ImRich
Veteran
Location: Derry, NH USA
| Extreme Range TestEveryone will experience different results as lot depends on the particular equipment which you are using.
Also one person's succesful results is another persons 'not acceptable'. I'm very fussy and do not want interference at all. We have to remember that we are flying aircraft that can cause damage if they crash into something of value. If you fly now where near anything of value and there is nothing near you for miles, then you do not need to be as careful as someone who flies in the middle of a crowded city.
My early tests were with an FM receiver and using a 200mw 900 MHz transmitter. This was done about 10 years ago. The 900 MHz RF was getting into the wiring of the receiver and servo's causing servo jitter at times.
I can't stress enough that your particular installation may be different. Do not think that just because one person didn't have a problem that you will be ok. You must validate your own installation!
At the time I had to sheild my 72MHz RC receiver and place ferrite chokes on all leads (except the antenna) going into and out of the receiver. I also had to position the transmitter antenna as far away from my receiver and servo wiring as I could. This combination solved my interference problems.
To detect if you have a problem lurking, you must do what I call an 'extreme range test'. This procedure is similar to the following:
Before making any changes to your system or installing your video downlink equipment perform the following test. Also be sure to start with a charged battery each time so that battery voltage will not affect the tests.
Note that this is for FM (PPM) equipment, a different procedure must be used for PCM equipment.
Extreme Range Test
- Keep your transmitter antenna collapsed as you do for a normal range check.
- Make note of your aircraft position. You must keep it in the same place for each test that will be made in this procedure.
- Start moving your transmitter away from your receiver, keep your transmitter antenna pointed at the same direction. Pointing it at the aircraft is ok as long as you do this consistantly through all tests. Also slowly move your control sticks from time to time. There should also be a period of no movement of your control sticks. Check to be sure that you have full control of your servos. Note that there is no jitter in the servos, especially when you are not moving the control sticks. it helps a lot to have a knowledgeable person carying the transmitter so you can listen to the operation of your receiver/servo's.
- See how far you can go until you just start getting some jitter in your servos. Note this distance.
- Keep moving away from your aircraft until you can find the point where control of your servos either stops completely or becomes very erratic (which would indicate total loss of control). Note this distance.
Now install your video downlink equipment including your video camera.
Turn on the RF transmitter, but do not connect the camera or turn it off. Do this so there is only an RF carrier (no video/audio modulation) being transmitted.
Perform steps 1 though 5 again being sure to have the aircraft in exact same position as the first time you made the tests. Also be sure to move the transmitter away from the aircraft in the same direction as you originally did. Make notes of the distances achieved in step 4 and step 5. If they are the same as the first step, then you are ok to continue. If the distance is significantly less than in the first test, then this means that the RF transmitter is causing some sort of interference. Try the test again with the RF transmitter off to confirm that you can repeat the same distances as you acheived before you installed your RF equipment. If so, then you must solve this issue before continuing.
The last step is to turn on your RF transmitter while transmitting video (and audio if you have audio abilities). Be sure that your camera is looking at a high contrast image. Creating a full image of a white sheet of paper with a lot of alternating thick black and white blocks is the best torture test. Or be sure that the camera is pointing at a brightly lit area which has both dark and light areas in it. Then perform all tests again making notes of the distances in step 4 and step 5 again. If there is no significant difference between your first tests and this last test then you are ok to fly and have no interference issues.
If the distances achieved in steps 4 and 5 of any test are significantly different than the first test which you made, then you must find out why and solve the problem before continuing.
If you want to be fully safe you will perform the last test one more time with your engine running at speed. This is a very difficult thing for a heli pilot to test as if there is any interference, then the heli may go out of control and someone could be hurt!
Special considerations for PCM systems:
These tests must be run with PCM receivers also, but it's difficult to detect the points in step 4 and 5. You have to understand how PCM will react differently than an FM (PPM) receiver when it is losing signal strength. At first a PCM receiver will start to lose frames, so you may notice that instead of jitter in step 4 you may see slower servo response. This may be very difficult to detect, it depends on your particular receiver, transmitter, servos etc. You may go completely to step 5 without finding the point in step 4. It's best to have your Fail Safe set to engine IDLE and perhaps have your fail safe collective set to full negative pitch for this test only. This way if you go into fail safe (loss of signal) you can detect it by the collective going to full negative pitch.
Do not be lulled into the sense of security if one person tells you that they tried a particular RF video transmitter and no problems. They may be telling the truth, but you may experience major interference even if you are using the same brand of equipment and have a similar installation. You must test your system to be sure that it's working well. You should also perform periodic tests to be sure that nothing has degraded to the point of causing interference when you were once interference free.
Another guideline if you have a PCM system is to program your Fail Safe to turn off the video downlink transmitter if your signal is lost. This may help you to recover an aircraft if you get some in flight interference. Just be sure to turn off the video with your transmitter control because if you regain control when the system goes into Fail Safe you do not want the RF transmitter turning back on and causing interference again!
I hope this procedure is clear enough to follow and someone finds it useful. I can't go into the details here about how to solve an interference problem, but hopefully this message will allow you to test to see if you have any interference issues lurking in your setup.
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Rich |
