FPV Video Transmitter Reliability

 equipment  Comments Off on FPV Video Transmitter Reliability
Aug 252015
 

fpv video transmittersIt sounds like an overheat failure

This picture shows the miniature 32 channel transmitters as supplied (before overheating) and a New Zealand coin to indicate size.

I have used, seen and repaired some video transmitters with very poor thermal design. Overheat failures are quite common. Many underestimate the need for cooling. The VTX produces heat even when the model is not moving; unlike the ESC.

It is probably a reverse polarity protection diode that has failed; if it is a series. If the board has not been badly burnt, it may be repairable.

Measuring the current draw before extended use is a good idea as it gives an indication that all is well. An antenna fault or connection problem can greatly increase VTX current draw and heating.

Video transmitters are very inefficient; transmitted power is a fraction of the power in. Therefore a lot of heat is dissipated in the transmitter. Voltage regulators on transmitters are almost all switching types now, to reduce the heating problems; but it’s only a partial solution.

I sometimes include a switch to turn the VTX off while the model is on the bench or waiting for GPS lock etc. It reduces internal heating when there is no flight induced air-flow; and it saves the battery. Also handy if you want to swap or adjust the VTX antenna without powering down the model.

I recently bought two 200mW and two 600mW miniature video transmitters from China (just arrived – pictured above) for my mini-fpv-wing project. VTX power supply and cooling are major design considerations. It will have RTL, but it’s always a good idea to include a reliable video transmitter.

FPV Monitor Problem

 equipment, fpv  Comments Off on FPV Monitor Problem
Aug 172015
 

fpv monitor testingThe Problem

This monitor-receiver combination looses the image to a snowy screen regularly, even when the 200mW video transmitter is within 10 meters.

The way it would suddenly come and go, it looked like a connection problem or bad joint on the video lead between receiver and monitor.

We were expecting a more consistent better image with occasional noise and some interference. For FPV, a noisy image is better than no image.

To eliminate the connections a direct soldered lead replaced the two leads and RCA connectors. But the problem continued.

fpv monitor testingTesting the Receiver and Monitor

I resurrected an old 10mW 5.8GHz transmitter with clover-leaf antenna and mini camera to provide a relatively weak signal that could be easily blocked to simulate a longer range. Also a second receiver and monitor to sit nearby just to keep an eye on the transmitter and as a comparison.

The RC-832 receiver seems to be fine. The receiver was compared against an older RC-305 and there was no noticeable difference in sensitivity or image quality.

The other receiver-monitor would indicate interference and noise but would not loose the image completely.

As a second comparison I connected two monitors to the RC-832 receiver; it has two AV outputs. The suspect monitor and another monitor that looks identical but is just from a different seller. The suspect monitor was cutting out while the other was displaying a noisy image. To be sure it wasn’t the AV output, the monitors were swapped over; the problem remained. So it is definitely a problem with the suspect monitor.

 

fpv monitor testingThe suspect monitor passed the bump/vibration testing,confirming it’s not a vibration or movement induced problem.It doesn’t take much interference to completely loose the image, which takes a second or two to return. Not much use for FPV.

Using a clover-leaf antenna on the transmitter and linear antenna on the receiver is not ideal and the problem is quite bad, even over just a few meters. Using a circular polarised antenna n the receiver (clover-leaf or skew-planar) noticeably improves it; fewer drop-outs. Which makes sense, but does not really solve the problem.

fpv monitor testingTest Conclusion

Image loss does coincide with signal level fluctuations and interference. It even completely looses image when simply moving about near the receiver.
It seems that this is just how this monitor responds to a noisy signal or interference. It is not very good at detecting the video sync of a noisy signal. Or it’s a left over of a blue-screen function.

Of 3 monitors that look identical, only one has had this problem. It is sold on various sites as a cheap 7″ LCD monitor for FPV. The problem now knowing how to avoid the dud.

Other Notes:

  1. I sat the test transmitter rig in our microwave oven to reduce the signal. It didn’t work. Those little holes in the screen behind the window are apparently not small enough to block 5.8GHz. Either that, or there is another leak somewhere. I haven’t seen anything glowing in the dark yet.
  2. Two apparently identical monitors behave quite differently. One has no blue-screen but cuts out with interference. The other does blue-screen but displays very noisy images and only blue-screens when the image is unusable. One displays much brighter and more colour when both at the same settings.