The nulls off the ends are incredibly accurate at pointing to the anomaly. It took me ~15min and is 72", center is tuned to 72MHz +-1MHz @ minimum -15dB on the network analyzer...
Doug McLaren wrote:
On Thu, Mar 31, 2005 at 11:29:07AM -0800, John Erickson wrote:
| We've has a couple of club members lose planes recently from | interference.
Personally, I've seen interference blamed for a lot of user errors too. Not that this is the case in your situation, but ...
| We have a scanner on the field. It shows a channel is on | (sporadically) although all our pins/radios are accounted for. We | suspect another flyer about a mile away but we've yet to find him. | There are both houses and an industrial complex nearby. I know they | do some robotics in the industrial area.
You haven't said otherwise, so I'll assume you're in the US. Your address suggests so ...
What sort of scanner exactly is it? Does it let you listen to the signal? Our R/C transmitters have a rather distinctive sound and it should stick out very quickly. (Of course, it's also possible that the signal isn't FM. Could be AM, or some digital mode that would sound best with a SSB receiver.)
Here in Austin, TX, there's strong pager signals (yes, people DO still use pagers!) on a few frequencies in between our R/C channels. They're intermittent and last only a second or two, but they're either strong enough or transmitted from enough towers to be picked up all over town. Being only 10 kHz from our channels, they could easily overwhelm a receiver and send the plane out of control if it can't reject glitches.
You'll also find wireless microphones and pagers of the sorts used by restraunts in the 72 mHz band. And I've heard that railroads and industrial uses can be assigned frequencies in the 72 mHz band for R/C use as well.
Pagers also have rather distinctive sounds. http://www.kb9ukd.com/digital/ gives a number of digital data transmission modes (and sample .wav files) amd you might be able to identify something by listening to it. You might even be able to decode it with the right software :)
[ Mental note: I need to send this guy some sound bites of R/C transmitters. ]
If your scanner/receiver gives you a signal strength meter, you could make a 72 mHz yagi antenna and use that to determine what direction the rogue signal is coming from, if it lasts long enough. Of course, such an antenna will be quite large -- roughly 2 meters by 2 meters.
And even if you find the source of the signal, there's likely to not be much you can do about it. If it has a fixed frequency, you could stop using the frequencies near that. If it's a guy with a park flier, you could ask him to fly somewhere else, or work out some frequency sharing arrangement with you, but ultimately he has just as much right to the frequency as you do, even if you're flying at an AMA club and he's not.
And yes, the ultimate solution to most of our frequency control issues will be some sort of spread spectrum setup. Alas, while this is completely feasable and practical technically (and even cheap!), politically there's still a number of issues to be overcome before it can become reality.
People *do* already sell spread spectrum R/C equipment -- it's out there now. But it's on the 900 mHz and 2400 mHz bands, and the power allowed is very low, so there's lots of interference of all sorts, and so the range will be relatively poor. And the AMA regulations don't permit it's use yet, so that means that AMA members can't really use it, at least not and still follow the AMA rules that you've agreed to follow.
Here's some --
http://www.auav.net/spread_spectrum_radio.htm http://www.ajhobby.com/product.asp?pid=2420
Not that you should buy it, but it is out there.
| Would it be possible to start your flight with reduced range (3/4 signal | strength) and then flip to full strength in an emergency?
3/4 signal strength would be almost identical to full strength.
Even 1/10 strength will get pretty far -- which is why rubber duckies and similar antennas work well in most cases.
If you really wanted multiple power modes, I'd suggest a 1/1000 th power mode for doing simple adjustments to your plane without interfering with others (but even then there's the possibility, so it's not such a great idea) and full power. But the danger is that somebody would accidently take off in low power mode ...
| Better yet, a way to boost signal strength?
Sure, you could make a 20 watt transmitter. It would be illegal in most of the world, and would cause problems for your fellow fliers, as well as getting really hot in your hands, but it could be done. I wouldn't suggest it.
What can you do that's reasonable and legal? Keep using your scanner. If the interference is always in the same place, stop using those channels near it. If possible, find out where the signal is coming from and talk to the people in charge of it -- maybe you can work something out, maybe not. (And remember, you probably can't force somebody to do anything. And if you try, you could make them angry enough to deliberately shoot down your planes.)
Have people use receivers that reject glitches -- either PCM, or the new `smart' PPM receivers that have become popular. These aren't a magic bullet, but at least when the interference hits your plane's controls won't go to random positions, but instead will stay right where they were or maybe eventually go to a failsafe location.
-- Simon Van Leeuwen RADIUS SYSTEMS PnP SYSTEMS - The E-Harness of Choice Cogito Ergo Zooom
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