What antenna type is closest to isotropic? - Page 1
Forum section: electronics / rf-microwave

author: fonograph
author_rank: Frequent Contributor
timestamp: August 22, 2018, 05:13:52 pm
content: I read that perfect isotropic antenna is impossible,what antenna type is closest to the theoretical ideal isotropic antenna? Is it the half wavelenght dipole?

author: metrologist
author_rank: Super Contributor
timestamp: August 22, 2018, 06:34:04 pm
content: I imagine an antenna array filled with directional antennas that each radiate to a cone, which collectively radiate to a sphere.But on earth, the magnetic field has influence.

author: fonograph
author_rank: Frequent Contributor
timestamp: August 22, 2018, 06:49:56 pm
content: Quote from: metrologist on August 22, 2018, 06:34:04 pmThat is possible,but only if the individual antennas are atleast half wavelenght apart,otherwise they will mutualy couple and it will work as single antenna and single antenna cant be isotropic.

author: coppercone2
author_rank: Super Contributor
timestamp: August 22, 2018, 07:39:46 pm
content: a hot object? if you heat a ball with a torch it will radiate pretty isotropically so long its polished well

author: hagster
author_rank: Frequent Contributor
timestamp: August 22, 2018, 09:59:48 pm
content: Yes a hot object can be isotropic, but I would argue its not an antenna. There is no coherent signal being emitted. Akin to lots of dipoles emitting different signals.Placing many dipoles on a sphere just creates lots of seperate antennas with overlaping beam patterns. If you combine them into a single port, i dare say you end up with a null somewhere. Not done the math, bit fairly sure this would happen. Obviously you can have 2 or more radio receiver channels connected up provide diversity.Making a dipole shorter does make it more isotropic, but at the expense of efficiency and/or bandwidth.I think its possible to make a quadrifiller helix fairly isotropic. You end up with a single null at the feedpoint though. Skewplanar(cloverleaf) antenna are also fairly good.

author: AF6LJ
author_rank: Unknown
timestamp: August 24, 2018, 03:05:22 am
content: Placing many dipoles in a sphere won't be very isotropic when they all start interacting with each other.

author: metrologist
author_rank: Super Contributor
timestamp: August 24, 2018, 03:24:25 am
content: Quote from: AF6LJ on August 24, 2018, 03:05:22 amThat is so true and profound. I guess we all don't know?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 24, 2018, 11:23:28 am
content: In fact it is provable: mathematicians like to say, "you cannot comb a sphere".Why is this applicable?Light is polarized. It travels in some direction, and is polarized with respect to the directions perpendicular to the direction of travel. The polarizations can be phase shifted, so you get linear polarization of whatever angle, or circular of whatever ellipticity and axis angle. (Basically, polarization can be any Lissajous figure you can plot by varying the amplitude and phase of two sine waves of the same frequency.)However we arrange an antenna array, we must emit some sort of polarization in every direction. The polarization and amplitude must change smoothly from point to point.We can map these parameters onto the surface of a sphere, because we're looking for an isotropic -- spherically symmetric -- radiation pattern.If we do this, then we must have the amplitude equal at all points. Fine. And we can have most of the polarization however we like, like say it's vertical all the way around the equator (like the case for a dipole's main lobe, a torus-shaped radiation pattern, the polarization is always axial).We can keep polarization constant all the way to the ends, but then something happensatthe poles: looking down at the pole, to the left, polarization is aligned right; and to the right, polarization is aligned left. At the pole, the polarizations cancel, and there is an amplitude null at the pole (which violates our assumption of equal amplitudes at all points).(In a real antenna, the width of the null corresponds inversely with the overall size of the antenna, in terms of wavelengths. We could make a very small null, with a carefully crafted, very large antenna; but that would be economically prohibitive.)Or if we swirl the polarization, so it is vertical at the equator, and bends over (east-west) as we get closer to the poles, we get linear polarization around the equator and circular polarization from the top and bottom. But again we get a null at the poles, because polarizations are opposite on opposite sides of the poles.A more accurate correction to our assumptions: we must have amplitude and polarization continuous at almost all points, except for finitely many discontinuities. A sphere with two poles (poles in the mathematical sense of an extreme point, as well as in the geographical sense) is such an example.These are just two of many possible configurations, but it turns outall possible configurationscan be ruled out, and thus thehairy ball theoremcan be proven.Incidentally, the example on there (a smoothly combed torus) is a dipole's pattern, down to differences in what angle the vectors are drawn at (the polarization is linear and axial), and the dimensions of the torus (a dipole's radiation pattern is a "horn torus", i.e., the inner diameter is zero). That's a perfectly consistent result.In practical terms:Not only will you always have nulls in the antenna response (and not be able to afford the space, and money, to shrink the nulls closer to the mathematically ideal points only), but you will have nulls due to unpredictable reflections in the environment as well.Sidestep the problem entirely, by using several complementary antennas. Complementary in terms of polarization and radiation pattern, as well as spacing within the ambient multipath pattern. Do not combine them directly -- because that would create the same problem again -- but detect them separately. (The IF signals can be coherent -- that is, converted with the same LO, but passed through independent RF-mixer-IF-detector signal chains.) This is called diversity radio. The final output signals can be used directly, added together however you like (with whatever considerations might be necessary, of course).The demodulated signals should be strong and coherent at this point, even if the antennas are in positions that would've otherwise interfered with each other (if simply connected together at RF). The case where diversity doesn't help much, is when multipath is strong (specular reflection off a metal building, say?) and the detected signal from one antenna, thus delayed, interferes with the other. (This kind of multipath was quite obvious on analog television, appearing as a shadow image, shifted right by the delay, overlaid on the main image.) A lot of digital comms include active delay compensation filters for this reason.Tim

author: fonograph
author_rank: Frequent Contributor
timestamp: August 24, 2018, 01:32:53 pm
content: If we have dipole antenna,does making it bigger make it more or less isotropic?

author: cdev
author_rank: Super Contributor
timestamp: August 24, 2018, 03:14:50 pm
content: Aquadrifilar helixcan be made to be quite omnidirectional (or unidirectional) depending on the parameters you use. However it is circular polarization, so there is a slight loss when communicating with a linear polarized antenna. Still, you're likely to get far less fading than you would with a dipole.A Lindenblatt antenna is an array of tilted dipoles fed out of phase (to answer the question about what happens with multiple dipoles. Another multiple dipole antenna is a turnstile.) You need a means of shifting phase 90 degrees, which can be as simple as a coaxial cable stub.The kinds of antennas that deliver an isotropic pattern are the ones used on satellites for the kinds of applications that require illumination of the entire earth and maybe even other satellites. (like GPS) without directionality.So a QFH is basically a starting point from which one might want to look at a bunch of similar antennas, all more or less variations on that theme.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 24, 2018, 05:55:22 pm
content: Quote from: fonograph on August 24, 2018, 01:32:53 pmThe higher modes of a dipole are more directional, with a radiation pattern trending towards a sinc(theta) pattern I think.Quote from: cdev on August 24, 2018, 03:14:50 pmThat would be an incredibly expensive and inefficient decision for an already-$100M piece of equipment.Tim

author: rfeecs
author_rank: Frequent Contributor
timestamp: August 24, 2018, 07:47:33 pm
content: I vote for short dipole or small loop.But here is an interesting discussion of some possibilities:https://physics.stackexchange.com/questions/251971/how-do-you-make-a-spherical-radio-wave/311456#311456Links to the papers mentioned:"Isotropic radiators":https://arxiv.org/pdf/physics/0312023.pdf"Experimental Verification of Isotropic Radiation from a Coherent Dipole Source...":https://www.ece.nus.edu.sg/stfpage/eleqc/PRL_isotropic.pdf

author: metrologist
author_rank: Super Contributor
timestamp: August 24, 2018, 08:04:55 pm
content: I think in the real world we approximate theory via creative mechanism and processes. Like, you cannot really multi-task with an 8-bit micro, but if you cycle back and forth between two operations quick enough, you've created the same effect.What about pulsing each directional radiator in an appropriate sequence so a certain number of radiators with sufficient isolation from each other are active at one time, decrease the cycle period between active sets of radiators, and the effect may well enough be mimicked.

author: hagster
author_rank: Frequent Contributor
timestamp: August 26, 2018, 04:54:09 pm
content: Quote from: metrologist on August 24, 2018, 08:04:55 pmYou would need to switch at a faster rate than the data stream(so only really plausible for slow data rates). You would need to make sure the phase centers of the antennas were fairly close to avoid adding phase steps onto the signal.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 27, 2018, 03:20:54 am
content: Quote from: metrologist on August 24, 2018, 08:04:55 pmYou could also transmit three copies of the signal on three closely spaced but non-overlapping frequency bands, each with a differently oriented antenna. That turns out to actually be quite similar to what you suggested. Your rapid cycling at some frequency f_sw creates sidebands with spacing f_c +/- f_sw. In your case, each antenna broadcasts the carrirer and each sideband, I suggest just putting each sideband on a separate antenna (and dropping the second higher order sidebands).If you are transmitting to only one station at a time a better idea is to use the antenna diversity and MIMO tricks like in 802.11ac where the transmitter and/or receiver have multiple antennas and select the one that gives them the best signal, or even transmit coherently across multiple antennas to cause beam forming.

author: rhb
author_rank: Super Contributor
timestamp: August 30, 2018, 05:00:32 pm
content: An isotropic antenna is not physically realizable, but an infinitessimal dipole approaches that. It has the problem of being very inefficient. For a practical antenna, the best you can do is omnidirectional in some plane. A vertical half wave dipole for example.I've been reading "Antenna Theory" by Constantine Balanis, 2016. This is just an initial review before getting down to serious work. It's pretty good. I just finished the first 7 chapters.

author: coppercone2
author_rank: Super Contributor
timestamp: August 30, 2018, 06:43:17 pm
content: does anyone have any thoughts on antennas and EM in higher dimensions? Would EM have a third (fourth w/polarization) component since it seems like its 2 dimensional compared to our seemingly three dimensional universe.Do you end up with two polarizations? So 5 quantities to describe a EMX field at some point?Like what is to a EM wave where cube is to hypercube.Shit my brain just fucking overloadedhttps://physics.stackexchange.com/questions/21678/maxwell-in-multiple-dimensions-what-happens-to-curlThis started because I was thinking about hawking radiation since someone mentioned infinitesimal dipoles, given that the black hole is spinning there must be some kind of deformation of the infinitesimally small point? Like a rotation could only cause a disk but since the black hole still interacts gravitationally the other stuff might widen the disk in another axis and cause some kind of (particle?) flow inside of it? And maybe external forces could cause some kind of impedance gradient to develop between conductive poles, so it approximates some kind of really shitty dipole like thing? I mean that the deformation would occur in some other parameter then space since its supposed to be infinitely small, unless there is some kind of unknown pressure.Otherwise I think you would get infinite stress and rigidity and that seems harder to swallow then small size, as I think infinite rigidity would mean consequences for information transfer.

author: cdev
author_rank: Super Contributor
timestamp: August 31, 2018, 02:34:30 am
content: Linear polarization and isotropic patterns don't seem to happen together in the real world but a quadrifilar helix can act fairly like one with the caveat that the radiation is circularly polarized in one or the other direction but not both.These antennas do have applications for which they are ideal, for example, when you cant have holes in the pattern. Gain is obviously impossible-unless we are talking about gain over the opposite circular polarization (i.e. rejecting reflections) also gain over linear polarized signals. Every little bit counts.

author: Cerebus
author_rank: Super Contributor
timestamp: August 31, 2018, 02:57:54 am
content: Quote from: coppercone2 on August 22, 2018, 07:39:46 pmHow many times have I told you? If you want to play with tools, especially hot ones,you must put your trousers on first.

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2018, 07:33:50 pm
content: I used plasma cutters and acetylene torches in my under ware when I decided to do something when its 36C and 90% humidity outside. I wonder how the hell professional welders (especially with gas) can function in Africa, Australia and the middle east. I would probably be welding naked.What did I learn? You can get away gas welding very carefully. Don't use any kind of electricity based splatter heavy welding or ANY cutting process in your under wareCould probably be a new look for redneck daft punk