Effect of frequency on power in a wire?

In summary, at 60 Hz, copper wire radiates power. Above a certain frequency, wire becomes an antenna and radiates power.
  • #1
Evil Bunny
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We have relatively low power losses in copper wire at 60 Hz, but when you get up into the MHz range of frequencies, we start radiating and losing power and we switch from wire to something like coaxial cable or heliax and call it transmission line instead. We ground the outer conductor (shield) to "contain" the signal in the center conductor... Do I have the basics of that right?

But anyway, my question is at what frequency range does our ordinary wire start turning into an antenna and start radiating?

Any good sources on this?
 
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  • #2
Once the length of a wire is approaching a quarter wavelength of the signal then it can radiate very well. To cut out the radiation you need to use a screen around it (as in coax) or use two wires - such as a parallel, balanced, transmission line or twisted pair.
In a coax cable, the 'return' current flows along the inside of the screen but you get leakage through the braid and the small currents that flow on the outside cause a small radiated signal and loss.

There isn't a simple answer to your "how long" question. It depends on the layout.
 
  • #3
Lambda/4 is a reasonable rule of thumb.
 
  • #4
Evil Bunny said:
But anyway, my question is at what frequency range does our ordinary wire start turning into an antenna and start radiating?

Any good sources on this?

Any length at all will radiate and at any freq. Simple as that :)
It just radiates the best when the length is at resonance with the freq concerned. Which is what is aimed for specifically with an antenna. As the freq in question rises then the resonant length decreases.

Dave
 
  • #5
Okay... so I've heard the 60 Hz hum in certain equipment with unsheilded cable so I know that it must be radiating on some level. But apparently what you guys are saying is that once we get to a quarter wavelength, we will be in a scenario where most of the power is "radiating" instead of bein "contained" within the wire? That's probably not the best way of saying it, but it's the only way I can think to explain what I'm thinking.

Thank your for your replies!
 
  • #6
There are other coupling mechanisms at very low frequencies (60 Hz qualifies). The hum you hear can be coupled electrostatically or magnetically, as opposed to by propagating EM waves. The ballasts in fluorescent lights, for example, generate magnetic fields (primarily at 60 Hz and the 3rd harmonic 180 Hz) that induce emfs in any conductive loops. Those loop can exist between wires, between traces on a PCB, or as a ground loop. This is different than the radiative coupling of a length of wire at RF
 
  • #7
Evil Bunny said:
Okay... so I've heard the 60 Hz hum in certain equipment with unsheilded cable so I know that it must be radiating on some level. But apparently what you guys are saying is that once we get to a quarter wavelength, we will be in a scenario where most of the power is "radiating" instead of bein "contained" within the wire?
At low frequencies, you are in the 'near field' and nowhere near the distance where you need to consider an EM Radiated wave. You are just talking 'coupling' between elements, which can be magnetic or electric.
This whole question is very "how long is a piece of string?" and doesn't have an answer until you actually address a specific scenario.
 

1. How does frequency affect the power in a wire?

The power in a wire is directly proportional to the frequency of the current flowing through it. This means that as the frequency increases, the power also increases, and vice versa.

2. What is the relationship between frequency and resistance in a wire?

The resistance of a wire is independent of the frequency of the current flowing through it. This means that changing the frequency will not affect the resistance of the wire.

3. Can frequency impact the heating effect of a wire?

Yes, the heating effect of a wire is directly influenced by the frequency of the current flowing through it. As the frequency increases, the wire will heat up more due to the increased power.

4. How does frequency affect the transmission of power through a wire?

The higher the frequency of the current, the more power can be transmitted through a wire. This is because higher frequencies have shorter wavelengths, allowing more energy to be carried through the wire.

5. Is there a limit to how high the frequency can be for a wire to transmit power effectively?

Yes, there is a limit to the frequency that can be used for effective power transmission through a wire. This is because at very high frequencies, the wire may experience high levels of resistance and energy losses, making it less efficient for power transmission.

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