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Radio communication

  1. Aug 17, 2005 #1
    What is the basic concept behind radio communication?
    How does one vary range in it ?
     
  2. jcsd
  3. Aug 17, 2005 #2

    berkeman

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    Staff: Mentor

    The basic concept/trick (discovered by Marconi?) is that when you use a tuned antenna, you radiate real energy in the form of an EM wave. And a tuned receiving antenna will generate a terminal voltage that tracks the EM wave.

    The simplest antenna is a tuned dipole or tuned monopole. The tuned dipole is basically a balanced transmission line that terminates in two opposing elements that are each a quarter wavelength long at the frequency of transmission. The input impedance of a quarter wave dipole antenna is around 73 Ohms real, which means that all of the energy that goes into it from a 73 Ohm transmission line will get radiated away into space as an EM wave. A tuned monopole is just a quarter wave antenna above a large ground plane, and it has half of the 73 Ohm real input impedance.

    Once you realize that you can radiate real energy (as opposed to just having reactive energy trading off for a non-resonant LC type structure), then you can work on modulation schemes on the carrier frequency. I think probably simple AM was the first modulation scheme, with audio frequency voice information amplitude modulating the RF carrier that was resonant with the transmitting and receiving antennas.

    As for range, there are lots of variables, including multipath reflections and whether the frequency band you are using is mostly line of sight, or can follow the earth's curvature. Basically more power into the antenna gives longer range, as well as a better (quieter) receiver gives better range through sensitivity. You can also make antennas with very high directional gain, like the parabolic antennas used for microwave communication with sattelites, for example.
     
  4. Aug 17, 2005 #3

    Excuse my ignorance.
    What it a tuned antenna,terminal voltage,real energy ?
    Is range dependent on power comsumption ?

    thanks for help
     
  5. Aug 17, 2005 #4

    Ouabache

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    berkeman gave a very nice concise description of radio communication.
    though I believe continuous wave (syn. CW) was the first form of information sent on an RF carrier, made by switching the carrier frequency ON and OFF. It also was our first form of digital radio communications, sending alphanumeric and punctuation characters by MORSE code.

    Perhaps I can lend some insight on your first question.
    A tuned antenna directly relates to its resonance. An antenna of
    any given dimension will resonate at a specific frequency. If you want that antenna to resonate at a specific frequency (for example the RF frequency you are transmitting) it is useful to tune the antenna. On easy way to tune an antenna is to change its physical dimensions (shorten or lengthen the radiating element).

    An analogy to appreciate resonance; think of a tuning fork. When you strike it so that it vibrates, it resonates mostly at a frequency you can hear (as you bring it close to your ear). This frequency is in the audio range and is the resonant frequency of that specific shape of metal.
     
  6. Aug 18, 2005 #5

    berkeman

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    Staff: Mentor

    As mentioned, an antenna is most efficient at radiating energy when the antenna is at resonance for the RF frequency of the transmission. And the same holds for the receiving antenna -- it convert the EM radiation flowing by it most efficiently to an output voltage (at the terminals of the antenna) when the antenna elements are tuned to resonate at the RF frequency of interest.

    By "real" energy and "real" input impedance, I mean that the imaginary part is negligible. For example, you can have an L-C tuned circuit that resonates at 300MHz, but if it is small physically, it will not radiate much energy. The energy just keeps bouncing back and forth between the inductor and the capacitor, and any losses come from the resistances of the wires in the circuit. But if you drive a current into a dipole antenna that has its two radiating elements about 1/4 of a meter long, then you will radiate away much of the energy that you are driving into the antenna. To radiate it all, you would need a balanced 73 Ohm transmitting source to match the real 73+j0 Ohms of input impedance of a balanced (2x) quarter-wave dipole antenna at its resonance.

    Another thing I should point out to avoid confusion -- most RF sources are unbalanced 50 Ohm sources, and most receivers have unbalanced 50 Ohm input connectors. Since quarter wave dipoles have an input impedance of 73 Ohms real, and quarter wave monopoles have an unbalanced input impedance of 73/2 Ohms real, neither of those popular antennas match traditional RF sources and receivers. So you use a small component called a "balun" to match the 50 Ohms to 73 Ohms (or whatever the antenna Z is), and to also do the ubalanced-to-balanced conversion for you. Like, on the back of your TV set, there is probably a coax input jack for VHF and one for UHF, and since coax has an inner conductor that is surrounded by the big shield tube, coax is considered an "unbalanced" or asymmetric transmission line. But "rabbit ears" antennas and loop antennas are "balanced" (the two sides are symmetric), the balun that you buy at Radio Shack to connect your rabbit ears to your coax input on the TV has the Z conversion circuit in it, and the balanced-to-unbalanced conversion characteristic as well.

    All antennas have input/output terminals, either coaxial or some balanced means of connecting to them. When I mentioned "terminal voltage", I mean the voltage that you would measure at these connections to the receiving antenna. Typically, though, power numbers are used both for the transmitting antenna and the receiving antenna, and typically (but not always) this power is referred to 50 Ohm systems. The units are usually dBm (dB above a milliWatt), and you can talk about the transfer function between the TX and RX antennas in terms of the number of dBm loss. The loss increases with distance, and is affected by multipath and other factors, depending on the frequency range of the transmission.

    BTW, there is another thread in this forum titled "Reciprocity applied to complementary antennas" where Antiphon posts a good link to tutorial info on antennas. Check out that thread and Antiphon's link for more info. -Mike-
     
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