electromagnetic transmission power relativistic effects

...although, as Joerg has mentioned, there are Real Live Professors in Real Universities who still inform their students that this is how transmitters are designed, astonishingly enough!

I did get the "antennas are 50% efficient" bit in school, although -- as with the Kraus quote -- it may have been given by someone who was aware of certain assumptions behind that statement which I was not.

Maxwell's equations are the relativistically and quantum-mechanically correct description of the electromagnetic field in vacuo, at least at ordinary frequencies and ordinary power densities. Photons come into the picture when you're talking about the interaction of electromagnetic fields with matter.

There are some processes that can make nonclassical states of the electromagnetic field, for instance parametric downconversion, where two photons get entangled--they're physically separated, but their combined polarizations and energies have to obey certain conservation laws. (The whole field is so Hollywood that it's surprisingly difficult to get the straight story.) Maintaining any measurable quantum behaviour in a macroscopic system is very difficult in general.

In my last couple of years at IBM, I collaborated with David DiVincenzo, whom I admire for many things, among them that he's one of the gurus of quantum computing. I heard about some pretty unintuitive stuff, that's for sure. (We were trying to use antenna coupled tunnel junction detectors to make entangled photons in superconducting transmission lines. It's a good thing he understood it, because I sure didn't.)

Maxwell's equations are fully local, which means that the antenna interacts only with the fields in its immediate location, not something at a distance. So I'd say no, there isn't any spooky quantum mechanical stuff happening with light bouncing round between antennas antenna.

Of course the really deeply mysterious thing about quantum mechanics isn't entangled states and so on--it's the identification of the squared modulus of the waveform with the probability density of finding the particle. I have no idea why that is, and as far as I know nobody else does either. It's just one of those questions where we run off the end of human knowledge.

Cheers

Phil Hobbs

Much more than 50%....

of professors are leftist weenies ;-)

Thus their utterances are more often than not utter nonsense ;-) ...Jim Thompson

The radiation resistance is the same sort of animal as the characteristic impedance of coax. Both are nondissipative, and both merely reflect the voltage drop that results when you dump a given current into the input terminals.

Radiation resistance is a slippery concept that has led many many people to this sort of silly conclusion--there are two degrees of freedom in an antenna feedline, not just one as in a lumped element circuit.

And if the argument were really as conclusive as you say, it would apply to all antennas without exception, rather than just to wire dipoles. All antennas have some value of Re{dV/dI}, after all.

Cheers

Phil Hobbs

Okay. So, if I have understood your reply, there would still be a time delay between cause and effect and we can discount quantum effects? That is, photons still travel at the speed of light?

Thanks, John

Electromagnetic fields propagate at c, but I don't think that photons travel at all. A photon is an elementary excitation of a normal mode of some set of electromagnetic boundary conditions--it's not a thing, it's a property of something else.

I admit to using photons conceptually for some bookkeeping purposes, e.g. using conservation of energy and momentum to show that in an acousto-optic modulator, light that gets diffracted in the direction of the acoustic wave gets upshifted, and light that diffracts the other way gets downshifted--notionally because the first photon absorbed a phonon and the second photon emitted one. You can show the same thing using fields, but it's much more math. (I always feel like washing afterwards.) ;)

Folks imagining EM waves as photons rattling back and forth *never* get the right answer. That's just not how it works.

Cheers

Phil Hobbs

No. The radiation resistance of an antenna is the result of the propagation of the rf from the injection point to the end of the antenna and the reflection of the wave back to the injection point. Think about it. The simplest antenna to think about is a 1/4 wave monopole. You inject a signal at the base. It takes time for it to go to the end. While the wave travels from the source to the end there is radiation. It is there reflected. It will come back to the injection point exactly 180 degrees out of phase. So, the source will see a short circuit? NO! Upon being reflected, more radiation occurs on its trip back to the source. The difference between what was sent and what returns is sort of considered radiation resistance. What else could explain the loss of energy?

John

Okay. I think you are just jerking me off now. I think you could have answered my question in a more straightforward manner rather than jerking me around.

Cheers, John

Radiation resistance isn't an explanation, it's just a name. Antennas are linear, which means that dV/dI is a constant, and you can call it a resistance if you like. An infinitely long reel of lossless coax is exactly the same sort of thing.

Cheers

Phil Hobbs

Not meaning to. On the other hand, this is Usenet, so questions and answers are often not that closely related.

Cheers

Phil Hobbs

Usenet is a good excuse. Thank you.

Read his original question. He was specific about the numbers, not "however miniscule."

This has nothing directly to do with quantum mechanics. This is simple conservation of energy stuff.

At the remote site, if there were no receiver, the RF would just fly out into space. But if a receiver intersects a microwatt of that power there, the transmitter neither knows nor cares. There's no requirement that the transmitter power supply output an extra microwatt to make up for the microwatt the receiver steals from space.

If a reflector did fling a microwatt back at the transmitter, the transmitter antenna impedance would change slightly and the power supply might notice. But far less than 1 microwatt worth. And reflected power is not absorbed power anyhow.

As several links have shown, there's no reason an antanna has to reflect half of the incident power. Maybe a simple dipole does, but that's not the issue.

John

Yikes! Mr Photon Budget doesn't believe in photons! I am seriously disillusioned.

John

So am I. And, it's not the only thing he is wrong about. He just has too much ego to listen to others.

John

No. I know Phil, and that's not the case. His problem, if you can call it that, is that he's about 17 times as smart as us mere mortals. He wouldn't be wrong about basic stuff like this.

John

Oh, crap, John. How minuscule is a microwatt compared to 1kw? 10^12 makes me think minuscule. What does it take for you to think minuscule?

Fine. That is really the way I think of it, too. I'm trying to answer Jamie's question without imposing things such as "Why are you concerned? It's too little to measure anyway."

True.

How do you know that? Please prove it.

Of course not. The fact that an object changes the loading on the transmitter's antenna does not obligate the transmitter to change anything.

Absolutely true. But, that is not the question. Reread his post and

First, you do not know the type of antenna that the OP had in mind. So, my input is at least as valid as yours.

Second, the links you refer to are no better than the authors I have cited. Perhaps less valid.

Do you understand a source, source resistance, and load resistance, all in series? Don't deny that you do, because I know better. Draw and analyze that.

John

Anybody can be wrong. What credentials does he have in the field of antennas? Can he be compared favorably with the likes of John Kraus and the other giants in the field?

Please show citations.

John

Roughly a million times yours. Literally.

Can he be compared favorably with the likes of John Kraus and

Google them yourself. They're not hard to find.

John

Really? In the field of antennas? Your reply was not an answer, John. One can always say "Sure, go look for it."

Please show citations or go f*ck yourself. I am beginning to think Thompson and his cadre are correct about you. All you want to do is argue.