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Dipole voltage basic query

Discussion in 'Hobby Electronics' started by Martin Lewicki, Apr 23, 2006.

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  1. I'm basically knowledgeable in electronics eg E=IR, right-hand rule (or
    is that left-hand) etc...

    A Tv antenna dipole receives electromagnetic radio waves from the
    transmitter. As the E and B components interact with the metal a small
    oscillating current/voltage is set up in the dipoles in tune with the
    arrival of each EM wave front. Correct?

    What is the level of current/voltage induced in the dipoles? I expect it
    would be very tiny - milli, micro, picco. What might be the actual
    figure? I understand that distance from transmitter will affect strength.

  2. atec77

    atec77 Guest
  3. Overall Systems theory can help a little here.

    tranimitter puts out power - through cable -out of antenna, there is
    attenuation of radiowave propagation to the receive antenna and loss down
    the cable to the receiver.

    a tranmitter = power gain (say in units of dBm)
    b cable = power loss (attenuation in dB)
    c Tx antenna = power gain or loss (usually gain in dB)
    d radiowave attenuation = power loss (in dB)
    e Rx antenna gain = power loss or gain (usually gain in dB)
    f cable loss = power loss (in dB)
    g receiver measured power (in dBm)

    Overall Radio system equation( sum of losses and gains)

    a -b +c -d + e -f = g (there are other factors also)

    g the received signal, receivers have a typicall threshold , for say some
    mobile phones it migh be -118dBm

    So what must be received at the receiving antenna must be -118dBm + f -e
    for the receiver to work above it's threshold.

    If we assume cable loss is as 10dB and antenna gain is 10dB then there must
    be -118dBm at the receiving antenna.

    -118dBm can be converted in to uV (based on sa a 50ohm load - because dBm
    is a power measurement and uV is a voltage)
    eg P = V x V / R ==> Hence V = Square root ( P / R).

  4. Phil Allison

    Phil Allison Guest

    "Martin Lewicki"

    ** The signal voltage picked up by a TV antenna is generally in the order of
    a few millivolts per received ANALOGUE station. Digital TV signals are
    considerably weaker. This voltage is delivered into an impedance of 75 ohms
    at the set -so 1 mV equates to about 13 nanowatts !

    If because of distance or unfavourable local geography the signal falls to
    less than 1 millivolt, the picture becomes noisy - ie affected by "snow".

    The power output, location and radiation pattern of TV *transmitters* is
    deliberately arranged so that viewers in the intended zone will get a signal
    of a few millivolts via standard TV antennas on their roofs.

    Those living in close proximity to the transmitter site can use a very basic
    ( small dipole) antenna while those on the fringe of the range will need a
    larger antenna than usual with designed in "gain" to boost the level of the
    signal voltage.

    ........ Phil
  5. Thanks Phil, that's what I was looking for.
    Also thanks for the others, though I'd need more technical background to
    follow it.

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