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Why are there not millions of usable frequencies?

Discussion in 'Radio and Wireless' started by EnricoFermi, Jul 26, 2018.

  1. EnricoFermi

    EnricoFermi

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    Jul 26, 2018
    Hi guys. It’s my understanding the only theoretical necessary bandwidth separation for frequency distinction is on the order of the Planck constant, h, that relates energy to frequency by E=hf where h comes from the solution of the blackbody radiation problem circa 1900. For energy of broadcast frequencies is this separation not extremely small, like on the atomic scale? Ten to the minus ten meters? What are the challenges of frequency filtering in real life? And if anyone could point to any further ready, papers, etc .. I would really appreciate it. Thank you.

    Here is some physics for anyone interested https://physics.stackexchange.com/q...elengths-of-light-is-the-em-spectrum-continuo
     
  2. Ylli

    Ylli

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    Jun 19, 2018
    A pure RF carrier occupies zero bandwidth. But as soon as you add information to that carrier (modulate it), then it uses bandwidth. For example, an AM (double side band) broadcast signal carrying modulation in the audio range up to 5 KHz, will use 10 KHz of spectrum.
     
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  3. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Getting a filter of 'zero bandwidth' would be impossible therefore you could never separate the signals for reception.

    Bit pointless having 'infinite' transmit channels if you can't separate them at the receiver.......

    Given that the RF spectrum is many 100's of GHz wide you could transmit MILLIONS of signals and separate them very easily using todays technology so, there already ARE millions of useable frequencies..
     
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  4. EnricoFermi

    EnricoFermi

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    Jul 26, 2018
    Thanks @kellys_eye. From what I understand from comparing the EM waves of light to those of radio is that the "eye" for radio waves has a small brain which seems to amount to a current alternating in a circuit. Please let me know if this is not accurate. The human brain is able to make sense of multiple different overlapping frequencies but not radios. Radios see a limited number of frequencies which are often interpreted as ones and zeros (i.e. pulse radio). This does not seem efficient given an infinity of modulation options. If each radio had its own computer brain what would that brain do to form an efficient global wireless mesh of all radios? I am interested in this topic and would appreciate if anyone could point to more technical or mathematics related reading in this area. Thanks everyone.
     
  5. davenn

    davenn Moderator

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    cant really make sense of what you were trying to say there ?
    There is no difference between radio EM and visible light EM other than frequency ( and of course wavelength)


    again just a little off base
    A radio receives a frequency(s) for which it is designed to do so


    not really inefficient
    Radios are channelled for specific frequencies for which a specific user has been allocated/licenced to use
    We don't want two different users using the same/close frequencies in close proximity to each other, it will cause interference commonly known as intermodulation.


    There are some specific pieces if RF test equipment that do cover very large parts of the RF spectrum.
    They are called Spectrum Analysers …. my spec an. has a reasonably narrow range 10kHz to 2 GHz, because I don't have $10's of 1000's to spend on a really wide band one
    I would love one that went to 25 GHz as I do a lot of work in the 1GHz to 24 GHz range
    The top ones will go from 10 kHz to 50 - 70 GHz, maybe higher

    I also have a scanner that continuously covers the range from 100kHz to 2 GHz


    Dave
     
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  6. kellys_eye

    kellys_eye

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    Jun 25, 2010
    If you imagine the eye trying to discern a particular colour (frequency) you can see that it's not very good...... this is because the human eye has a very wide 'bandwidth' and the filter (the brain) isn't very efficient in this respect.

    Technologically we can design filters with very narrow bandwidths - certainly MUCH narrower than the human eye/brain can achieve. If we take the eye and colour as the example, there are devices that can identify millions of individual colours in the visible spectrum whereas the human eye could only achieve (say) 16 different shade of any one colour.

    There are online 'eye colour recognition' programs around that can show how good 'your' eyes are at discerning colours/shades - try to find one and have a go! Then Google 'digital colour detector' and you'll find the likes of 16-bit resolution devices that can identify (literally) MILLIONS of individual colours.

    back to radio - the only difference between colours and radio is that you can't see the radio signal but the human ear can pick out individual signals even if they overlap - that's something the human brain is actually better at than digital processors. But we can make filters that limit the passband and filters of sub 100Hz are readily available.

    Divide that 100Hz across even just the HF radio spectrum (DC to 30MHz) and you've got 300,000 'channels' right there. But the channels usefulness is limited by it's bandwidth as the data it carries requires 'bandwidth' to work - the more complex the data the wider the channel needs to be. Morse code requires (theoretically) zero bandwidth so you could get an infinite number of such channels however data (sent as morse code) is limited in speed/content. Analog TV signals require around 6MHz of bandwidth so you are limited (again) in how many channels can fit in a given space unless they 'overlap' - at which point, as @davenn points out, you get interference and the data content is corrupted.

    Modulation techniques often use encryption to compress the data being transmitted - which is why there are now more digital TV channels than analog - but the complexity of modulation is proportional to the complexity of the equipment needed to decode it. I'm sure quantum computing will increase the number of available channels (overall) but that's progress for you. Once upon a time we only had basic radio and a handful of signals but as technology progressed we increased that exponentially - and continue to do so.
     
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  7. hevans1944

    hevans1944 Hop - AC8NS

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    This is absolutely NOT correct. The human eye-brain system is easily fooled and/or confused by a myriad of optical "illusions" based on color, contrast, and spatial relationships that appear to be "hardwired" into the system. No such limitations are deliberately introduced into radio systems.

    As far as "multiple different overlapping frequencies" are concerned, this is one of the most powerful "spread spectrum" means of transmitting information via electromagnetic radiation occupying the same spectrum without mutual interference. When multiple, different, overlapping frequencies are generated and transmitted as electromagnetic radiation, each frequency is independent of any other frequency even if two frequencies are nominally the same frequency. The only criterion necessary to distinguish and separate the frequencies is that they cannot be temporally coherent with each other.

    The frequencies in the spread spectrum modulation, however, are most likely deliberately coherent with each other and with a "master" frequency from which they are each derived. This has the effect of imparting "intelligence" or a priori information to the spread-spectrum signal, said intelligence being used to recover the original spread spectrum modulation signal through coherent demodulation and other signal processing techniques. Much of this technology, or at least specific implementations of it, is highly classified because of its application to secure communications and control. The math however is not classified. Start your journey down the rabbit hole here.
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009
    The question had been asked and has been answered multiple times

    Thanks to all the responders for their good info :)

    thread closed
     
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