flyinfriend

Hi,

I have searched the forums for this but found nothing conclusive. While standing a couple of feet away from my plane and pointing the ant directly at the plane all of my servos twitch wildly. From a distance this does not happen and while close up it has to be pointed at one exact spot. Does anyone know why this would occur?

My radio is a Hitec with spectrum synthesizer and the reciever is a 7 channel Futaba. All servos respond normally and this only happens in one spot while pointing the ant at the reciever.

Thank you for your help,

Tom

GaGeeBees

My Feedback: (23)

Taken directly from the manual that came with my Hitec Eclipse: "It is a good idea to avoid pointing the transmitter antenna directly at the model at all times, since the signal is weakest in that direction." I'm no expert on what the signal field coming from a Tx transmitter looks like so I can't speak to why this happens. Every Tx manual I've read contains a similar statement about never pointing the antenna directly at your airplane.

BarracudaHockey

My Feedback: (11)

Its called the cone of silence and it decreases as you get farther from the transmitter.

RTK

My Feedback: (1)

I thought that was only in Get-Smart

Radio wave propagating from the ant looks like a butterfly wing attached to the ant with the cone of silence starting at the end of the ant.
I hope that was understandable.

Dizzy Pilots

Besides the output being weakest from the tip of the antenna, you also have strong reflected signals that are out of phase causing multipath distortion. When you are close to the model the reflected signals can be as strong as the signal radiated from the tip of the antenna and can distort or cancel each other out.

I've noticed it also and assumed it to be caused by strong multipath signals.

Flypaper 2

If it goes away when you move away from the reciever, sound like simple swamping, to strong of a signal at close range.

Earlgrey

Keep your antenna pointing towards towards your receiver at approx 45 degrees and you won't be far out.

JPMacG

My Feedback: (2)

I think Flypaper is correct - it is swamping caused by too strong a signal at the receiver.

Dizzy Pilots

If he is overloading the receiver, then explain why it only happens with the tip of the antenna pointing directly at the plane, at one exact spot?
I'm sure with the transmitter in other positions the signal is even stronger and the servos don't twitch.

Flypaper 2

Explain why it doesn't happen when he backs away from it.

JPMacG

My Feedback: (2)

I think there probably are other positions where the receiver will glitch when the transmitter is held close to the receiver. I know there are with my airplanes.

I completely understand that a monopole or dipole has a null off the axis of the wire, but this is in the far field in free space. The situation flyinfriend describes is much different.

JPMacG

My Feedback: (2)

OK, I'm now going to eat my own words.

I just ran an experiment. I turned on an airplane and transmitter and placed the transmitter antenna parallel with and very close to the receiver antenna, so that the signal from the transmitter was as strongly coupled to the receiver as possible. I could not produce a single glitch.

The cause of the problem must be multipath or polarization mismatch.

Dizzy Pilots

I wonder if it's possible that the receiver is picking up the signal from the each side of the transmitters antenna out of phase.

BarracudaHockey

My Feedback: (11)

Like I said, its the cone of silence, as you back away from the model the transmitting pattern starts to fill in the cone. If I had some pictures of the radiating pattern it would make sense. If it was swamping it would occur in all orientations.

Dizzy Pilots

I think there is more to it then the cone of silence.
Here is a picture showing where I think the receive antenna would be in the cone.

JPMacG

My Feedback: (2)

I'm thinking the jitter might be caused by large swings in amplitude envelope of the rf signal at the receiver. The tx is close to the rx, so the direct and multipath signals are both strong and add or subtract to produce large variations in signal level. Maybe the automatic gain control circuit in the receiver can't keep up with the large variations (inadequate slew rate of the AGC?). This would not a problem at greater distance because the signals are smaller so the variations are smaller.

Interesting topic and one I've always wondered about but never seen discussed before. All my receivers do it.

BarracudaHockey

My Feedback: (11)

Dizzy you're close.

Dizzy Pilots

In that one position the signal is trying to cancel itself out.

AllTheGoodNamesAreTaken

My Feedback: (180)

Not trying to Hijack this tread or anything, but I had a glitching problem today. I brought 4 planes with me and all but one started glitching at one point or another which makes me think that it's in my transmitter. Anything I should check? I'm thinking about sending it in for service. I'm afraid to fly now since I almost lost one plane on landing.

Just for S&G's, I checked all of my planes agian now that I'm home. No glitching. ?????? I've read somewhere that hot radio gear can cause glitching. It was over 80 today and I did have my transmitter in the sun for a while. Could heat be the problem?

dirtybird

My Feedback: (5)

AS pointed out above this antenna pattern is not accurate for the near field. You must be many wavelengths from the antenna before the beam starts to form.

Dizzy Pilots

I did a little experiment using my Yaesu FRG-9600 receiver with a telescopic antenna and the 20 DB attenuator on.

The receiver read between S40 & S60 with no distortion no matter how I held the transmitter. I almost gave up trying to find the right spot, then I found it at about 50 inches tip to tip at 90 degrees, the signal dropped down to S7 and became very distorted.

Maybe the received signal is just 90 degrees out of phase.

Dizzy Pilots

It's possible that someone might have commercial transmitter on the air 10KHz away from the frequency that you fly on.

About 8 years ago I started getting glitches on channel 58 and never did before. It turned out to be from a commercial transmitter 10KHZ away.
We have a lot of very strong commercial transmitters in-between the 72MHz RC channels around here and they do cause glitches when they come on the air.

If you can find someone with a receiver that will let you tune around the 72MHz band and see if you pickup anything near your frequency.

If your transmitter passes a range test on the ground it's probably OK, unless it has an intermittent problem. I don't think having it in the sun got it hot enough to affect it any.