Part one is on the fundamentals
of electromagnetic waves (you are here!)
Part
two is on radar cross-section physics.
Part
three is on radar absorbers and absorption mechanisms.
Here is the index to this page:
Some
definitions
Fundamentals
of electromagnetic waves
Click
here to go to our main page on absorbing materials.
Some
definitions:
carc
Chemical agent resistive coating.
far-field
When you get far enough from an antenna so that its radiated field
wave can be considered planar. Also called the Fraunhoffer
Region. See rule of thumb below.
H-pol
Horizontal polarization.
magram
Magnetic radar absorbing material, made of a synthetic rubber
material loaded with iron particles.
near-field
When you are close enough to an antenna so that its radiated field
must be considered mathematically spherical rather than planar.
Also called the Fresnel Region. See rule
of thumb below.
PEC
Perfect electrical conductor.
PMC
Perfect magnetic conductor.
Poynting
vector
The cross-product of the E-field and H-field, it "points"
in the direction that energy is moving. Units are power/area (W/m2).
radar
cross-section (RCS)
The area of a perfect "mirror" that would give the same
radar return as a complex object in the far field.
RAS
Radar absorbing structures.
RCS
Radar cross-section.
R-cards
Resistive cards.
scatterer
The object that you are trying to reduce the radar cross-section
of.
Specular
reflection
Mirror-like reflection, the worst kind when you are trying to
reduce RCS. Similar to when an SUV behind your car nails your
rearview mirror with its headlights while you are driving at night.
TE
Transverse electric.
TM
Transverse magnetic.
V-pol
Vertical polarization.
Fundamentals
of EM waves
Electromagnetic (EM) waves
are created by time-varying currents and charges. Their interactions
with materials obey the boundary conditions of Maxwell’s equations.
EM waves can be guided by structures (transmission lines) or by
free space. An antenna is a material structure that directs EM
fields from a source into space, or, by reciprocity, from space
to a receiver. The shape and size of the antenna controls the
transition from the near field to the far field.
Near-field behavior is most
clearly seen surrounding small antennas; the electric dipole is
a capacitive object:
The near field consists of
the reactive near field, also known as the quasi-static near field,
and the radiating near field also known as the Fresnel zone or
Fresnel region. In the quasi-static near field we see fields that
strongly resemble the electrostatic fields of a charge dipole
for a dipole antenna and the fields of a magnetic dipole for a
loop antenna. In large antennas the quasi-static field can be
seen near edges.
In the Fresnel zone the waves
are clearly not plane and may have phase shifts that do not vary
linearly with distance from a (fictitious) phase center.
From the near field to the
far field, EM radiation changes from spherical waves to plane
waves. The far-field is sometimes called the Fraunhoffer region.
Microwaves101 Rule of Thumb
You are in the far-field if
your distance from the source is greater than 2d2/
.
The small loop is the magnetic
dipole, an inductive object.
Common to all
electrically small (less than a wavelength) antennas is that the
near field excites the environment in which the antenna resides.
It’s the antenna combined with its environment that radiates EM
waves. Electrically small antennas include:
|
microstrip
patch
|
cell phone
|
folded
patch
|
Before tackling the near field,
it’s best to understand the behavior of the far field. Far from
the source, the spherical EM waves flatten out and can be treated
like plane waves. The power density is given by:
Ptransmitted/Area
spread out = W/m2
In the far field,
E-field and H-field are proportional to 1/radius. The Poynting
vector (power/area) is given by E x H. Therefore, power density
drops as 1/radius2.
Electrically large antennas
are good sources of plane waves in the laboratory. A large antenna
focuses the power, this is called directivity, or directive gain.
Here are some large antenna examples:
|
Cassegrain
antenna
|
Slotted
antenna
|
Waveguide
horn
|
The antenna creates a large
(D>>)
area of electromagnetic field, with nearly uniform phase.
The paragraph below was
updated February 16, 2009 for clarity:
Below are two pairs of field
pattern plots of a 1 x 1 foot aperture, at one GHz and then at
five GHz, representing an "electrically small" aperture
and an "electrically large" aperture. The left plots
are amplitude while the right plots show phase. Note that the
5 GHz signal becomes "far-field" in a shorter distance
than the 1 GHz signal (that is, the phase lines become flatter).
The large aperture (5 GHz) quickly allows plane wave fronts to
form, but there is interference inside the Fresnel zone (the peaks
and valleys you see in the amplitude). The small aperture (1 GHz)
generates a wave much like a small point radiator, so the wave
is spherical near the aperture (there is no clear transition to
a Fraunhoffer region).
Footnote: the computation
that generated the figures beyond the 1x1 foot aperture is credited
to PhD student Zhichao Zhang of ASU.
Much of the technical material
on this page was prepared by Dr. Rudy Diaz of Arizona State
University, for ARC Technologies, Inc. This Microwaves101 page
was contributed and is sponsored by:
