electromagnetic transmission power relativistic effects

Hi,

For a radio transmitter that is transmitting a 1MHz sine wave at 1kW power into free space, and then a 1MHz receiver is turned on 1 km away, which draws 1uW? of power from the transmitted wave, will the transmitting power supply increase by 1uW at the time that the receiver is turned on or at a later time proportional to the distance to the receiver? I was wondering about this because from some interpretations of quantum mechanics it seems that the transmitter power supply would increase its power draw by 1uW at the exact time the receiver turns on, but this should be relatively easy to test if there is an actual delay.

cheers, Jamie

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Reply to
Jamie
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It may not change at all. If the antenna+receiver reflects no power back towards the transmitter, the transmitter doesn't know they are there. Power that is absorbed isn't reflected.

If the antenna did reflect something back, only a tiny bit of that would hit the transmit antenna. But it would take about 3 microseconds for any reflection to propagate back to the transmitter.

Incidentally, that transmitter is spitting out about 1.5e30 photons per second.

John

Reply to
John Larkin

For a perfectly matched antenna/receiver system, half of the incoming power is re-radiated and half is absorbed.

John

Reply to
John KD5YI

Well, the receiver system will absorb half the power and reflect the other half.

As far as the transmitter increasing its load, it depends on the phase of the re-transmitted signal from the receiver. It could actually reduce load on the transmitter.

You may have given enough information to calculate that, but it is late and I am tired. Perhaps, if you think about it long enough, you can solve it yourself. It is a rather complex problem.

Cheers & 73, John - KD5YI

Reply to
John KD5YI

I think my brain got sidetracked on your question which will explain my last post.

If you accept the fact that light is the ultimate speed limit of everything in the Universe, then you know that cause-and-effect can happen no faster than that.

1 km would cause the transmitter to not know that the receiver is re-transmitting power for about 6 us at the distance you propose. If cause and effect were instantaneous, yagi antennas would not be possible, for example. If the sun were to suddenly extinguish, would we know it instantly?

Do you have a copy of QED written by Richard Feynman? It will introduce you to quantum effects and is well worth reading.

Cheers, John

Reply to
John KD5YI

Why is that? There's no fundamental reason why an antenna couldn't absorb 99% of incoming power over some aperature.

John

Reply to
John Larkin

How about the entanglement principal that is supposed to transmit information instantaneously? Know anything about that? Sounds sci- fiction to me.

-Bill

Reply to
Bill Bowden

This sort of transmitter doesn't produce entangled states so the transmitter will only see the change in load after a return trip light travel time has elapsed. And chances are compared to the grass and trees moving about the change would be totally undetectable.

Early atomic clock based VLBI systems quickly noticed systematic errors easily measurable in the public time signals from Rugby which varied diurnally according to the amount of dew on the ground.

There was a famous one where allotments at the bottom of a UK TV transmitter mast were heating their greenhouses with radiated power. The authorities noticed the load changing when more and more tapped into the "free power". They were told to cease and desist.

If you have a process for example radioactive decay or matter antimatter annihilation that by symmetry must create a pair of particles with the opposite sense of spin or parity then in quantum mechanics the system state is undefined superpositions of all the possibilities until you actually measure it.

Measuring one of the particles and getting an answer automatically means that you know the state of the other one at that exact instant too. You could interpret it as information transmitted instantaneously.

The same entanglement principle is being used to build quantum computers with registers of N qbits of information that can represent all possible numbers up to 2^N simultaneously. It has very serious implications for cryptography if it can be made to work for large N.

And article in Nature on the public access side of the firewall shows that this is *not* science fiction however weird it may sound.

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Technology from 50 years in the future can look like magic.

Regards, Martin Brown

Reply to
Martin Brown

John Larkin wrote: : On Mon, 23 May 2011 20:16:08 -0700, Jamie wrote: : >For a radio transmitter that is transmitting a 1MHz sine wave at 1kW : >power into free space, and then a 1MHz receiver is turned on 1 km away, : >which draws 1uW? of power from the transmitted wave, will the : >transmitting power supply increase by 1uW at the time that the receiver : >is turned on or at a later time proportional to the distance to the : >receiver?

: It may not change at all. If the antenna+receiver reflects no power : back towards the transmitter, the transmitter doesn't know they are : there. Power that is absorbed isn't reflected.

With an ordinary transmitter configuration the case is like John described: each transmitted photon either gets absorbed in the far-away receiving antenna or travels to infinity. No information gets carried back to the transmitter about which way the photon went.

It is a different story if each photon gets created by a special process, which guarantees that the photons are created in pairs in such a way that a conservation law guarantees one of the photons being 'up' while other photon being 'down'. (Ordinary rf transmitters do not create photons in that way.) You can then somehow store the one photon locally for future reference while transmitting the other photon away. Now, if an observer in the far-away receiving station catches the photon (sort-of-absorbs it, but the 'dumb' unobserved absorption is not enough to cause interesting effects) and measures its spin to be 'up', the sender can be sure to have his stored photon to be 'down'. (note that the transmit rate must be slow enough so that the sender and receiver can be sure that the photons belong to the same pair).

This still is not enough for anything interesting. It is equivalent of splitting a coin in the darkness of your pocket, without looking, and giving one half to your friend, still without looking. Your friend then travels oversees and there looks at his half. If it is tails, you can be sure (without looking) that your half is heads. However, it would be silly to say that your coin half magically transformed into heads in your pocket because of 'superluminal quantum correlations'. There is nothing quantum in this effect yet.

The strange part comes about only when you and your far-away friend are given the freedom to measure whether the photon spin is left/right instead of up/down. Such a measurement accesses quantum superpositions. Even then you and your friend must come together and compare your results in order to notice that something weird has happened. So, even by utilizing quantum correlations, there is no way to detect at the transmitting station, superluminally, whether someone has absorbed a photon which you have transmitted. But many *other* counterintuitive quantum effects demonstrably do exist.

Note that I did not go into details of what happens when one *does* measure in the left/right basis - the reason is that two-photon correlations that the OP was groping for give rise to just statistical correlations and hence sound vague and uncomfortable to a layman (at least at the "Bertelmann's socks" level). With three photons (the GHZ experiment) it is possible to get more assuring bang-bang kind of results.

Unfortunately there is no way to measure the split coin in other than the heads/tails -basis, to make an analogy which would convince a layman...

Regards, Mikko

Reply to
Okkim Atnarivik

y,

er

According to these guys, approaching 100% is theoretically possible but difficult to achieve in practice - 50% or less is more likely. It comes down to the relative receiving and scattering directivities of the antenna.

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-- Joe

Reply to
J.A. Legris

That's nice. The folks who propose beaming microwave power down from solar satellites would pave square miles with rectenna arrays, and would certainly need above 50% efficiency.

John

Reply to
John Larkin

The idea is that, if your antenna represents a source with a 50ohm impedance, to extract the most power from it, you match it into a 50ohm load, right?

Great. But at that point, invoke reciprocity... that 50ohm antenna source impedance also represents a 50ohm radiation resistance when the antenna is transmitting. Hence, for every watt that you transfer to the load, a watt must be hitting that radiation resistance and re-radiating as well.

Or something like that. :-)

---Joel

Reply to
Joel Koltner

I think John D. Krause says it better than I. From "Antennas For All Applications", 3rd edition, Krause & Marhefka, page 29:

"When the antenna is receiving with a load resistance Rl matched to the antenna radiation resistance Rr (Rl=Rr), as much power is reradiated from the antenna as is delivered to the load. This is the condition of

*maximum power transfer* (antenna assumed lossless)."

From page 30: "The above discussion is applicable to a single dipole (lambda/2 or shorter). However, it does not apply to all antennas. In addition to the reradiated power, an antenna may scatter power that does not enter the antenna-load circuit. Thus the reradiated plus scattered power may exceed the power delivered to the load."

As Joel said, just the act of supplying current to the load causes that same current to flow through the antenna's radiation resistance and that causes radiation.

Cheers, John

Reply to
John KD5YI

Joe posted a link to a paper that discusses the issue. Some antennas do a lot better than 50%.

At optical wavelengths, one can absorb 99% of incident radiation.

Here's a rectenna that has 82% efficiency converting RF to DC:

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I've seen claims of 90 and 91%.

John

Reply to
John Larkin

As a practical matter, it won't increase (because transmitter power is regulated by law, and the transmitter will adjust to deliver the 'right' amount of power). In theory, however, the impedance of the antenna will have a response to that receiver, in the form of a time-delayed dip. That's because a receiving antenna is in no way the same as 'free space'. The current in the receiving antenna generates another wave, and that wave interferes with the original transmission in such a way as to remove that microwatt of power.

That outgoing wave is also the reason RADAR works. Even if the responding antenna doesn't have any receive electronics attached, it makes a response.

Reply to
whit3rd

y,

er

Yes - I think this is incorrect as well. I would have chimed in sooner, but thought maybe John (the other John) thought this thread was about a half-simplex arrangement, or what not. Still, either way, it looks wrong to me.

And in fact, why not absorb more than 100%? Need we restrict ourselves to TEM mode? Just thinking out loud.

Reply to
mpm

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rce

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att

My question is actually pretty simple: (to pose, that is...)

Does an antenna behave exactly the same way for recieve as it does for transmit? Before trying to answer this, (and I don't know the answer for sure, and neither do some extremely bright folks I've discussed it with), it would obviously help to know where, precisely, the electrons leave the active element (if they leave at all). For simplicity, maybe we could stick to a dipole, or a folded dipole for discussion purposes.

My contention is that exciting a conductor in free space is different, the instant you attach anything to it. (And I'm not talking about tuning here.) Think about a wire at resonance, and then try to use it for a transmitter or receiver. What happens to the electromagnetic current (compared to before you tried to use it).?

What happens if you were to take this entire contraption, and place it inside another electromagnetic field? Hummmmm......

Reply to
mpm

Nonsense. That argument would also mean that you couldn't get a SWR better than 2:1 when you connected two coax cables together, and that a stealth aircraft's radar cross-section couldn't be less than half its projected area. It's all about mode matching.

All you need is to have a scatterer that cancels out the re-radiated beam. I've built antennas coupled to dielectric waveguides that had better than 95% efficiency.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
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Reply to
Phil Hobbs

Phil, would you agree that an oscillating (accelerating/decellerating) current causes radiation? Further, would you agree that, in a receiving antenna, the incoming signal causes an oscillating current? Still further, can you explain why the receiving antenna would then not radiate?

John

Reply to
John KD5YI

And one can absorb the same amount of RF by some means but not by a metallic conductor dipole with a load in the center.

Why do you think NIST uses such high resistance at the load point of their reference dipole?

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Reply to
John KD5YI

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