# A Level Physics~Communication

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#1
I understand the first highlighted section till "wire pairs are used for short distances". What does "with low frequencies" imply here?

Similarly, what does a "high frequency signal" mean in context of being used for large distances?
Last edited by cruduxcruo9; 11 months ago
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11 months ago
#2
high frequency signal is the electromagnetic signal from 3 to 30 megahertz
low frequency signal is radio frequencies in the range of 30 kilohertz to 300 kilohertz
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#3
(Original post by cruduxcruo9)
I understand the first highlighted section till "wire pairs are used for short distances". What does "with low frequencies" imply here?

Similarly, what does a "high frequency signal" mean? Highlighted in the second text. Does it mean a signal within the range of 500 kHz (the bandwidth for a wire pair) but high enough?
Joinedup
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11 months ago
#4
(Original post by cruduxcruo9)
Joinedup
for comparison he frequency of radio 4 longwave is 198kHz
The transmitting aerial for that station is made of long wires hanging between pylons - they put 500kW of electricity at a frequency 198kHz into those wires and it almost all radiates off as radio waves.

500kHz is definitely in the range of frequencies where an electrical signal in wires is going to radiate off as a radio wave, it's a good thing if you're trying to send a radio wave but a bad thing if you're trying to send a signal through wires.
it gets more pronounced as frequency increases - there isn't a sharp cut off frequency
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#5
(Original post by Joinedup)
for comparison he frequency of radio 4 longwave is 198kHz
The transmitting aerial for that station is made of long wires hanging between pylons - they put 500kW of electricity at a frequency 198kHz into those wires and it almost all radiates off as radio waves.

500kHz is definitely in the range of frequencies where an electrical signal in wires is going to radiate off as a radio wave, it's a good thing if you're trying to send a radio wave but a bad thing if you're trying to send a signal through wires.
it gets more pronounced as frequency increases - there isn't a sharp cut off frequency
I see

So, lower frequencies are used to transmit signal to shorter distances? And higher frequencies for longer distances, reason being so it's attenuated not too quick?
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11 months ago
#6
(Original post by cruduxcruo9)
I see

So, lower frequencies are used to transmit signal to shorter distances? And higher frequencies for longer distances, reason being so it's attenuated not too quick?
The section in the book is about electrical signals in a long wire.

You've got it the wrong way around - low frequency signals are transmitted well by a long wire but high frequency signals in a long wire are partly radiated as radio waves - meaning there's a lot less of the original signal left at the other end of the long wire. The energy you put in to the wire goes off into space as radio waves.
The problem gets worse as the wire gets longer and as the frequency gets higher.- you could think about it as alternating electricity being less happy about staying in a wire and wanting to be a radio wave instead. and the higher the frequency the more the electricity wants to escape from the wire as a radio wave.
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#7
(Original post by Joinedup)
The section in the book is about electrical signals in a long wire.

You've got it the wrong way around - low frequency signals are transmitted well by a long wire but high frequency signals in a long wire are partly radiated as radio waves - meaning there's a lot less of the original signal left at the other end of the long wire. The energy you put in to the wire goes off into space as radio waves.
The problem gets worse as the wire gets longer and as the frequency gets higher.- you could think about it as alternating electricity being less happy about staying in a wire and wanting to be a radio wave instead. and the higher the frequency the more the electricity wants to escape from the wire as a radio wave.
Oh, it's all clear now! Thank you so much

But does high frequency mean lower attenuation compared to a low frequency?
Last edited by cruduxcruo9; 11 months ago
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