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Adjacent Channel Interference

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Old 07-24-2006, 11:34 AM
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coupar
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Default Adjacent Channel Interference

I fly at a small club so last weekend was the first time we've had two adjacent Channels (45 and 46) being used at the same time. We noticed that, when a transmitter is being used with antenna extended it causes serious glitches on the adjacent channel's receiver UNTIL that channels transmitter is switched on. Both receivers were exhibiting the glitches, both were Hitec but both were dual conversion. One Futaba transmitter, one Hitec.

Is this normal? We did'nt risk flying together but how serious an issue if we did?
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Old 07-24-2006, 11:43 AM
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Default RE: Adjacent Channel Interference

As an RF engineer that does transmitter and receiver design (not model equipment), I would consider that pretty normal. You were probably pretty close to the receivers with the transmitters, and even though model transmitters and receivers are pretty good these days, operating a transmitter only one channel away in close proximity to the receiver, without the proper TX being on, would cause some glitching. Turning on the proper TX would drive the limiters in the FM receiver into saturation (which is desired) and effectively reduce the gain, to the point of the sidebands from the off-channel TX no longer being big enough to cause any trouble.

It's not a "normal" situation to have an off-channel transmitter very near your receiver with your own transmitter off. I would have been more surprised if there wasn't a little glitching in this environment!

If you have proper narrow band receivers, properly tuned transmitters, and don't make a practice of flying five feet from the other guys's radio when you're 100 yards away, you shouldn't have any trouble with adjacent channels.

We had a Q500 race this past weekend, with radios on frequencies next to each other, and didn't have any trouble from radios. I think you're probably ok..

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Old 07-24-2006, 12:54 PM
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Default RE: Adjacent Channel Interference

I have flown in a group with channels 11, 12, & 13 all close together with no problems at all.
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Old 07-24-2006, 03:37 PM
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Default RE: Adjacent Channel Interference

We often have flyers with adjacent channels with never a problem. Neither of the two reported problem units had the transmitter crystals changed without the proper check, did they?
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Old 07-24-2006, 05:43 PM
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Default RE: Adjacent Channel Interference


ORIGINAL: coupar

We noticed that, when a transmitter is being used with antenna extended it causes serious glitches on the adjacent channel's receiver UNTIL that channels transmitter is switched on. .

Is this normal? We did'nt risk flying together but how serious an issue if we did?


Yes its a normal response. Anytime your rx is on without the Tx on then its looking for anything out there to respond to. Severe glitching can occur when you do this and can even damage servo gear trains thats why all manufacturers suggest the Tx is first on and last off.

Fly your airplanes and enjoy.

John
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Old 07-26-2006, 06:15 AM
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Default RE: Adjacent Channel Interference

Hi Fizz,

I am in Hong Kong and use 72 MHZ FM - I suffered really bad interference for 20-30 second and was lucky not to lose my plane.

The Hobby shop calmly explained that interference on FM in HK is common.

Apparently my flying site is near a radio transponder, Taxi and truck drivers have FM radios, some scanners may block bands. He gave me an example that a Chinese Truck driver using an FM booster to communicate to a base on the mainland could wipe out most of 144 band, 1/2 of 72, 1/4 of 36 etc. Apparently almost without exception experienced Hong Kong pilots get PCM systems...

Is this explanation sound accurate and valid.
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Old 07-26-2006, 11:16 AM
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Default RE: Adjacent Channel Interference

Thanks for the responses. They are reassuring.

The TX crystals wern't changed at all. The interference was happening at 25ft with the antenna extended.

It's probably something we just did'nt notice before.

Anyway, thanks again.....now I have some names to blame if planes start falling out of the sky!

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Old 07-26-2006, 04:43 PM
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Default RE: Adjacent Channel Interference

ORIGINAL: coupar

now I have some names to blame if planes start falling out of the sky!


I'am afraid that having names would not even buy you a cup of cold coffee or a stale glazzed donut at the Flightline Cafe.

John
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Old 07-26-2006, 05:22 PM
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Default RE: Adjacent Channel Interference

mmmmmmmmmmmmm, donuts...
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