is this an EM wave

Be sure that you are not mixing up radiative coupling with inductive or capacitive coupling.

Bill

-- Ferme le Bush

As long as we assume an infinitly long and straight conductor, why not assume an infinite voltage to go along with it? ;-)

Are they long WRT wavelength? ;-)

any voltage from zero to infinite into a zero ohm load will result in zero transfer of energy.

look up antenna radiation resistance

infinite voltage into a finite load would result in infinite power and probably destroy the entire universe.

if it's a very long wire, but not infinite, you can use it to talk to submarines (see project elf)

if its long but not extremely long, load it up on 160 meters (see beverage antenna)

oo/0 is undefined.

Have you a zero ohm antenna?

...and would take the entire universe to gen...

Of course, but now you're going into the mundane real world. AIUI, ELF really modulates the Earth's magnetic field.

If it's anything like l/20 load it up to whatever you want. If it's a "beverage antenna", isn't it you that gets loaded?

Think about this:

If the antenna is infinitely long (or even just awfully darned long in reference to what we normally imagine for antennas) then you can look at say the first half-wavelength of wire and consider the remaining (still infinite by definition of infinity because you can't make something that is infinite become finite by subtracting a finite amount from it) length of wire to constitute an open circuit. You have a half-wavelength antenna with an infinite resistance at the far end of it.

So you do that and get an answer.

See: the question seemed to be reasonable and the answer meaningful.

But now you look at some other finite length (say a quarter-wavelength) in a similar way and get another answer.

You find the two answers are contradictory. Both cannot be simultaneously true.

You conclude one or more of your assumptions is in error, and in this case, it is the infinite length of the wire.

No meaningful information can be obtained from the assumption of an infinite antenna length.

I will not nit-pick the infinitely long/perfectly straight assumption.

Good exercise.

Chuck

dysfuncti> if i pass alternatin current through an infinitely long and perfectly

Ok, but it it's a little shorter than oo-lambda... ;-)

Yes it does, if the wire is very long compared to the voltage applied. However, a finite (unterminated) wire will eventually reflect the voltage you stuck on it.

Where is the contradiction?

As long as twice the length of the wire (times 'C') is longer than my life, no, I really don't care. It's close enough to infinite length.

Bullshit. I can deduce the characteristic impedance from an infinite length antenna. I can measure the voltage and current at the antenna terminals. That tells me quite a lot.

As long as it's longer and straigheter than our time of interest, it's "infinite". The concept of infinity has meaning. If I take an infinitely long wire passing over an infinite plane, and connect an ohm meter to one end and the plane, what do I measure.

"Infinite" wires are good exercises. I thought you just said there was nothing to be learned from contemplating them. No?

I don't follow what you're saying here. The OP asked about whether he would get an EM wave. An end-fed half-wave wire with an open circuit at the other end will surely radiate.

My point was that depending on how you set up the problem, you can get different answers. To me, that suggests contradiction.

Your position is that if someone shows you an infinitely long antenna, then you'll deduce its characteristic impedance?

Will you also be able to describe radiation patterns for it?

Where will you measure e and i? One terminal to the wire and the other to some convenient place?

Could you deduce the characteristic impedance without having the antenna before you?

If I take an infinitely

What do you measure? Are they in infinite space?

They do tend to be monotonous.

I thought you just said there was

Not exactly. I'm still not sure what I've learned here. I'm waiting to hear your answer to the OP and to see the reasoning behind it. So don't give up yet . . . I'm anxious to learn, kidding aside! ;)

Chuck

It doesn't have to be a half wave to radiate. An infinite wire will radiate, as will a 1/20 wavelength. Launch a step (or impulse) down a wire and tell me that it's not going to radiate. The EM wave is moving town the t-line; it's going to radiate.

No contradictions.

Yep. Measure the current and voltage (S11).

Pattern, certainly not. I'm only looking at one terminal.

Ok, if you have no "ground", are you driving the wire? If a tree...

Could "I"? Likely not, but it seems that the math's gotta work.

"Infinite" to as good of an aproximation as I care to test.

I know, like this thread, they go on forever. "Are we there yet?"

Contemplating the infinite tells us a lot. ...like the ohmmeter on the infinite transmission line.

An end-fed half-wave wire with an open circuit at

Rob, go back to my original post to see where I was going with this.

Let me ask you whether a wire of finite length that has something with infinite resistance (i.e., an infinitely long wire) on one end is of finite or infinite length?

Chuck

Robbie Mayhem wrote:

As you state above, it is clearly a wire of finite length. But an infinite resistance on one end? Hmmm! Not sure about that! Mathematically speaking, you should try and avoid dealing with infinities where possible. I suppose you could talk about how the wave function behaves as it tends to infinity.

...but an infinitely long wire doesn't have infinite resistance. If you hook an ohmmeter to it it'll show it's characteristic impedance (until the D/R components bleed the current on the line). If you hook the same ohmmeter to a finitely long wire terminated with an infinite resistor, the ohmmeter will read the characteristic impedance until twice the length of the wire and then see the reflection from the end.

Please don't top-post. It makes following your point more difficult.