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Strength of signal and 3IM

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Old 03-29-2004, 11:53 PM
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tailskid
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Default Strength of signal and 3IM

Being somewhat brain-dead concerning electronics.....can there be a strong enough signal created (3IM) if two of the radios (Ch. 54 and 37) are activly flying (antennas extended) when the third tx is turned on - with antenna collasped. Can this cause loss of control of channel 54? The distances between the two active transmitters was greater than 50', with the third radio about 75 feet from the pilot on ch. 54.

The plane rolled over inverted and at approx. 10 degree angle struck the ground. This happened directly in front of the pilot and about (I'm guessing) 75-100 feet from his tx.

I told ya I wasn't too swift on this electronic thingy......

Comments?

Jerry
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Old 03-30-2004, 01:33 AM
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Default RE: Strength of signal and 3IM

the radio that was turned on was also on 54? if so, then there is a very good chance it was the direct cause of it. even with the antenna collapsed, it still emits RF.
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Old 03-30-2004, 07:24 PM
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Default RE: Strength of signal and 3IM

No the ch. 54 was flying, channel 20 was turned on with the collasped antenna.

Jerry
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Old 03-31-2004, 11:29 AM
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Default RE: Strength of signal and 3IM

I could see that SS could work on 2.4 GHZ where there is plenty of bandwidth.
But how can it work on our 50 spot frequencies?. And with several other channels mixed in?
Is there something I am missing here?
I don't know how SS works. If you do let the rest of us know.
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Old 03-31-2004, 12:05 PM
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Default RE: Strength of signal and 3IM

Are you asking about intermodulation products (IM)? If so, I'd say the answer is that it is extremely unlikely.
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Old 03-31-2004, 02:03 PM
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Default RE: Strength of signal and 3IM

In my opinion there is little to no possibility that the crash on Ch 54 was caused by the turn on of a Tx on Ch 20 with the antenna collapsed. Even with the antenna fully extended there should be no interference...........RJ
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Old 03-31-2004, 02:40 PM
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Default RE: Strength of signal and 3IM

I'm not an Rf expert, but...

What was the RX in the crashed plane? The RX has more to do with getting "hit" from other radios coming on than anything.

Unless the RX was a rather old single conversion, or AM RX, I'd say the chances of the radio casusing the crash are basically near-0. Espeically since the model was fairly close to it's TX, and specifically closer to it's TX than any other TX (if I understood correctly).

Most of these IM effects are worse when the plane flys away from it's TX and close to the interferring TX's like on a long low pass down the runway with the pilot at one end and the other TX's at the other end. That's your worst-case situation for interference of all types.

(My GWS park flyer RX (single conversion, FM) on channel 27 gets really unhappy when some other TXs come on, espeically if they are on channel 50, even with their antenna's down. While all the JR, Hitec, and FMA duel conversion FM receivers I have routinely handle much worse when it comes to multiple radios on at once. (I flew a combat contest two weekends ago (Paris, TX) where we had as many as 13 planes up at once, with pilots standing all over, sometimes closely spaced. I didn't see a single radio hit, or hear anyone complain about interference. Our modern gear is amazingly good at rejecting all kinds of garbage).

A possible cause for the crash could be a mis-labeled crystal on one of the TX's in question. I've heard of this before, though it's not common.

Sounds to me like someone dumb-tumbed his plane, and is looking for an excuse .
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Old 03-31-2004, 09:54 PM
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Default RE: Strength of signal and 3IM

The receiver that was 'hit' was a JR 700. I still don't understand the 'strength' of the third tx. - its antenna was down, but the ch 54 and 37 were extended and flying when the ch 20 was turned on.

I thought RF was tied very closely to the strength of the signal.....

Jerry
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Old 04-01-2004, 02:14 PM
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Default RE: Strength of signal and 3IM

Ok, no way that a functioning JR 700 RX on channel 54 or 37 was hit by a radio on 20. Even if the ch20 TX had it's antenna up and was right next to the other two TX's, I wouldn't expect a problem.

That crash must have been caused by something else, bad antenna, low battery, loose or failed crystal, loose nut on the TX stick, something, but not that radio coming on, antenna down, in the pits.
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Old 04-02-2004, 09:45 PM
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Default RE: Strength of signal and 3IM

If ch. 37 and 20 are on, it creates ch. 54! EXAMPLE: 37 X 2 =72 - CH. 20 = 54!!! At least that is what some are claiming!

Jerry
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Old 04-02-2004, 10:46 PM
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Default RE: Strength of signal and 3IM

LOL... that's funny. Sort of the right idea, but you have to use the frequencies in MHz, not channel numbers.

If you use MHz to do the intermod calculation (N*F1±M*F2) where N and M are integers you will find that N and M have to be very large to put an intermod product on another RC frequency. The strength of the intermod product goes down rapidly as N and M increase.

Bottom line... don't worry about it.
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Old 04-02-2004, 11:27 PM
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Default RE: Strength of signal and 3IM

That's the type of info I need (being electronically 'challanged' ).....we had this incident at the field and some are running around like Chicken Little!!!!! "The Sky is Falling..."

Jerry
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Old 04-03-2004, 02:30 AM
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Default RE: Strength of signal and 3IM

Let's see.....

2 x 72.53 = 145.06
145.06 - 72.19 = 72.87

It doesn't matter whether you use channel numbers or frequencies. If I measured the frequency in radians per second it would also come out the same.

To save you all some time

Ch 20 is 72.19 MHz
Ch 37 is 72.53 MHz
Ch 54 is 72.87 MHz.
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Old 04-03-2004, 07:04 AM
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Default RE: Strength of signal and 3IM

ORIGINAL: tailskid

If ch. 37 and 20 are on, it creates ch. 54! EXAMPLE: 37 X 2 =72 - CH. 20 = 54!!!
If that was the cause of the crash, then imagine how complicated their frequency pin board must be...
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Old 04-03-2004, 08:13 AM
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Default RE: Strength of signal and 3IM

And that's the point....some want to revert to what they call the 'White Pins" where only 5 TX can be on at one time....which wouldn't have made a difference if the '3' TX were 37, 54 and 20!

Go figure!!!!!

Jerry
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Old 04-03-2004, 08:35 AM
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Default RE: Strength of signal and 3IM

Oops... very sorry, I goofed. Thank you Phil. Please disregard my previous post (as Phil says, you can use MHz or Radians per second or whatever, as long as you are consistent and the units are 'linear' (our channel numbers are ok since they are evenly spaced).

So we are speaking of third-order (or 5th or 7th, ...) IM. There is a possiblity of interference except that the transmitter will be much stronger than the IM source. Typically more than 60 dB (one million) times greater and more likely 100 dB. Would you all agree?
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Old 04-04-2004, 01:04 AM
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Default RE: Strength of signal and 3IM

Pretty much.

If the problem at Jerry's field was IM, and all the equipment was in good order, then at least one of the signals must have been quite strong.

The intermodulation can occur in the transmitters or receivers, but they have to be really close. The reason flight stations are spaced apart is so adjacent transmitters don't intermodulate each other. If a receiver intermodulates, it is either very badly designed, or is receiving signals much stronger than would be case if it was in a model that is actually flying.

I just got back from a day of racing. 35 races were run with five pilots standing just far enough apart so we didn't poke each others' eyes out. No radio problems in well over 100 flights by 30 different planes. If intermod was going to be problem in normal use, we were very likely to have seen it.
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Old 04-10-2004, 10:39 PM
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Default RE: Strength of signal and 3IM

I belong to the club in question, and saw the incident, and also remembered the 3 IM formula, which resulted in two different "pairs" that could have produced a signal on crashed Ch. 54.

I don't know any more about radio technology than Tailskid. However, I lived through the "growing pains" with AMA when the new frequencies were phased in from about 1983 through the early 1990s. I know that one of the causes of interference that AMA was concerned about was 3 IM.

If inquiring minds would like to research this some, go to the AMA site www.modelaircraft.org
and click on the archives section they have just installed. Do a search on "Third Order Intermodulation". I think I got hits on some 30 articles from about 1983 to about 1990, many written by George Myers, one of the movers and shakers that helped get our new frequencies.
The question is not whether there is such a thing as 3 IM, but whether the newer radios are so good they can reject a weaker signal on their frequency.

And.....what about the older receivers that I am sure exist in a club of some 300 members?

Our club installed the "white pin" system in the mid-eighties when interference was a real problem all over the country during the phase-in of the new channels. You had to take your frequency pin, plus one of five white pins before turning your radio on. This restricted the number of frequencies on at a time, which statistically reduced the chances of getting hit. It also cut down on the number of people with radios on in the pits adjusting their planes while as many as five other pilots were flying at the flight stations. It seemed to work very well for us.

Clair Sieverling
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Old 04-25-2004, 04:52 PM
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Default RE: Strength of signal and 3IM

Azcat59,
Some procedures that were is use in the past just are not needed today. Many times, I have seen people flying while standing and or sitting right next to each other with no concern as to who is on what channel and no problems were encountered. When the new channels were introduced, a lot of clubs banned the odd channels for fear of problems for the wide band receivers. After widebanbd rceivers were fazed out, the odd channels were allowed and no problems have been encountered since.
It seems that from the replies to this post and another recent thread started by Tailskid, there is no longer a problem with 3IM. It may not be what you want to hear, but there it is. The equipment now is far superior that what was used in the 80s.
You know, in the past, some Doctors probably thought that we should alway use blood letting as a cure for most ailments before they knew any better...
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Old 04-25-2004, 08:45 PM
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Default RE: Strength of signal and 3IM

I also wonder about glider contests....they put up 15+ planes at a time without any problems - at least none that I'm aware of. My last glider contest had 18 planes airborne at one time - and what a scene at the landing zones

Jerry
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Old 04-25-2004, 09:55 PM
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Default RE: Strength of signal and 3IM

Gentlemen, as I said, I am no electronics or radio expert. Go read the AMA archives to understand that there was a serious problem with 3 IM during the frequency phase-in in the mid-eighties. Maybe something has changed on the planet that I don't know about. Maybe the newer receivers are so good there is no problem. I know that not all of a 300 member club are using the latest receivers. I don't know how spaced out the glider pilots were in the example above, and I am told the interfering "pairs" need to be within 10 or 20 feet of each other. I know one can always find some anecdotal incident to "prove" any theory. You are not telling me "something I don't want to hear".....I fly mostly PCM receivers and am not very exposed to this problem anyway.

And finally, there was more involved in this "white pin" system than simply minimizing 3 IM.

Clair
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Old 04-26-2004, 06:48 AM
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Default RE: Strength of signal and 3IM

Clair,
In your first post you stated that you used a formula to come up with 3 IM as the cause of the crash described by tailskid. The airplane that crashed had a JR narrow band receiver and it has been stated that 3 IM could not have caused this crash. Why are you bringing up the possibility that out of a club with 300 members, someone might be flying old wide-band receivers? It seems that for some other reason, you are not happy with the "white pin" system no longer being used. This is evident in you last statement about "there was more involved with the "white pin" system than symply minimizing 3 IM". This is not what you stated in your first post.
In you last post, you stated that we should "go read the AMA archives to understand that there WAS a serious problem with 3 IM during the frequency phase-in in the mid-eighties." This is 2004 and there is no such problem now.
I am not attacking you, just trying to understand your reasoning. By the way, isn't it great that we now have the internet and be able to get information instantly? Wish we all had it in the eighties...
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Old 04-26-2004, 09:39 AM
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Default RE: Strength of signal and 3IM

Maybe something has changed on the planet that I don't know about.
Yep, a lot did change.

Maybe the newer receivers are so good there is no problem
That's what changed. That, and better, more narrow transmitters as well. There is a WORLD of difference between the gear available when those articles on the AMA site were written and the quality of even a cheap 4-channel box today.

PCM is nice, and might help, or it might not. I wouldn't say you are imune to problems just because you have PCM, that's for sure.

However, there are lots of things going on where many TX's are running, sometimes close together, with no problems. Sounds like the glider guys might actually be harder on their RX's then we are in combat, they sure fly a LOT farther away at times.

Anyway, in the case that started this thread, I'd only believe 3IM if there was some other faulty equipment involved, like an old RX, a very weak TX getting interferred with by a couple of overly strong ones, something like that.

But if you want to limit the number of planes up, power to you. It's not necessary, and since you're not at my club, it won't bother me.
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Old 04-26-2004, 10:45 AM
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Default RE: Strength of signal and 3IM

ORIGINAL: Azcat59

I belong to the club in question, and saw the incident, and also remembered the 3 IM formula, which resulted in two different "pairs" that could have produced a signal on crashed Ch. 54.

I don't know any more about radio technology than Tailskid. However, I lived through the "growing pains" with AMA when the new frequencies were phased in from about 1983 through the early 1990s. I know that one of the causes of interference that AMA was concerned about was 3 IM.

If inquiring minds would like to research this some, go to the AMA site www.modelaircraft.org
and click on the archives section they have just installed. Do a search on "Third Order Intermodulation". I think I got hits on some 30 articles from about 1983 to about 1990, many written by George Myers, one of the movers and shakers that helped get our new frequencies.
The question is not whether there is such a thing as 3 IM, but whether the newer radios are so good they can reject a weaker signal on their frequency.

And.....what about the older receivers that I am sure exist in a club of some 300 members?

Our club installed the "white pin" system in the mid-eighties when interference was a real problem all over the country during the phase-in of the new channels. You had to take your frequency pin, plus one of five white pins before turning your radio on. This restricted the number of frequencies on at a time, which statistically reduced the chances of getting hit. It also cut down on the number of people with radios on in the pits adjusting their planes while as many as five other pilots were flying at the flight stations. It seemed to work very well for us.

Clair Sieverling
AMA 15654
I don't find an archive section on that site.
Is under the members only heading?
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Old 04-26-2004, 10:57 AM
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Default RE: Strength of signal and 3IM

I am suprised to hear that IM was ever a significant problem. I don't understand why IM was a problem in the past and not now. I don't see that narrow-banding the receivers should improve IM as IM occurs exactly on frequency. IM products, while always there, should be thousand, more likely millions of times weaker than the transmitter signals. Maybe I'm just not understanding it.

Could it be that in the past IM was confused with something else? Maybe another transmitter operating on the image frequency of a receiver?
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