Effect of frequency on power in a wire?

Evil Bunny

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?

sophiecentaur

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.

marcusl

Lambda/4 is a reasonable rule of thumb.

davenn

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

Evil Bunny

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!

marcusl

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

sophiecentaur

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.