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
>
>
>
>
>
>
>
>
> -------------------------------------------
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