An excellent explanation -- I've seen these issues explained many times, either over simplified or overly detailed. The explanation below is a superb balance. Thanks....
Michael Hopkins KeyTek -----Original Message----- From: Nick Rouse [mailto:100626.3...@compuserve.com] Sent: Thursday, November 30, 2000 9:06 PM To: brian_kunde Cc: EMC Subject: Re: How does RF travel through outer space? RF, meaning radio frequency radiation, is electromagnetic radiation just like light. At the classical level this can be thought of as an electrostatic field that is in a direction that is at right angles to the direction the radiation is propagating. Coupled to this is a magnetic that is at right angles to the electric field and the direction of propagation. If you stand at one point, the strength of the both the electrostatic and magnetic fields vary sinusiodally with time. The number of times a second the fields swing through a full cycle (positive, negative and back to positive or north, south and back to north) is called the frequency. If you examine the fields at different points in space at any one instant you find the strength varies sinusoidally along the direction of propagation. The distance between peaks is called the wavelength. A simple way to think about the radiation is to consider that the changing fields induce each other, the changing magnetic field induces the electrostatic field and the changing electrostatic field induces the magnetic field. Since both electrostatic and magnetic fields contain energy this radiation contains energy. As long as nothing absorbs the energy of the two fields this process continues in a self sustaining way and the radiation propagates energy through whatever medium it is in. A more sophisticated explanation comes from solving the differential vector equations for electricity and magnetism that James Clerk Maxwell developed. These have a solution in the form of a propagating wave. You can add various layers of sophistication to this. You can include the fact that the energy associated with these fields cannot be increased in a completely smooth manner but must increase in small jumps or quanta. Incorporating this will give the Schrödinger equation and adding the effects of special relativity will give Dirac's equations in spinor form that are at the heart of quantum electrodynamics (QED), our best theory. There are no known circumstances in which this theory can be applied and in which the experimental results are in conflict with the predictions of this theory. Returning to more simple matters; if you can convince yourself that electrostatic fields and magnetic fields operate in a vacuum them you should not have too great a difficulty accepting that electromagnetic radiation can propagate through space. It is fairly easy to show that these fields do operate in a vacuum. The gold leaf electroscope, beloved of primary level physics lessons, works just as well if you pump the air out of the conical flask it is traditionally set up in. A steel ball slid gently into the vacuum flask you use for picnics can be moved around by a magnet outside the flask with the magnetic field operating through the vacuum. These fields (or the equivalent quantum mechanical variable) are all there is to electromagnetic radiation. They are not some extra bit added on to the 'real' nature of radio, x-rays, light etc. The strength, frequency, direction etc. of these fields give all the properties of these effects. Electromagnetism is a single phenomena. The range of frequencies that have been observed cover 36 orders of magnitude. Not surprisingly the properties vary enormously over this expanse but the variation is smooth and continuous. There are no real boundaries. The limits given in books for x-rays infra-red and the like are as arbitrary and man made as country and state borders. Light is only special in that man has evolved organs that are sensitive to electromagnetic radiation with frequencies in the range of about 430 to 850 THz and sensing such radiation gave it a name. Even here the boundaries are fuzzy. The sensitivity of the eye peaks in the middle of that range and tapers away at the ends in a bell shaped curve with no absolute limit and variation between individuals. Having said that you should be able to accept that electromagnetic radiation propagates though a vacuum I should say that in the ninetieth century many imagined that light must propagate through something. Since it had a fixed velocity it must be a velocity with respect to something. An all pervasive 'luminiferous ether' was postulated as the fixed reference for this velocity. However the famous experiment of Michelson and Morley in 1881 showed practically, and Einstein's special relativity of 1905 explained theoretically, that electromagnetic radiation propagates at the same speed with respect an observer irrespective of whether that observer is moving towards or away from the source of that radiation. By showing that this and all other physical effects could be explained without the need for a special frame of reference, the need for a luminiferous ether was removed and using the principal of Occam's razor was discarded. I don't know of any doubts in mainstream science as to the explanation of the propagation of light given by QED. Various proposals have made to overcome difficulties in other areas that would effect the propagation of light such as the variation over cosmic time of the principal 'constants' or free space being slightly dispersive (like a prism) but these are highly speculative and have, as far as I am aware, no experimental backing. Heterodox thinkers of very widely varying credibility have challenged most established theories at one time or another. Who knows one of them may be the prophet of the scientific future but the odds are against them. If 11 dimensional super string theory lives up to its promise and becomes the master explanation of all physics the description of light will look very different but it must reduce to something close to the present formulation in most experimentally accessible regions if it is to agree with all past experiments. The inverse square law only applies to distances that are large with respect to size of the source. It does not imply any loss of energy, only the spreading out of that same amount of energy over a larger volume. Imagine the energy in a spherical shell 1cm thick at a radius of say 10m from a small antenna or light bulb and then imagine that this energy propagates out radially in all directions so that some 90ns later it occupies a spherical shell still 1cm thick but 40m radius. Since the radius has increased by a factor of 4 the surface area of the shell will have increased by a factor of 16 (A = 4 .pi .r²) Since the thickness is the same the volume over which this energy is spread out has increased 16 fold. If no energy has been lost and all of it uniformly spread out the energy per unit volume must have been reduced by a factor of 16 giving the inverse square law relationship. Note that this is not a peculiarity of electromagnetic radiation but applies to any spherical spreading out of energy such as sound from a suspended loudspeaker an heat conducting through a solid from a point source. Conversely in does not apply even to electromagnetic radiation from a source that is large with respect to the measurement distance. The field from a long wire radiator will fall as 1/r for distances short with respect to the wire length and does not fall at all from a flat plate radiator at distances short with respect to the plate dimensions.(assuming all points on the radiator in phase or distances small with respect to the wavelength) Followers of this forum will know that close to the irregular shaped equipment they have to deal with, the radiation pattern can change in the most complicated way. I know this does not count as anything like simple but simple sounding questions often have complicated answers as Benoit Mandelbrot found when he asked 'how long is the coast of Britain?' I hope you can distil some simple if simplified answers that will satisfy your class. Nick Rouse ----- Original Message ----- From: "brian_kunde" <brian_ku...@leco.com> To: "emc-pstc" <emc-p...@majordomo.ieee.org> Sent: Thursday, November 30, 2000 8:50 PM Subject: How does RF travel through outer space? > > > > > Hello, > > I'm sorry if this is too simple of question... "How does RF travel through outer > space?". > > I will be teaching a class in which this question will come up. I want to be > prepared with all the basic science behind this principal. I need an > explaination that is simple and easy to understand. > > People seem to have no problem understanding how waves can travel through mass > such as a body of water but can not understand how it can travel where there is > no mass. I also understand that there is a lot of debate over how Light travels > through space (photons and all). > > Also, I understand that RF signals degrade at a rate of 1/distance(squared). > What force is causing this attenuation? > > Try to keep it simple for my audience it not all that technical. Appreciate the > help. Please forgive any improper punctuation or word misuse. > Brian > > > > > > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > To cancel your subscription, send mail to: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Jim Bacher: jim_bac...@mail.monarch.com > Michael Garretson: pstc_ad...@garretson.org > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
