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