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Some questions about Radio signals.

  1. Feb 12, 2012 #1
    Hi All,

    Always wondered a few things about radio transmission, hoping to get some questions answered.

    1. Is there a reason why certain frequencies are free to use by everyone and certain ones are reserved? Is it something to do with the fact that the cost (or means) to send signals on certain frequencies are more (or less) available? Also, what stops someone from broadcasting something on a reserved frequency, is it some kind of encryption that radio stations use?

    2. When i am listening to 100.1 Mhz FM means you are frequency modulating a carrier wave of frequency 100.1 right?

    3. When buying bandwidth we usually buy a small slot of frequencies (and not just one). I know one of the reasons for that is there should be no interference from one broadcaster to the other. Is another reason for this that "while frequency modulation of the signal, frequency of the carrier signal may change (say +/- 5 Hz) so they need that gap (or bandwidth) to transfer data. More the (gap or) bandwidth, more the data they can transfer." Is this correct reasoning?

    If this reasoning is true is it correct to say that AM broadcasters don't need as much of a gap (or bandwidth) as FM broadcasters as they are using amplitude and not frequency to encode data so in effect its unlikely they will interfere with other frequencies.

  2. jcsd
  3. Feb 12, 2012 #2


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    Most of that is correct.

    In most countries, ALL frequencies of the radio spectum are controlled by Governments for very good reasons.
    Mainly it is done this way since the same frequency should not be allocated to two or more broadcasters if their listeners can possibly hear both stations. This would cause interference or non reception of one or both stations.
    Since broadcast stations pay a lot of money for their licences, they expect to have exclusive access to a frequency.
    Penalties for illegal broadcasting are usually severe and may include fines or time in prison.

    Governments PERMIT access to certain frequencies. Power limits and types of equipment permitted are specified.
    There are small allocations in the regions of 433 MHz and 2400 MHz and these are intended for low powered, low interference potential transmissions.
    There are also frequency allocations for "Citizen's Band" radios on about 27 MHz and 460 MHz and maybe others.

    In recognition of the huge educational and civil emergency benefits of Amateur Radio, there are generous frequency allocations for Radio Amateurs who have devoted the time needed to become proficient at radio theory.
    Entry level exams for Amateur Radio are designed so that anyone who has a genuine interest in radio or electronics should have no problem qualifying. Morse code is no longer required.
    Amateur Radio equipment that is home-built is permitted on these bands, while all other frequencies usually require commercial eqipment.
  4. Feb 12, 2012 #3
    Assuming you live in the U.S., all the frequencies have restrictions on them. You should look at part 15 of the FCC regulations to find out what they are.
    http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=83afa677f9100ca1c3d1359962b61d94&rgn=div5&view=text&node=47: [Broken]

    The thing that keeps someone from broadcasting on a reserved frequency is that if someone complains to the FCC and the FCC determines the broadcaster has violated the rules, they issue a very steep fine on the offender.

    Some services like police and fire are encrypted but most are not. Still you are prohibited from repeating or using information you hear over the radio for any purpose unless it was meant for public consumption.


    There are different ways that one can acquire frequencies. One is by auction and those frequencies are purchased and they can be very expensive. With other frequencies such as the Industrial/Business, one applies for a license to use a particular frequency. That application must go through a frequency coordinator who makes sure your frequency doesn't interfere with others. Other licenses like Citizen's Band or Amateur Radio you get a license to use any frequency in the band.

    Your reasoning is correct. The faster you want to transfer data the more bandwidth you need.

    Yes, generally that's true. AM requires a bandwidth equal to double the highest modulated frequency broadcast. FM uses a bandwidth somewhat wider depending on the deviation used. By using more deviation, the receiver is more able to lock out lower power signals on the same frequency. This is called the capture ratio.
    Last edited by a moderator: May 5, 2017
  5. Feb 12, 2012 #4
    Each station are required to separate from each other because of the side band of the modulating signal. For example if you have a 1MHz carrier and you modulate with a signal ( music or data) that has a BW of 10KHz, the sideband of the carrier after modulation is between 990KHz( 1MHz-10KHz) to 1100KHz(1MHz+10KHz). So if any other channel with sideband that get into this frequency will have interference. Let say there is another station want to transmit in around this frequency and the modulation signal BW is 10HHz just like the other station. The second station will cover +/-10KHz from the carrier frequency. THEREFORE, the closest frequency that can work from 1MHz is 1MHZ+ or - 20KHz(10KHz from each side) which is 980KHz and 1200KHz. Anything close than that will cause interference.

    This is true whether it is AM or FM. The only difference between AM and FM is defined by FCC in this country. AM modulation frequency BW is limited by FCC to 5KHz and FM is much higher ( I don't remember, it's over 10KHz), so You can put two channel closer in AM than FM. But this is only because of the FCC requirement, nothing to do with the nature of the Am and FM.
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