Dipole voltage basic query

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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.

Marty

Re: Dipole voltage basic query


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http://www.google.com.au/search?hl=en&q=determining+dipole+voltages&btnG=Google+Search&meta =


Re: Dipole voltage basic query


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).


Regards
JG

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Re: Dipole voltage basic query



"Martin Lewicki"
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** 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








Re: Dipole voltage basic query



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

Martin

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