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Cell Phone Interference

Discussion in 'Electronic Design' started by GregS, May 11, 2007.

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  1. GregS

    GregS Guest

    I am wondering what those "hospital approved" phones are that I see. They
    say regular cell phones cause interference. Is it a scam or true. I
    do see where cell phone bans are being lifted as findings indicate there
    is no interference from regular cell phone use..

  2. mpm

    mpm Guest

    Interference is possible and difficult to perdict in advance.
    I have not personally seen a "hospital approved" advertised cell phone
    and would not know what to expect in same. Perhaps there is some
    other aspect besides RF interference that they are complying with?

    FYI - There is (was?) a magazine called "Compliance Engineering" that
    routinely reported on RF interference to unintentional "receivers". A
    classic case is motorized wheel chairs being interfered with by taxi
    cab radios. In another case (documented!), a patient died in the
    Operating Room because of a P5 or P6 VHF Paging Transmitter located on
    the rooftop. Later analysis concluded the paging transmitter was not
    malfunctioning and the problem was traced to conducted interference.
    (Not all of the O/R Suite equipment was battery operated.)

    That said, beware of "spin". I'd be curious to see the ad.
  3. GregS

    GregS Guest

    These phones are bigger, and yellow and black here.
    Look more like a portable house phone.

    By the way, I wish they made some SMALL portable house phones.

    I'll try to search for those hospital phones some more.

    There is an area here where they put up signs warning of any RF device.
    The manufacturer of some very important equipment made that suggestion
    after failure of said important system. Seems like they need a better design.

  4. GregS

    GregS Guest

    A long while back, a researcher requested the pager antenna be moved from
    his room area. The antenna was probably about 25 feet away. It obviously
    made some interference. I have known cars to malfunction driving near a TV
    tower. Seems like things are much better. On a British TV show they zapped a car
    with artificial lightning, and it did OK.

  5. GregS

    GregS Guest

    I did find some info. There appears to be very similar to portable cell phones,
    but the power level appears to be 100 mw or less.

    Phones by Spectralink

  6. Joel Kolstad

    Joel Kolstad Guest

    There was a lecture hall in college in a building adjacent to a main
    thoroughfare. The wireless microphone system in the room would pick up the
    taxi cabs if they keyed up just as they whizzed by... :)
  7. GregS

    GregS Guest

    Back in high school days, there was this
    Ham opperator, a woman, using 1000 watts on 6 meters, on the military
    freqs, well she was heard for miles around, I think it was every Sunday night.
    Everybody used outside antennas back then.
    Gracy K3JTH

  8. Rich Grise

    Rich Grise Guest

    Did anybody talk to her about it? Usually, hams are very concerned with
    interference - I've even heard of hams who, when their neighbors
    complained about picking him up on their TVs, _GAVE_ them filters to
    filter his signal out of their cheap TV front ends, and came over and
    installed them!

  9. GregS

    GregS Guest

    Don't remember exactly, but her affiliation as a Mars radio station probably
    gave her some immunity.

  10. mpm

    mpm Guest

    I wasn't going to bring up all the weird things I've seen (I used to
    have a high-profile job dealing with lots of towers in the US)...

    I recall one episode where a music teacher was teaching students on
    her piano.
    She called in complaining she could hear music coming out of her
    piano. (Duh!??)
    (At first, I couldn't figure out what she was talking about until it
    dawned on me she had a digital piano!) Of course, she was in a
    blanketing contour and the audio she was hearing was coming from an FM
    on the tower. I don't think the station ever resolved the problem.
    She was late reporting it, and they were under no legal obligation.
    (This was from an FM upgrade in power)

    Another guy swore up and down our tower was causing interference to
    his TV reception. (The tower was completly empty - no tenants - no
    antennas!!) I asked him to send me a video of the interference.
    After which, I suggested he call the local power company and get the
    pole transformer replaced! (That fixed it.) I understand this is a
    big problem with low-band hams, like broadband over power lines
    promises to be...?.

    And then you have the truly weird ones...
    A paging operator calls me up and says they can't receive a GPS signal
    at the tower.
    (Which is unusual to say the least. ...weird because there's clear
    view of the sky and this particular complaint had "never" came in
    before. (Trust me, we heard 'em all.) And besides, you could pretty
    much guarantee the GPS satellites were working!) But the tech swore
    it was "us".

    Sure enough, a low-power TV station on an adjacent tower had removed
    their 2nd harmonic filter. I forget exactly which channel, 47 I
    think? (We didn't own the station.)

    Reinstall filter, slap engineer on wrist, "Don't do that again!".
    Problem solved.

    Next time, I'll tell you about the kitchen sink (actually a metal
    frame base slop sink) at an AM station that was causing interference
    200 miles away! We were really lucky to find that one. Had been
    going on intermittently for years. I didn't find it myself, but I
    understood the problem was so unbelievable (and yet definitely
    repeatable), that the whole sink was unbolted, packed up and shipped
    off to the engineer in charge at the affected FCC monitoring point.
    True story.

    So "Yeah", if someone tells me they have a "Hospital Approved" cell
    phone, I'll listen.
    If a bit skeptical. -mpm
  11. mpm

    mpm Guest

    ....OK just one more, 'cause this one is just too damned funny!!

    Customer calls in saying the shelter is "nuclear hot" inside. Air
    conditioner is running, but not blowing cold.

    So we get out there to find an old, busted A/C compressor.
    We get that replaced, and about a month later someone else calls in
    with the same complaint.

    This time though, it looked like some livestock had kicked or knocked
    over the compressor. This was at an AM station, which are notorious
    for lack of metal fencing. So we built a nice, solid wooden fence
    around the (now new) compressor. If you're counting, this is
    Compressor #2.

    Several weeks go by and sure enough, the call comes in again.
    Only this time, the tech on the other end says the wooden fence is all
    busted up and he's actually witnessing a bull HUMPING the A/C

    Needless to say, compressor #3 went on the roof !!

  12. Rich Grise

    Rich Grise Guest

    It shouldn't, but then again, I've taken note of your earlier comment,
    "on the military freqs" - if it was actual military stuff, they probably
    just didn't give a shit.

  13. joseph2k

    joseph2k Guest

    That is also the the way legisltion and regulation also read; if your gizmo
    is important, it had better be hard to interfere with. Just the same
    incorrect installation may ruin everything.
  14. Scam? I doubt it. I can pick up GSM cell phones signaling over FM radios
    from several yards distance.
  15. Used to entertain myself with the 900 mhz radio and teenagers'
    cheap radar detectors back when I was driving a cab. Hitting
    the talk button lit up their detectors. Always good for a chuckle
    when they slammed on their brakes at 80 mph.
  16. joseph2k

    joseph2k Guest

    Then it is, by definition, a poorly designed radio receiver. Being
    disrupted by relatively moderate emissions at 8 or 19 times the normal
    receiver operating frequency is clearly an inferior design.
  17. Didi

    Didi Guest

    Scam? I doubt it. I can pick up GSM cell phones signaling over FM radios
    The cellphone here is the prime suspect for a weird thumping the
    TV set is doing occasionaly (sounds like its power supply does
    something, several thumps over 3-4 seconds, once a day or so
    if the phone is at the TV's table). I don't have the statistics on
    that so
    I could not swear it is the cellphone, but it has not done it for
    a week or two since the phone is not put there and it has never
    done it before without the phone there.
    What would the phone transmit once a day or so I don't know.
    It does not interfere with the TV during normal communication,
    this must be talking to some more remote location. Losing the
    local station shortly and trying to get in touch with another?
    Transmitting compressed data it has gathered over the day?
    Anyones guess :).

  18. mpm

    mpm Guest

    Typical cell phones are in communication with the base station even
    when you are not using the phone for conversation. This is how the
    cell phone company "knows" which cell you're on in case you are
    moving. (They do not ring every cell tower in the world. That would
    be a waste of spectrum.)

    There are "channels" for signalling, and "channels" for voice
    If nothing changes, your cell phone might stay on a single signalling
    "channel" for weeks.

    If your phone experiences a fade, or loss of signal, the reacquisition
    may occur on another signalling "channel". (This can, and does, often
    happen many times a day.) It can also happen if the cell site
    transmitter drops the channel, or if they dynamically reallocate
    spectrum to deal with cell site congestion issues, etc...

    The point is, when your cell phone (lying peacefully on the TV table)
    needs to get updated, it will broadcast a short burst of data back to
    the serving cell site. This is probably what you are experiencing (TV
    Interference) when the phone is not in Communication.

    To extend this reasoning, when you are talking on the phone, First,
    you are using a different "channel". One other than the Signalling
    "channel", but very likely in the same general band. But many things
    have changed. The orientation to the TV for one thing. (Presumably
    you are further away unless you put your ear on the TV table?) Also,
    the cell site may have instructed the cell phone to reduce output
    power on the voice channel, etc...

    Unintentional receivers in close proxmity to cell phones and the like
    are usually bombarded with fairly high RF signal strengths, which
    their front ends were not designed to deal with. While it is
    certainly possible to make receivers that would be immune to this
    proximity effect, it is not often economical, and may impose physical
    size restrictions (for the front end filtering) which consumers could
    find objectionable. In short, you just have to live with it.

    BTW, I offset the word "channels" in quotes because in the strictest
    sense, not all cellular systems (air interface standards) use discrete
    channalized bandwidth, but the intent should be clear from the above.

  19. mpm

    mpm Guest

    Probably front end overload, or direct radiation in the receiver IF
  20. Didi

    Didi Guest

    Thanks for your cellphone behaviour analysis.
    that's a viable guess, could be the cellphone is trying to talk louder
    to reach some more distant station. I have not watched its reported
    signal strength when this happens, I may still do it.
    Could also be many other things, of course.
    Quite unlikely, the area is not densely populated and it also happens
    during the night when most people are asleep.
    No, this can be ruled out. When the phone gets messages and is
    at that table there is no interference. I doubt it uses a different
    level to acknowledge messages. Also, it does not drive the TV
    mad when it rings.
    It is not the front end. It is, like I said, the TV power supply
    The thing (the whole TV set thumping madly) happens even when
    the power is "off" (on enough to see the remote control and to
    be messed up by the phone), the signal power from the phone must
    be orders of magnitude higher than it is during normal transmission
    to achieve that.

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