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

"Martin Lewicki"

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