn’t mind bumping that up to a larger one.

  3. Ramon said

    Thanks for the non-addressed kudos, guys :0) !

    In the beginning of this month (January 2013) we did a videolink supported KAP session but it was no success, videolink wise. We came to the conclusion that the step-up concept didn’t work well enough so I removed the step-up voltage converter and the AAA batteryholder from my videolink TX.

    Now we’re using a 9V E block in its own neat case, cabled to the videolink TX. But obviously we needed to step down this time, which is a much better way of getting to the required voltage than stepping up. So I grabbed an 78L05, 2 capacitors and a diode and 15 minutes later the whole thing was ready for a testflight. Soooooooooooo much better!!!!!!!!!!! The TX only draws about 35 milliAmps so this 9V block is gonna last forever.

    Just a small piece of FR4 epoxy board and ready you are for 10 minutes of soldering violence. It doesn’t take more. It looks like this… . The new situation is as follows: and

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