What are the repercussions of using 2 transmitters on same frequency? Tx are ~50 feet apart and the freq is 447.800 mhz.

  • $\begingroup$ It depends on the modulation, FM, AM etc. Airplanes use AM because the "doubling" or capture effect doesn't happen, and the control tower can sort of hear that there is two voices. With FM, the signals can either cancel out, or get a weird whiny sound, or there is sometimes things called "winning" when one radio's signal is stronger and overpowers the other, thus hearing one voice. $\endgroup$
    – Skyler 440
    Feb 13 '14 at 2:30
  • $\begingroup$ These are digital signals. $\endgroup$
    – John
    Feb 13 '14 at 2:53
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    $\begingroup$ @john you should edit the question to include that clarification. It's a very critical piece of information. $\endgroup$ Feb 13 '14 at 3:29
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    $\begingroup$ Are the two transmitters synchronized using the same oscillator(s) and/or clock(s) or otherwise phase locked (PLL) to one reference frequency? $\endgroup$
    – hotpaw2
    Feb 13 '14 at 5:43
  • $\begingroup$ Or, if digital, are they using CDMA or TDMA to separate the data streams? $\endgroup$
    – SDsolar
    Sep 9 '17 at 6:07

In the simplest case, the result will be useless. It's like having two people with identical voices shout at your simultaneously, except that radio receivers are even worse than human brains at separating signals.

If you want to have multiple transmitters operating simultaneously, you need to multiplex them somehow. There are many ways to do that. The simplest way is usually to put them on separate frequencies. This is precisely why companies that make money on wireless communications exchange a lot of money for the right to use a frequency.

  • $\begingroup$ Thank you guys for all the info. Let me give you a little background on whats going on. I am the IT Director for a medium sized nursing home. Well budget/department cuts have thrown the nurse call system under our hat. Our nurse call works by sending out pages to these pagers everytime a resident pushes their call light button. Well the problem we are having is some of the calls are not being received, or they are being received but the output is gibberish. The transmitter sends the pages out on 467.800. We have another transmitter that controls reader boards and clocks, also on same freq. $\endgroup$
    – John
    Feb 13 '14 at 14:16
  • $\begingroup$ Just for giggles, yesterday we turned off the reader board and clocks transmitter and knock on wood, it seems things are doing ALOT better. I will get with my director of nursing today and see if the complaints of them not working have gone down. I know the freq is adjustable on both transmitters and all devices. How far apart do the frequencies need to be apart so that they don't interfere? $\endgroup$
    – John
    Feb 13 '14 at 14:20
  • $\begingroup$ @John That depends on the bandwidth of the signals of each. Without the specifications for each of these systems, it's hard to say. If you make pessimistic assumptions in the absence of proper specifications, then the thing to do is separate the frequencies by as much as possible. $\endgroup$ Feb 13 '14 at 14:31
  • $\begingroup$ I will give both companies a call today and see what they recommend. $\endgroup$
    – John
    Feb 13 '14 at 14:42

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