>"Importantly, that cylinder can be pulled out into a long skinny configuration about one foot tall (305 mm), or pushed down to form a ring about one inch tall by five inches across (25 by 127 mm).
In its long state – and when connected to electronics such as a transceiver, ground plane and battery – the antenna emits a low-power signal in all directions, allowing for radio communications with ground-based team members. In its short state, it sends a high-power signal in a specific direction, allowing for satellite communications.
The frequencies utilized in either state are determined by the exact dimensions of each individual antenna."
Isn't that weird and interesting?
An antenna which can transmit in two distinct "dispersion modes" depending on shape and frequency...
In "wide-dispersion mode" (for lack of a better term), it is a standard transmitting antenna, probably subject to the inverse square law (https://en.wikipedia.org/wiki/Inverse-square_law), that is, not unlike the radio frequency analogue of incandescent light -- as might be emitted from an incandescent light bulb...
In "narrow-dispersion" (AKA "focused" AKA "beam") mode -- it is no longer subject to the inverse square law(!) -- and is not unlike the radio frequency analogue of a laser beam!
What's amazing (to me!) is that apparently (if this article is true!) frequency makes all of the difference between dispersion modes -- relative to size and shape.
In other words, perhaps it is possible to get a laser to act more like an incandescent light soruce if its frequency is changed, and conversely, perhaps it is possible to get an incandescent light source to act more like a laser, again, if its frequency is changed. (Of course, in the latter case, we'd need to start with a single frequency since incandescent/white light is by definition multiple frequencies...).
And perhaps this same effect is possible across all frequencies (RF, infrared, ultraviolet, etc., etc.)...
Related (future) question: Under what conditions, exactly (exceedingly rigorous definition required!) is a coil (AKA "inductor") in a circuit also an antenna -- and conversely, when exactly is an antenna in a circuit also a coil?
The "Hopf Fibration" -- might be related to all of this:
Why does the intensity of the light emitted by an incandescent light bulb fall off with the square of the distance (the inverse square law) -- but the light of a laser beam does not?
The light of a laser does diminish at the square of the distance but the cone is very, very narrow. This is because laser light is collimated. The production of laser light occurs in an optical cavity that where uncollimated light is reflected back into the cavity. I would really just be quoting wikipedia so I included the link.
A parabolic antenna also collimates the energy, reducing the size of the cone that the energy is spread over. This allows things like point-to-point communication and narrow-field radio telescopy.
The "power" of a transmitter is the effective radiated power so a transmitter using 1 watt might spread that out over a wide area, but with low power in each direction, or collimate it to a narrow area but with relatively high power in that one direction.
Cones are still subject to inverse square. What matters more is that the focal length is much longer (effectively infinite) with a collimated beam -- that is, the (virtual) tip of the cone is very far behind the light generating element. Inverse square only applies at distances from the source much greater than the focal length.
Laser beams at short distances don't fall off inverse square and may even increase in intensity (decrease in spot size) with distance.
But in the end, beam dispersion/the diffraction limit wins and the power density is inverse square.
(I can focus a big light down to a smaller spot; but ultimately the light is going to be spreading out. This can be true for radio, too, with weird things happening close).
OK, so if it is coherence -- then what's the generalized method to make a given EM wavelength or frequency band (light specifically, all EM wavelengths generally) coherent?
That is, in Physics, how would one take an EM wavelength/set of wavelengths/frequency/set of frequencies (or even more broadly speaking, "energy") -- and make it coherent?
The antenna is already coherent because it is driven by a single signal. The lightbulb isn't because individual electrons are being shaken randomly by thermal noise. The laser is coherent because it is taking advantage of stimulated emission which makes the output photons coherent with the environmental field.
(I wish I could find a better writeup of the spatial interferometer, it's actually a pretty simple concept and a simple experiment but I've never seen it explained very well, even in print, when I was studying physics.)
>"The antenna is already coherent because it is driven by a single signal.
1) Did you mean laser or antenna?
2) By single signal, did you mean single frequency? (If so, I get it. If not, please elaborate...)
>"The lightbulb isn't because individual electrons are being shaken randomly by thermal noise. The laser is coherent because it is taking advantage of stimulated emission which makes the output photons coherent with the environmental field."
3) If thermal noise is the reason that a lightbulb's light cannot be made coherent -- then could you suggest a method whereby the thermal noise in the lightbulb could be removed such that the light emitted could be made coherent?
4) What do you mean exactly by "environmental field"? (A Google search for that term in the context of Physics -- seems not to yield any results -- but then again I lay no claim to being the best Google searcher out there...)
(2) the signal of a radio transmitter is (usually) more or less a sine wave that is either modulated by varying the amplitude or the frequency. You could feed the same signal to multiple antennas. For instance in this photo
there is a radio antenna used for emergency responder comms. Note that there are several arrays of antennas stacked on top of each other. If you feed the same signal into an array like that the radiation pattern becomes focused around the horizontal plane so that energy is not thrown into the ground and the sky.
(3) It is the shaking by random vibrations that makes the black body radiation of a light bulb. If you stopped that shaking there wouldn't be any light.
(4) By "environmental field" I mean the electromagnetic field inside the laser that an active molecule or atom inside the laser experiences.
Note if I hooked up 50 antennas to the same oscillator that would be coherent, but if I hooked up 50 antennas to 50 different oscillators that would be incoherent.
there are two ways to build a phased array. A passive phased array has one transmitter and an collection of phase shifters that delay the signal to create a controlled wavefront. In the first case the emissions of all the antennas are coherent because they come from the same oscillator, in the second case the antennas are coherent because the oscillators are synchronized to a common timebase and controlled by a computer.
Note the "magic" of that kind of phased array is similar to the "magic" of a hologram (they do similar things to wavefronts.) Light other than laser light has a certain amount of coherence though it is a complicated subject, see
Note in Figure 10 they show that you can get enough coherence out of an LED to make a hologram. When they use a real laser the picture is really sharp but you see a speckle pattern that's caused by interference of the light with surface roughness. The LED image is blurry but doesn't have the speckle.
Coherence is not a necessary property of collimated (nondiverging) light (and doesn't cause it to "spread out"). You can produce noncoherent collimated light from any point source using e.g. a parabolic mirror.
I think it's very neat! There's calculators online to design your own QFH antennas and they're popular for amateur use to e.g. receive images from weather satellites. And the calculators can tell you that if you make it more squat, it's more directional.
But I don't think anyone had had the idea before to let you vary those parameters by pinning/scissoring it.
In its long state – and when connected to electronics such as a transceiver, ground plane and battery – the antenna emits a low-power signal in all directions, allowing for radio communications with ground-based team members. In its short state, it sends a high-power signal in a specific direction, allowing for satellite communications.
The frequencies utilized in either state are determined by the exact dimensions of each individual antenna."
Isn't that weird and interesting?
An antenna which can transmit in two distinct "dispersion modes" depending on shape and frequency...
In "wide-dispersion mode" (for lack of a better term), it is a standard transmitting antenna, probably subject to the inverse square law (https://en.wikipedia.org/wiki/Inverse-square_law), that is, not unlike the radio frequency analogue of incandescent light -- as might be emitted from an incandescent light bulb...
In "narrow-dispersion" (AKA "focused" AKA "beam") mode -- it is no longer subject to the inverse square law(!) -- and is not unlike the radio frequency analogue of a laser beam!
What's amazing (to me!) is that apparently (if this article is true!) frequency makes all of the difference between dispersion modes -- relative to size and shape.
In other words, perhaps it is possible to get a laser to act more like an incandescent light soruce if its frequency is changed, and conversely, perhaps it is possible to get an incandescent light source to act more like a laser, again, if its frequency is changed. (Of course, in the latter case, we'd need to start with a single frequency since incandescent/white light is by definition multiple frequencies...).
And perhaps this same effect is possible across all frequencies (RF, infrared, ultraviolet, etc., etc.)...
This antenna/coil design seems very similar to something called a "Caduceus Coil", which I first read about on the now defunct Keelynet BBS/website, a copy of one such file is here: https://www.newphysics.se/archives/keelynet/energy/caduceus....
Related (future) question: Under what conditions, exactly (exceedingly rigorous definition required!) is a coil (AKA "inductor") in a circuit also an antenna -- and conversely, when exactly is an antenna in a circuit also a coil?
The "Hopf Fibration" -- might be related to all of this:
https://www.google.com/search?q=hopf+fibration&tbm=isch
As might the ancient notion of the "Norse World Tree":
https://www.google.com/search?q=norse+world+tree&tbm=isch
Related:
https://www.mail-archive.com/ctrl@listserv.aol.com/msg31218....
Anyway, a very interesting article!