I have been flying at my local club for about three months now. My trainer was the victim of this phenomenon. There is a large metal building about five hundred yards away from the runway. I got close to the building at a low altitude and lost all controls. It did a nose dive into the yard of this building and hit a stack of I beams. Totaled. Since then I have seen two more fall out of the sky near this building. They weren't mine as I stay very clear of it. Most of the people at the field claim that the receiver in the plane begins to get two singals from the transmitter. One that is being sent and one that has bounced off of the building and then is received. Has anyone heard of anything like this? A couple of old timers can count many planes lost near the building and consider it a sort of Bermuda Triangle area and warn people to stay away.

Seems odd. I have not heard of it, but that doesn't mean it can't happen. Seems like there should be some way to test it to see if signals are bouncing off of it. If it is, I would think about moving the field or making it a no fly zone so people are not loosing planes there.

It is a no fly zone, unofficailly. What's really wierd is that it does not happen all the time. I've seen people get gutsy and fly in the same place where I lost mine and nothing happens. However I have seen people fly in a different place than I lost mine, and they lose theirs. I know there's not always an explination. I got my plane back and everything worked. There was no other radio at the field that day on my frequency, and my batteries were still charged. And since there are so many stories about the building, I'm sure it has something to do with it, sometimes.

IT SOUND LIKE THE BUILDING WITH STEEL BEAMS IN THE YARD MIGHT BE A STEEL FAB COMPANY AND THEY DO A LOT OF WELDING. THE ARC FROM THE WELDERS OR THE TRANSFORMORS IN THE WELDERS MIGHT COURSE RADIO INTERFERANCE

They do fabricate metal. They make irrigation parts. I can't believe I didn't think of welding. You're exactly right, so we're not going to know when they are and aren't welding. We'll just have to stay out of the bermuda triangle! Thanks a lot. I'm going to tell everyone I found out what the problem is. Even though we can't really do anything about it.

I have read a well documented case where a metal grain silo caused the same problem, so it may not be due to welding. Your radio signal can bounce off the building (like an echo), but it will be delayed. The echo signal will then conflict with the true signal and it will be the same as if another pilot had turned on his radio on your frequency.

RADIO WAVES TRAVEL AT THE SPEED OF LIGHT A COUPLE OF HUNDRED FEET WOULD BE A MILLIONITH OF A SECOND ECHO I DO'NT THINK THE RECIEVER WOULD NOTICE IT IN ANY WAY A FEW MILES MAYBE

I agree with you in theory, but the fact of the matter was the grain silo had no machinery in it, just grain. No other possible source of interference for miles (remote area, on a farm). It only happened when the pilot flew between himself and the silo. Perhaps the silo distorted the signal before it bounced it back... perhaps the electical capacitive property of the silo caused an RC (resistor-capacitor) phase shift which would explain the delay. It only takes 2 milliseconds to shift one servo channel's signal to the next servo.

strato911 is correct.

The phenomena is called reflective sum and difference.

If the radio signal bounces off a large metal object it may create a secondary signal that is delayed in time.

If it reaches the receiver delayed and 180 degrees out of phase, it effectively cancels the signal. Reaching it in phase, it becomes additive.

Reflective difference is subject to frequency, soil saturation (yes), size of metal building relative to the frequency being acted upon, transmitters elevation and distance between the reflected signal and the receiver and receivers vertical height.

The difference between a hit (difference) and nothing (sum or none) can be a matter of inches. That may be why some get a hit and in the "general" area and others do not. It must be exact, horizontally and vertically. Also the pilot's height will change the reflection angle and thus the area where the "difference" appears.

Kinda black magic, but it happens more often than people think.

Yes, this happens quite often. Anytime that the reflected signal is 180 degrees out of phase with the prime signal and of nearly equal amplitude, the signals cancel. It is just like turning off your transmitter as far as the receiver is concerned. This can also happen if the transmitter is near a good reflector so that the signal bouncing off the reflector is out of phase with the transmitter.l We had a chain link fence near the transmitter area and, if you stood at the right spot you could predict where the plane would experience a dead band. By standing right next to the fence, no problems. Usually, if the plane is high enough and moving in the right direction, the interferance or drop out will be brief and you can recover as the plane flys thru the dead zone.

That must be why many fields use plastic snow-fences. For those of you in warmer climates, that's a plastic look-alike of chain-link.

same thing happens with sound waves.

if you want to try it get two subwoofers and hook them up right next to each other.

now play a nice low note.

now reverse the polarity on one sub and play the same note.

it will be a whole lot quieter due to the reversed sound waves canceling each other out.

also it is possible to bounce sound off a wall and get this effect buy harder to predict and "tune" .

If you like canceling sound waves by inverting the original signal, you'll like these headphones

Many years ago We had the same problem at a fiels where we flew. A building with a big metal roof was causing loss of signal. It really does seem to happen.

It's a no-fly zone for an even better reason than just interference... Safety.

There's no reason to be doing low passes near a building, especially one that's 500yards out. At that distance it is hard to judge your distance accurately, and you shouldn't be flying over their property. There's probably something about this issue in the AMA saftey rules, and your flying field's club rules. As a Canadian, I have never read the AMA rules. We have our own set of rules (MAAC).

I'm also glad to hear about the chain link fence, because I mentioned losing control with our chain link. I got control back and everything was OK, but on RCU people said they had not experienced that and it must have been something else. Now I believe it can happen, but with all the variables, no one can predict where and when.
We have a small metal roof at our field (radio impound) and I advised people that we had enough metal, than you very much. They thought I was crazy, but it sounds like a club shouldn't get too carried away with metal installations at the field.

approximately 20 years ago i belonged to a club that flew aboout 1000 feet from the main road and farm buildings and sometimes when flying near the building with metal roofs some planes would respond to signals bouncing off the metal roof. we just kept the situtation in mind and kept our distance from the buildings.
KitKat

The RF energy travels at the speed of light.
Even if the signal did bounce off the metal roof it's going to reach the RX at the same time and the same frequency.
What you need to do is go inside that building and see what is running.
I found a building that had a swimming pool seam sealer that had the bar covered with copper.
That bar was actually an antenna for the 440 band and it generated 15,000 watts and that created heat between that bar and the ground plate under the plastic.
If their running something thats in violation of FCC then you can have them shut the equipment down.
I worked with QRM tracking for many years and I'd sure like to see what's in that building.
Go in and le them know what is going on.
Just a suggestion

Whirley Bird:

Speed of light has absolutely nothing to do with it.

Look at a couple of everyday examples that are not directly r/c.

If you watch TV and are not on cable you have seen the effect of large airplane propellors messing up the picture. Or if you are driving arould town with the FM radio going you'll have places where the sound goes to pot. All the same thing with the r/c plane.

As strato911, Crash_N_Burn, and Rodney were pointing out, it's phase shift. Propagation at the speed of light has nothing to do with it.

The building, or the silo, or even a water tower can act as a reflector - like a radio mirror.

Don't fly around large metal structures.

So long as the distance from the transmitter to the plane is less than approximately 1/3 of the distance from the transmitter to the reflector and back to the plane you should have no problem.

Don't fly around large metal structures.

But as the difference decreases you are going to reach a point where the primary signal has weakened due to inverse square, and the reflected signal is near the same power to the plane, then at just the right distance and angle you crash. End of argument.

Don't fly around large metal structures.

Just ask any "Ham" operator who works six meter or higher about wave reflections. Or ask your local FCC agent. Or a broadcast engineer from your local TV/FM station about the fun they have with antenna design. It's all known.

Don't fly around large metal structures.

Just iin case I haven't made it clear:
Don't fly around large metal structures.

If only you were inside the plane,
. You could take a hit, and get it back again.

Bill.

PS: Don't fly around large metal structures. wr.

Whos argueing Bill?
I know several TV engineers that work in NY and are hams like me
They Know a lot more then me and agree with me.
Read your letter again.
I fly from a fram and the large metal barn there never had an effect with my system.
READ THE BOOK AGAIN!!!
.We know about skip,long wave,short wave ,etc.
Read your book again.
Keep the topic hot.
I'm going skydiving

Whirley Bird:

I used "Argument" in the classic sense, meaning, roughly, supporting statements. Did not mean to ruffle feathers.

And with that Metal barn at your field, you've either been fortunate or never flown too close. I would hope the latter is the case.

Oh,and by the way, Don't fly around large metal structures.

Haw.

Radio interference in your aviation?
. Don't worry, not a Martian invasion.

Bill.

Bill,
After leaving the airport we went flying and I not only flew near that large metal barn,
I flew right through the front door and out the back door.
The guy also has some really high silos there made of metal.
So far no problems.
A few weeks ago when we had some heavy rains a local flyer was flying his heli inside the barn and that building is 100% all steel.
So?????
Well I E mailed the Engineer that workes for a TV station in NY and he'll let me know what happened.I sent him the other persons post.
Hey,
It would be nice to see whats in that building.
I think it was Stratto911 that said there was no reason to fly 500 yards close to a building.
(I AGREE 100%)
I'll be speaking to my friend on 2 meters tomorrow night.
Thanks for the post

Actually, it was: "There's no reason to be doing low passes near a building, especially one that's 500 yards out."

At 500 yards, the human eye has a very difficult time perceiving depth enough to prevent HITTING either the structure or people in the vicinity.

Hi Strato911,
Yeah,
That a long range and you can't see the plane that well.
Sure would be nice to know whats in that building though just to see whats bringing the planes down

I think you guys are missing the point why is it i have to have my fm ant. pointing a certain direction to hear my station . and where I fly we stand by a metel hanger and when the wind is from the west we stand to the south of the hanger because if you stand in front of the building there is this 4 foot by 4 foot box 20 feet off the ground and several hundred feet to the east that makes planes eat stubble.
nothing around for 1 1/2 miles