Current flows only when circuit is closed, right? but for antennas

[dexterdev]

Hi pf,
I thought that current will flow in closed loops only , but the figure attached shows working of antenna with current in open circuit, but how?

-Devanand T

 

[Bobbywhy]

Have you studied electronics? Normally they teach D.C. (Direct Current) theory first, and later they teach A.C. (Alternating Current). The former requires a closed loop for current to flow. The latter, at radio frequencies (R.F.) driving an antenna, for example, does not. That R.F. current "goes back and forth" or "up and down" inside the antenna, depending on the orientation of the antenna.

Cheers, Bobbywhy

 

[dexterdev]

Any references, books , links etc...please

 

[gnurf]

[DC] requires a closed loop for current to flow. [AC], at radio frequencies (R.F.) driving an antenna, for example, does not. That R.F. current "goes back and forth" or "up and down" inside the antenna, depending on the orientation of the antenna.

The loop is closed in the AC (RF) case as well, when you consider the parasitic capacitance between the two poles of a dipole.

 

[sophiecentaur]

Hi pf,
I thought that current will flow in closed loops only , but the figure attached shows working of antenna with current in open circuit, but how?

-Devanand T

Electrical signals take time to get anywhere. A 'closed loop' in a DC circuit has settled down to a steady state when your simple principle can be said to apply.
If you flick on the switch, it takes a finite time (a few ns or perhaps more) before the circuit can 'know' whether it's open or short circuited or what resistances are there. In the case of an antenna, the oscillations are fast enough for the signal to travel to the far end of the wire and to find an 'open circuit', get reflected back again and to set up a standing wave (as in those pictures of yours). So some current can flow into the input without current flowing 'out of' the ends. With an antenna, of course, there is some power radiated and, after things have settled down (several cycles of the RF) the same amount of input power from the transmitter is radiated into space and there is a lot of energy just sloshing up and down the wire - like the oscillations in an organ pipe. The reason for using antennae of the length they usually are is that they radiate more efficiently when the antenna is approximately tuned to the transmitted frequency (half wave resonant dipole). But any old piece of wire will do it- just not so well.

As I don't know your level of existing knowledge then I can't suggest anything suitable. There really is no short cut to this stuff. You need to read textbooks at an appropriate level for you and slog through the basics or what follows is unlikely to make sense. Your OP question is some way down the line!

 

[gnurf]

Any references, books , links etc...please

Sections two and three in Antennas for Non-Specialists should shed some light on your question. Check it out!

 

[sophiecentaur]

A good link. I'm not sure if it's really for "non specialists" but it would depend on the definition. "Poynting Vector" could spoil someone's day, so early on in the topic.
I like the "blowing bubbles" style picture in Fig 11. It makes the idea of radiation sound reasonable.

 

[jim hardy]

Antennas require one to re-examine his concept of 'current'.

Remember - current is amount of charge passing a point per unit time.

We are taught Kirchoff's laws, about current returning to where it started.

Now - hold your two index fingers together and imagine a single free charge on the end of your right fingertip. hold that thought-----
Now wiggle your right fingertip about an inch either side of your left..
Has not the charge that you imagined moved back and forth past your left fingertip? That would be alternating current and it didn't flow in a loop.
If you could move your fingertip at speed of light you'd have the mechanical analogue of an antenna.

So long as the wires in a circuit are short compared to distance light travels in one period of highest frequency present, you will not have any antenna effects so the current must flow in a loop as you were taught. That's what Sophie was saying with his transit time delay...

This applies to cross county power lines - they must be kept short enough to not become an antenna and radiate power into space at 60hz...

hope this helps.

 

[dexterdev]

Oh Great... I thank you all for the help...I am feeling heaven here...Thank you God for giving me such friends and Teachers... :)