How EMR interacts with matter and how antennas differ to other rods

toneboy1

Hi,
So I was thinking, if you've got a metal rod of random size, then any EMR wavelength that the rod (antenna) is a similar length to will stimulate an AC current inside the rod as a complete mess. (It would be the sum of all the frequencies the rod can resonate with)
IS THIS ACCURATE??

But THEN you put an LC circuit on the end of the rod and you tune it (the LC circuit becoming in essence part of the rod! (antenna)) so that ONLY ONE frequency now resonates in the rod.
IS THIS ACCURATE??

If so, how would a one atom antenna resonate? A frequency the wavelength of the atom?
Is the difference between an antenna and other matter that it resonates from EMR but a person doesn't?

FOIWATER

EDIT - corrected

toneboy1

So a rod is a complete non-resonating mess? But an antenna obviously, resonates.

gnurf

In the absence of another nearby conducting object, the only currents I think will be induced in your rod are Eddy currents. If, on the other hand, you rod is connected to a receiver then the rod is a monopole antenna where the AC current will be capacitively coupled to the the reference plane (e.g., ground) to complete the circuit. The rod has resistance and inductance which means you'll have a series RLC circuit which in turn has a resonant frequency that is equal to a quarter wavelength.

gnurf

I think it's a stretch to say that antennas act as they do because of the skin effect. I'm also not sure what your trying to point out re the length of an antenna---the amount of energy you can put into or take out of an antenna is obviously related to the self-resonance, and hence electrical length, of the antenna. Maybe we're not on the same page here? :)

sophiecentaur

I'm not sure what the "skin effect" has to do with the way you can analyse the behaviour of a transmitting or receiving antenna.

Any piece of metal will interact with any em wave that happens to be passing by. It will modify the fields appreciably if it happens to resonate (in some way) at the frequency of the wave and this can reflect or deflect the wave. If you put an appropriate resistor (load) across certain parts of the piece then energy will be dissipated in the resistor. The piece of metal need not actually be self-resonant if you add the appropriate values of lumped reactive components into the circuit. Getting significant power into or out of an antenna of the 'wrong size and shape' may involve a great deal of power loss. Short monopoles tend to be inefficient but you can't make a quarter wave radiator at 200kHz so you just have to put up with it.
Essentially, you can match any frequency into the proverbial 'dead sheep' and get it to act like some sort of an antenna - but not a good one.

FOIWATER

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FOIWATER

Ok, I see what the OP means, once the circuit is tuned, it in essence becomes part of the antenna, and resonance only then occurs at one frequency,

I never thought of it that way before, but I see what you mean.

gnurf

The antenna is also self-resonant at a frequency that is related to the physical dimensions of the antenna itself.

Also, by going back and deleting your posts you're removing the context from other commenter's posts and making the thread incomprehensible.

FOIWATER

ok

so it is resonant with that frequency, the one attributable to the physical dimensions of the antenna itself, but what about all the other frequencies, are they attributable to the connected tuning circuit?

The OP asked if all frequencies would induce voltage into the antenna as a kind of "mess"

Are you saying that is not true, that there is one resonant frequency, and many other possible "tuned" frequencies?

I'm not sure..

sophiecentaur

You seem to be drawing too much of a distinction between the piece of metal and other 'pieces' of metal (such as coils and capacitors) that happen to be connected to it. In many cases, there is not such a clear distinction. Look at a base loaded monopole, for instance, that has a coil at its base which may constitute up to 10% of its height - or a top (capacity) loaded , 'T' antenna. Sometimes (often, even) it is possible to treat the antenna and its matching circuit as two separate elements but it isn't fundamental.
What counts is the currents that happen to be flowing in large parts of the structure and the way it's built will affect them.
Usually, the last thing you want is for your antenna to be highly tuned. Wide band is usually what you are after and wide band matching is a serious engineering art.

toneboy1

Well say we were comparing three things: a single atom (with I suppose no inductive or capacitive capability), an antenna we wanted to suit only one frequency and another we wanted to match a wide band of frequencies.
So the inductive / capacitive capabilities of: the first would mean the electrons didn't resonate with any EMR, the second did best for only frequencies related to the fraction of antenna length to wavelength and the third could selectively resonate the electrons depending on how the LC of the antenna (and circuit) were tuned?

Is that an accurate statement?

Thanks!

sophiecentaur

I think you are trying to reconcile two different models when what you are really dealing with is the old Duality thing. You either must talk about photons or about waves - choose.
The electrons can be regarded as resonating in a simple atom structure because you have line spectra and discrete energy levels; it is ok to discuss photon - atom interaction in that case. You do not have discrete energy levels in condensed matter (Pauli exclusion principle applies to Fermions so you end up with broad bands and not levels) and you need to talk in terms of the photons interacting with the whole structure (distributed charges throughout the antenna) and not individual electrons, in which case, any resonance will the the same as that which classical wave theory will give you (you can't solve it with quantum mechanics afaik).

toneboy1

1. What exactly do you mean by 'broad bands'? Like voltage nodes along the antenna?
2. I was looking up the Pauli exclusion principle, but I don't see how not occupying the same state means they are 'broad bands' (?)
3. To get a general idea. What sort of distance are these charges oscillating back and fourth on the antenna? Any analogy to relate it you like.

Thanks heaps for helping me get my head around this!

EDIT - How should I visualize the charges moving on the antenna as a wave?

sophiecentaur

Broad bands: Nothing to do with antennae - it's to do with energy (Quantum) levels. You've heard of the discrete energy levels in the simple Hydrogen atom? Condensed materials don't have single energy levels, they have a continuous range of possible energies - referred to as bands. Pauli applies because they can't all occupy the same state and a single line (corresponding to one state) will be broadened to allow all the closely spaced atoms to get somewhere near the level they would be in if they were on their own.

This isn't going to help at all if I tell you that the charges have virtually no movement at all. If you consider that the drift speed of electrons is a couple of mm/s and then you realise that, at 10MHz, they are changing direction every 10 millionth of a second. The ain't going to get far in that time and at that speed!
The secret of getting this stuff is to use the right terms and not your own ones. When you talk of antennae, you need to drop QM and use Currents, Potential Differences, Electric Fields and Magnetic Fields. There is an almost simultaneous thread in which we are trying to put to bed the idea that moving charges and their speeds are at all relevant. See: this.

toneboy1

That makes a lot of sense being tightly packed! Cool
Oh, yes that thread, I am familiar with that, I'll continue on there. Cheers