- Thicker absorbers (volumetric)
are more broadband than surface absorbers.
- Magnetic properties increase
the bandwidth of absorbers.
- High dielectric constant
reduces bandwidth.
- Magnetic properties are
frequency dependent, especially in the UHF to GHz range.
- An absorber that is broadband
in frequency is broadband in angle; conversely narrowband in
frequency implies narrowband in angle.
- Broadband behavior requires
grading the properties from light on top to heavy on the bottom.
- Wideband behavior can be
obtained by using transformer/filter design concepts – a combination
of interference and absorption.
- Most absorbers are backed
by a conducting ground plane. Graphite reinforced epoxy is one
example of this.
- Most RAS includes an unfortunately
thick skin, plus chemical agent resistive coating (carc) plus
paint on top; the high frequency behavior of the RAS is controlled
by this skin structure.
- Skin cancellation is possible
but it's not pretty.

Of course some generalities
compete with others. For instance, an absorber that is electrically
thin over its whole band of operation tends to behave the same
way at all frequencies. High dielectric constant makes kz inside
a material independent of angle of incidence so you can get internal
behavior that is broad versus angle. A thick homogeneous layer
of material has internal resonances that will make its behavior
narrowband.

### Classic
absorber design #1: Salisbury screen

Salisbury worked in the MIT
RadLab in WWII, and this technique was named after him.

An easy way to use the ohmic
loss mechanism is layered absorbers. The Salisbury screen consists
of a sheet of resistive material placed /4
over ground. Magnetic loss mechanisms are intrinsically narrow
band.

The Salisbury screen is narrowband
in frequency and angle.

According to one of our generalities,
to get more bandwidth you need to use multiple layers of R-Card
separated by dielectric spacers. According to another, the dielectric
constant of the spacers controls the maximum bandwidth of the
design. The lower the permittivity the larger the bandwidth. But
foam and honeycomb spacers give a physically thick sandwich structure.
To get a composite skin you can lay up fiberglass and R-card layers
and injection mold the whole thing.

### Classic
absorber design #2: Jaumann

Johannes Jaumann worked in
Germany before and during WWII, check him out in our Microwave
Hall of Fame! His absorber design was first used on submarine
periscopes to make them less visible to surface ships.

The Jaumann absorber can be
designed using transformer concepts. Think of it as a shorted
transmission line to which a sequence of shunt conductances are
added separated from each other by quarter-wave transformers.

Conductances add in parallel,
and the quarter-wave transformers can be used to invert the result.
Work on a Smith Chart normalized to 377/_{R}^{1/2}.
Start at infinite admittance (short circuit, right side of the
Smith chart), then add a quarter-wave
transformer to turn it into an open (zero admittance). Next add
G1 to move past the center of the chart (here it's 377 ohms).
Then add another quarterwave transformer to rotate back to the
right side, then add G2, and so forth...

The result of this approach
is a very deep null at center band, and twice the bandwidth of
a Salisbury screen, using the same dielectric.

To get broadband design, think
of the statement of the problem as being: "hide the groundplane."
In other words, if we could eliminate all echoes behind the front
face of the dielectric, the worst case echo would be the one from
the front face of the dielectric. A dielectric of impedance =
_{0}/2
has a front-face reflection coefficient of about –10 dB. Let’s
come up with a scheme for gently erasing everything else.

Note that after you accomplish
this you can convince your customer to add an anti-reflection
layer to the design and you’ll do a lot better than –10 dB

The approach: imagine the energy
entering this material as it passes G3, G2 and G1, hits the groundplane
and returns again through G1, G2, G3. Every time the wave interacts
with a G sheet it loses energy. How much energy does it lose?
Well if you treat it as a local phenomenon (kind of time domain
thinking – not frequency domain) then consider a sheet of G in
a sea of Ym.

By considering the rest of
the medium to the right as a load Ym behind G

It follows that:
= (Ym-(G+Ym))/(Ym+(G+Ym))

That is,
= -G/(G+2*Ym)

Thus, T=2*Ym/(G+2*Ym).

In this simple problem T is
the transmitted E field at the sheet and therefore that of the
transmitted wave behind the sheet. Therefore the power transmitted
is lower than the power incident by the factor: P=4Ym^{2}/(G+2Ym)^{2}.

At each interface you now have
a power loss factor and a local reflection coefficient.

Looking at the “ray” that makes
it all the way to the ground and reflects back…

The incident ray, Ray 1, is
down in power by:

Ray number 2 is
the one that crosses G3 and G2 reflects off G1 and travels all
the way back:

Ray 3 only crosses
G3 and reflects off G2 and crosses G3 again:

Finally Ray 4
just reflects off G3:

Experience with
the anti-reflection coating tells us that the way to get deep
nulls and good cancellation is to make the various echoes have
the same strength so that when they are out of phase with each
other they cancel exactly out.

This means, starting with Rays 1 and 2, that |P_{1}^{2}|
= |_{1}|.
Therefore:

Which becomes
a quadratic equation

In this example Ym=2/377

Solving this for G1 gives:
G1= 1.235Ym

From this we can calculate:
1
= 0.38

Now for Rays 2 and 3 to be
equal P2^{2}*1=2,
or:

Again a quadratic equation
that yields: G2=0.5897Ym

For Rays 1 and
2 to be equal, P3^{2}*2=3

Because G2 yields
2
= 0.22

Again a quadratic equation
that yields: G3=0.3711Ym

Implementing these:

And so there is
a closed form procedure for designing “gentle Jaumanns” that reduce
the reflection coefficient to that of the supporting dielectric.
The Jaumann is broadband in angle as well as frequency. In fact
the degradation with angle in TE is simply the increased reflectivity
of the TE case of Fresnel’s equations.

Like so many microwave circuits,
with the Jaumann absorber you can trade off bandwidth for performance:

So there is a design procedure,
but with exceptions.

Note that this design is upside
down: The top sheet (G3 because we are not using G4) is the highest
and the one next to the groundplane is the lowest in conductance

The End!

Much of the technical material
on this page was prepared by Dr. Rudy Diaz of Arizona State University,
for ARC Technologies, Inc.