rajul

My Feedback: (58)

I would like to ask the radio experts in this forum for their opinion on 72Mhz frequencies that are most susceptible to RF interference from cell phone towers. The reason I ask is our field is seriously affected by a recently erected tower, and a few flyers seem to get away with it but many have been shot down. We do not own a freq scanner which I believe can help us. A quick internet search revealed that the towers operate in a frequency range of 800 to 2200 MHz.

1) I am no expert but there is a huge gap in the frequencies of the tower and 72Mhz. What form of interference is taking place here ?

2) Can dual conversion and PCM Rx eliminate the interference from cell phone towers ? How does this work ?


Highly appreciate all inputs...........

FLYBOY

My Feedback: (11)

There was a big discussion that got pretty heated last week on this. You might want to search that so the arguing doesn't start again. It was pretty much agreed that the only problem you would have with the tower is if you hit it. The experts all agreed that it wouldn't be causing the problem. One even claimed his cell phone ringing caused all the computer radios to loose their memories.

In short, it can't happen. Search the site and read all the posts about it though. It is kind of interesting with all the different backgrounds though.

strato911

Flyboy - I missed it - it must have been in one of the forums I don't read.

I agree, the frequencies of the cell towers are probably not the problem. However, the metal in the structure will cause plenty of interference problems.

As for the ringing cell phone, some models put out VERY strong EM interference while ringing, and that can disrupt computer memory. Put your cell phone a foot away from your computer monitor and call it from a regular phone. If it causes the screen to waver, keep it several feet from the nearest transmitter. I've seen one cause the computer screen to jump from 4 feet away.

CurtD

My Feedback: (1)

We have a 400 ft (guess) cell tower within 1/4 mile of our field (it's behind the flying area so it's not a navigation problem). It's been there for about two years now and no one has reported any abnormal radio problems on any channel. That's not a scientific confirmation of non-interference from cell towers but it's good enough for our club. Cell phones operate at GHz freq.s so I seriously doubt there will be any problems with interference to our RC equipment. Also, several of us in the club have experimented trying to duplicate the "de-programming" of computer radios that's been reported when cell phones are used or are ringing with no results. Seems to be a "phantom" problem. :-)

FLYBOY

My Feedback: (11)

strato, the claim was that the phone rang and erased every computer radio within 75 feet. I am no expert but from what the experts say, it can't happen. I have mine on my hip all the time and answer it while flying all the time. It has never been a problem at the field while anyone was flying.

Just hate to see rumors floating around like that. Makes people that don't know scared and they take it as gospel.

Didn't want to have another urban legand start.

TriadSport

There is to be a cell phone tower to be built near our field in the near future. In preparation for a county commisioner meeting to approve the erection I contacted the AMA headquarters to see if this might be a problem. They assured me that cell phones operated in the 800 too 900 mghtz range and would not be a problem. What could be a problem is if the tower owners sublease space to operators of pagers or repeaters which do operate closer to our frequencies. At the commissioners meeting we did receive under oath, assurances from the AT&T reps that no problems would occur and any subleased operators would be held to AT&T high standards. If a problem does develop they provided guarentees that they would work with us to find and eliminate the problem.

FLYBOY

My Feedback: (11)

Yea, like tell you that you won't have a problem if you don't fly there. That is kind of scarey. I don't see you having a problem, but if you do, I also don't see them helping you in any way.

strato911

This may be an urban legend based upon OLD cell phone technology. I too have used my cell phones while flying, but I always use a hand's free headset for saftey. Then again, neither of my cell phones disrupt my monitor either.

As a Computer Engineer, I consider myself somewhat of an "expert", and a strong EM emission like some cell phones radiate while ringing could disrupt the memory - but not from 75 feet. If it doesn't disrupt your computer monitor, it won't disrupt your transmitter.

hvengel

The more likely cause of the problem, if it is related to the cell tower, is the above. Pagers operate in the 140MHz range and put out signals that are fairly strong. In fact much stronger than a cell trasmitter. Also 2 meter ham repeaters and business band repeaters operate in the 144MHz to 156MHz area and can also have fairly high power output - typicaly 10 to 40 watts. However repeaters are also by their nature well filtered, because they operate recieve and transmit on 2 close frequencies are the same time, and are therfore not likely to generate out of band output. Pagers on the other hand are a common source of interfearance. Just ask any ham.

If any of these sources (cell phones, pagers, ham or business band repeaters) is the actual source of the interfearance then the tansmitter is likely not in compliance with the regs and the owner/operator is subject to enforcement action by the FCC. The problem is to locate the offending transmitter so that is can be reported to the FCC.

In addition most leases for tower space have a clause that requires the operator to maintain thier equipement in a condidtion where it is in compliance with the rules. So in the case above if AT&T were to be informed that a specific transmitter on thier tower was causing a problem they would likely give the operator a chance to correct the problem and then discontinue the lease if corrective action had not happened.

Again how do you locate the source of the problem. You might do some looking arround to see if there is a local ham club in your area. If there is one I would bet you dollars to donuts that someone there would give you some assistance. Hams in general are very nice folks and asking them about something like this is almost like asking one of your flying buddies if he wants to take one of your birds for a lap arround the field. Most of these clubs actualy have a person or commitee that is in charge of RFI issues. So for one segment of the ham community finding and eleminating RFI is a major part of the hobby.

Hal
AC6VZ

Shortman

My Feedback: (21)

Our club... Fresno Radio Modelers in Madera, California has a cell phone tower no more than 1/4 mile away from the flying site, IF THAT. We have yet to experience any problems or interference.

Its an older tower though so maybe the newer ones are doing something different?

FLYBOY

My Feedback: (11)

Yea strato, that was my point, from 75 feet the ringer won't do it. I could see it if it were right on it. I have heard of the ringer causing a fire when it goes off and someone if fueling a car or truck. They do pretty strange things. I can see a microwave tower causing problems, but they are used less and less these days.

falcon6171

My Feedback: (5)

who makes freq scanners and were can you get them? we just moved to a new field and the first month of flying was fine but today two planes on to differant freq got hit bad , the only thing thats has changed recently is there installing a gas main near the field and there is section of pipe laying there that is about 3 foot high and so long you cant see the end of it in either direction im wondering if that pipe can be causeing the problem ? any thoughts? i thought maybe we had a signal bouncing off it

rajul

My Feedback: (58)

Take a look at these http://www.desert-wolfe.com/rcscan/
http://www.geocities.com/roger_forgues/Frequency.html

gene2586

My Feedback: (11)

The The tech maven at the AMA Recommended to
our club ( the Palm Beach Skyhawks) the purchase of a Yaesu Model VR-500 hand-held communications receiver. The manual can be downloaded from the Yaesu web site, it tells how to program the unit to
pickup any signal on the band, if you need to get more help, contact steve kaluf at the AMA
, [email protected] he talked our member through the programing to set up our unit properly.
our interference seemed to be on ch. 45 and51 We were told our worst problem was TV channel 5's
audio. the station said it could be a Harmonic and Did place a filter on the transmitter feed to the antenna.

hope this helps. gene blanck [email protected]

Montague

There is some commerical gear out there that operates on 72Mhz, between our RC channels. I'm pretty sure some of those are pager systems.

At my field, there are 3 (maybe 4, depends who you talk to) of these near enough to cause interference on some of our channels.

At least one of the transmitters that hits our field is off most of the time. But when it comes on, it comes ON, and can (and has) taken out planes.

And the commerical users on 72Mhz are "primary" users, we are "secondary", so if they interfere with us, we can't do anything about it, as I understand it.

branded

My Feedback: (9)

I'm an electrical engineer of more than thirty years and can tell you that your cell phone can indeed wipe your memory clean if in close proximity to he transmitter. The RF Field Strength can induce stray voltages into high impedance devices, such as flash memory, and change the contents. This is not a problem localized to the hobby industry and we in the defense industry spend lot's of your hard earned cash shielding against radiated fields such as would be caused by a nuclear explosion.

The fact is, the phone doesn't have to ring to do this as all cell phones transmit periodically even when not in use. This is how "they" know your phone is in service.......

As for the towers. Unless there is a faulty piece of equipment located at the tower that's "dirtying up" the transmitted signal, the tower is of no consequence.

hiramboesky

Yep, what he said!

Sport_Pilot

Yes but that is at a controlled frequency and a low powered one at that. I think the claim is that the ringer generates a strong EMF field from the current. IMO the cell phones don't have a large enough battery let alone ringer to cause such a strong EMF field. I have put the cell phone right on top of the transmitter ringing its normal musical tune with no problems.

smokingcrater

how about front end overload? towers most certainly can have an effect, even if they aren't on the same frequencies.


Ryan
kc0lmo

branded

My Feedback: (9)

They certainly do have enough power as I can hear my phone in the speakers of my car radio, as well as the speakers on my computer, whether or not it's ringing.

And yes, some have experienced memory loss when the phone is placed in close proximity to the transmitter, such as in the field box, in your shirt pocket, etc. Just because you didn't experience a loss of memory doesn't mean it's a safe thing to do. There are differing models of transmitters out there using a myraid of differing IC types.

The fact is, it is entirely possible for a cell phone to induce enough extraneous currents into certain devices to alter memory.

If you want to really experience some an altering of memory, place your tranny in a microwave oven! Pretty much the same concept, except the phone is at a much lower RF power level.

Montague

It makes sense that SOME cell phones will affect SOME transmitters. I'm sure there are lots of guys out there that have no problems, but I'm equally sure there are guys who have had problems. Blanket statements either way clearly aren't going to be accurate.

(personally, my cellphone, has not bothered my TX's or any TX that I've used when doing instruction. But it's not like I've tested it beyond haveing it in my pocket or field box, and had it ring a few times that way, including while flying.)

My cellphone is a Nokia 8000something (I forget the exact model, it's been a while),
radios I have that haven't been bothered include a Polk TrackerII and a JR x347.

Those of you who have had problems, what cell phone and transmitter? Just for curiosity's sake?

Sport_Pilot

Never heard any phone transmitt on any car speakers, sure you don't have a car fitted with electronics to interact with a phone, I hav one and that can happen even when not connected. Just the same it doesn't take as much for IMF frequencies to bleed into a car radio, especially the AM reciever. To alter computer memory you need EMF, no frequency, just pure magnetic power. Now their is some EMF energy involved with all radio tranmissions, especially a 700 watt microwave focused onto one small area.

Sport_Pilot

I have an old analog phone that doesn't give problems. Hard to imagine the new smaller and lower powered phones have these problems. I think it all comes from the medias hyping of the woefully increadable charge that those ultra powerfull and ultra high frequecy phones were burning holes through our skulls.

BTW is the spell checker permanantly down?

canavanbob

My Feedback: (20)

sport pilot
can you direct me to some of those media report of cell phones burning holes in peoples skulls? maybe some pictures?

branded

My Feedback: (9)

Sport Pilot, are you an engineer? I'm sure there are many things you don't understand. You're serving up a good number of those things in this thread!