On Fri, Apr 01, 2005 at 09:09:27AM -0800, John Erickson wrote: | I want to thank everyone for their help and advice with our situation. | Doug, we are in California to answer your question.
It wasn't really a serious question -- but people tend to get annoyed when you mention the FCC and the AMA, and they're in Europe. | I've seen many, many instances where the famous "I'm being hit!" gets | interpreted as interference when in fact it has nothing to do with another | signal, but has to do with something in the plane or transmitter. | | Tops on the long list include an unseen break in the receiver antennae | (usually at the solder joint on the circuit board), an unseated crystal, | poor solder joint on a battery lead, switch problems, unscrewed transmitter | antennae, a transmitter left on in the pits, and the (in)famous dumb thumbs | exacerbated by turbulent conditions. Yes. Though what I've seen is more usually user error (dead battery, transmitter in the pits, antenna down, dumb thumb, ignoring turbulence) than mechanical/electical failure (unseen break, poor solder joint, unseated crystal.) | I wasn't at the field during the latest episode, but I was there | when the first plane went in. We have an ICOM IC-R2 hand held | scanner. The transmitter was shut off but there was still a strong | signal on channel 16. The IC-R2 should let you hear the signal. Did it sound like a RC signal or did it sound different? Try both FM and AM -- it could be either. (It could also be something else, but the IC-R2 only does FM and AM, so that's all you've got to work with.) I've got a Yaesu VR-500 which is somewhat similar, and just because it picks up a signal on a specific channel, that doesn't mean that there's a signal on that channel. General purpose scanners aren't always good at picking up a signal at only a specific point on the dial -- often a strong local signal will show up at several frequencies on the scanner due to the way the internals of the scanner work. For example, my scanner picks up the local 93.7 radio station somewhere in the 72 mHz band, but I know it's not really there. (the strong signal is probably bleeding in somewhere where it shouldn't. And I think there is an intermediate stage at 10.7 mHz, and 93.7 - 10.7 * 2 = 72.3 mHz ...) ... | There is a church nearby that uses a wireless microphone set up. If that's it, it should be obvious. :) | My original post was just a wish list item. Using PCM will allow a | fail safe mode, but sometimes fail safe isn't so safe! For | instance, if you're in a spiraling dive and you go into fail safe | with flaps out, and all other surfaces level, you'll still crash, | maybe just not as hard. Second wish list item: Parachute | deployment? Some of the new PPM receivers give you the same failsafe functionality as PCM. Really, the big advantage of PCM is to let you turn your engine off -- which isn't so important on a glider, though turning your spoilers on may be almost as useful. As for putting the surfaces level and flaps down, if the plane is stable and has plenty of dihedral, the odds of it making a gentle landing somewhere are quite good if set up properly. If it's very nimble and not stable, no matter how you set it up, if you lose control, it's probably going to crash hard. I'm not sure I'd want a parachute. If you get control again, that parachute is going to hamper your attempts to bring the plane back. | I understand that spread spectrum is still a ways off. It seems | that there should be a way as Jim B. suggests that each one of us | could have a "digital signature", that the first signal the receiver | hears is the only signal it hears and all other interference is shut | out. I saw it demonstrated with a Berg receiver and it looked very | promising. It sounds nice, but I don't see it as being that useful, and I'll explain why ... The `old style' receivers, like the Futaba R127DF, Hitec 555 and many many others, will send your servos jumping madly if they can't pick up your transmitter signal anymore, or if it's corrupted somehow (like by another signal.) This is bad, as your nimble plane is suddenly out of control, or your power plane just went to full power and took off without you telling it to (happened to me once! :) On the bright side, the interference is hard to overlook. PCM only moves the servos if it receives a valid PCM signal. If there is no valid PCM esignal, the servos do not move, and if it persists, they go to their failsafe position if set up to do so. This is good, but it can hide interference issues. PCM does not solve the interference issue -- it just hides it. The more modern receivers (FMA M5, many Berg receivers, others) are still PPM rather than PCM, but they give you most of the PCM ability to ignore noise. If a frame comes in that's not valid, they don't move the servos. Some even give you failsafe functionality. They're not quite as good as PCM, as a valid PPM frame is a lot easier to constuct accidently (i.e. by noise) than a valid PCM frame, but for the most part they work very well. So almost no glitching. The very newest receivers (again, some Berg, FMA, others) have DSPs that `lock' onto a specific transmitter and will only respond to it. Assuming that it works as advertised, that's fine, but this still doesn't allow two transmitters on one channel. Due to the FM capture effect, an FM receiver will generally tune in the strongest transmitter and tune out the weaker one, so it'll usually receive one or the other, but not both. (This is good -- it's why a guy flying a mile away from you on the same channel generally won't cause you big problems until your plane gets around half way between you and him. With AM you'd lose control much earlier.) Back to the newest receivers -- if your receiver is picking up your transmitter, all is well. But if it starts picking up the other transmitter, or a mixture of the two (the FM capture effect isn't perfect), then the DSP will go `this isn't the right transmitter!' and will either not move the servos or go into failsafe mode -- nobody has control. So, if your plane is picking up the other transmitter stronger than yours, it's either responding to that other signal (newer receiver) or ignoring that other signal (but not getting your signal either) (very newest receivers.) Either way, you don't have control, so I'm not really sure that one is much better than the other, though I'd agree that going into failsafe sounds a little better than obeying somebody else's stick movements. (But just a little.) I'd also be a bit afraid of something changing my transmitter's signal enough to make the receiver's DSP not recognize it anymore and start ignoring it mid-flight -- as it warms up, or the battery drains, or switching to a buddy box. Or accidently dropping/jostling it while you're flying. I haven't heard any reports of this actually happening, but it would still make me wonder just what they're looking for in the very simple PPM signal that lets them identify a specific transmitter. | The new Shadow 3 sounds good as well. Yes it does. Though what really sounds good is the synthesized part, and how it can do both 72 and 75 mHz. The Shadow 1 could even do 50 mHz which was really impressive (though it makes me wonder how well it could reject other signals listening to so large of a range. -- Doug McLaren, [EMAIL PROTECTED] If my calculations are correct, SLINKY + ESCALATOR = EVERLASTING FUN RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off. Email sent from web based email such as Hotmail and AOL are generally NOT in text format
