Dipole voltage basic query

[Martin Lewicki]

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

 

[atec77]

Martin Lewicki wrote:

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

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

 

[Joe G (Home)]

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

"Martin Lewicki" <mlewicki@ozemail.com.au> wrote in message
news:444af211$0$16716$5a62ac22@per-qv1-newsreader-01.iinet.net.au...

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

 

[Phil Allison]

"Martin Lewicki"

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.

** 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

 

[Martin Lewicki]

Phil Allison wrote:

"Martin Lewicki"

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.




** 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

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