The Absorbers for Microwaves Frequencies
4.2 The Structures and the Material of Wave Absorbers
Our purpose is to examine the dynamics of high-frequency microwave absorption and efficient absorber structures that can tolerate high power while also possessing a structural simplicity for laboratory fabrication.
Here, we introduce two various wave absorbers, the pyramidal absorber and the
wedg
ge-shaped a e 4.1.
The purpos ormance. In rbers whic rbers are s owave is fix
The sizes o eguide. In t se fundame versions are re 4.1 show vectors repre magnetic fie een in [23].
absorber. Th
se of this sim the simulati ental mode avoidable w ws the field
esent the m eld. More pl
he features a
mulation is diminish th fluence the the same, p
tt.
are specifie ion, these a
TE10 cutof when the us distribution agnitude an lots of moda
and physica
to look for he dimensi
reflection particularly
ed according absorbers a ff frequency sed frequenc n of TE10 m nd direction
al field dist
al specificat
what kind o ional distin
and absor y the absor
g to the insi are settled i y is 21.081 cies are und mode in a re n of electric
tribution in Table 4 The dim and we
tions of them
of absorber ctions betw rption, mos bing area.
ide dimensi n the Ka-b 1 GHz so th der double c
ectangular w field and bl rectangular 4.1 mensions of edge shaped
m are show
has the opt ween these
st parts of The powe
ions of Ka-b band waveg
that most m cutoff frequ
waveguide.
lue dashed l r waveguide f the pyram d absorbers
wn in
whos
Besides the se the leng rber, which nged dimens
e absorbers The dispers
re
he cutoff f ensions a, b d, mostly, hi he field distr gure 4.1 mensions a,
es (dashed l
e standard d gth is chan h is closer to
sion of abs are fixed at sion relation
frequency o , as shown igh-order m ributions of Field plot b. Electric f lines).
dimensions nged and st
o the actual sorber and t 10 degrees n in a wave
modes canno f which are
ω of TE10 m field are red
as shown in tretched to situation in waveguide ot appear in analogous t ma
cm πc ω 0 =
mode in a d vectors (so
n Fig. 4.2, w find the p n an anecho
and the ta n the waveg
to TE10 mod ma
a rectangula olid lines) a
we also sim performance
ic chamber.
aper angles
ar waveguid horter side o guide except
de [23].
ar wavegui and magneti
mulate absor e of a col r. The “L” is s of these t
de with in of waveguid t the modes ide with in ic field are b
wave e emitted to and knew t d. Now simi rber-1 [Fig.
A conspicu the values o er resistivity er absorption rst contact, w the frequenc gure 4.2
tled in Ka ationships o rallel polari mbination o
1 we have o infinite fla that what ki
ilar calculat . 4.2(a)].
uous dip ap of return los y. However n performan which resul cy fit for re The wedge a-band wav of the dire
zation and f (a) and (b
calculated t at surfaces
nd of mater tion will be
pears at abo ss at the pro r, theoretica nce is. The lts in some w eflection bac
e-shaped ab veguide. T ections of f
case (b) is ).
the reflectio of substanc rial is applic
done again
out ρ/ρcu =1 oximity of th ally the larg fact is that wave reflec ck and forth bsorbers (a) The differen fields and
perpendicu
on and tran ces over a w
cable to wa n through H
105, particul his dip are l ger the resi the absorbe cted. What i
h at the two )-(b) and p nces betwe the absorb ular polariz
nsmission c wide resistiv
ve absorber HFSS for the
larly in Fig less than the
istivity of a er cannot ab is more, the o wedge-sh yramidal ab een (a) an
ing surface ation. Case
coefficients vity range [ rs at microw e wedge-sha
g 4.3(a) and e materials absorber is,
bsorb all po e reflected w haped space absorber (c) nd (b) are
other
r word, reso go back to es a descri mplish abso
The lossy m prising grap
lubricity a t to form wa m) and the ρ frequencies
conductors gure 4.3 riety of mate
onance occu wave port iption of t orption.
material we phite and m and good ad
ave absorbe ρ/ρcu is abo
over 20 GH s, which me
The absorp erials at (a)
urs at the pe so the pow hat materia
e used as th ass isopropy dhesion to ers. The con
ut 3.48×106 Hz. Microw ans that the ption perfor
24, (b) 26 a
eripheral of wer measur
al with ab
he wave abs yl alcohol.
metal. We nductivity o
6. Neverthe waves penet
ere is more rmance of and (c) 28 G
f absorber an ed is less t out ρ/ρcu =
sorbers is a This colloid can smear of this mater less, it belo trate dielect power can b wedge-shap GHz.
nd the refle than it shou
=106 are go
kind of car dal graphite it on a ben rial is appro ongs to elect tric materia be absorbed ped absorbe
ected wave uld be. Fig.
ood enough
rbon compo e compound nded condu oximately 1
trical condu als more de
d by dielect er-1 made
but m
most dielec est electrica netic, ferroe ake wave ab
The Si
A figure of ), defined as
re Pin is th ctivity (S11
ative numbe hoic chamb cted. An an ch means on
gure 4.4 e "L" is the
ctric materi al conductiv electric and bsorbing ma
imulatio
f merit for t s a ratio in d
R
he incident ) of these r but minus ber is at le nechoic cham nly one of te
The diagra changed di
als cannot ve material d ferrite mat aterials [24]
n Result
the power lo decibels (dB
10 (dB)= RL
power and absorbers s sign is ofte east below mber with r en thousand
am of the w mensions in
sustain hig l to serve a aterials com
].
ts of the
oss because B) is measure en omitted.
-20 dB. O return loss o d of incident
wave and w n the simula
gh power. A s our wave mbined with
Three K
e of reflecti
log10
10 Γ
=
e reflected ed by return The require Only 1 perc of -40 dB is
t power is re
wedge-shape ation.
As a result absorbers.
some are c
Kinds of
on is called
Γ2
power, res n loss whic ement of re cent of inc considered eflected.
ed absorber
t, we select The dielec commonly u
Absorbe
d the return
spectively.
ch is alway eflectivity fo cident powe d to be exce
r in wavegu t the
absor versu as sh
Fig GH Fig GH
Figure 4.5 rber-1 [Fig.
us frequenc hown in Fig
gure 4.6 Hz microwav
gure 4.5 Hz microwav
and 4.6 r . 4.2(a)] and
y ranging f . 4.4 and the
The return ves. A retur The return ves. A retur
respectively d wedge-sha from 22 to 2 e ρ/ρcu of us
n loss of we rn loss of -1 n loss of we rn loss of -3
y show the aped absorb 28 GHz. Th
sed materia
edge-shaped 4 dB is equ edge-shaped 5 dB is equ
simulation ber-2 [Fig. 4 he "L" is the al is about 3
d absorber-2 ual to reflect d absorber-1 ual to reflect
n results of 4.2(b)], plot e length of w
.48×106.
2 [Fig. 4.2(b tivity of 4%
1 [Fig. 4.2(a tivity of 0.0
f wedge-sha ts of return wave absor
b)] for 22 to
%.
a)] for 22 to 03%.
aped loss rbers
o 28 o 28
First, there is a clear tendency for every curve toward less reflection (more absorption) when the frequency of incident wave is enhanced increasingly. This is well demonstrated by the following equation. The time-averaged power absorbed on the surface of a good conductor is
2
||
* H
] 4 H E 2Re[
1 × ⋅ = μωδ
−
= z
abs
da
dP e (4.4)
and by substituting Eq. (2.14) into Eq. (4.4) σ ω
μω ∝
= H||2
8 da
dPabs
(4.5) where H|| is the tangential magnetic field that exists just on the surface of the
conductor. The power absorbed is proportional to square root of frequency of incident wave and return loss is in the same way.
Equation (4.4) demonstrates a more significant point that the power loss per unit area also depends on the tangential magnetic field (H||), and this can explain the difference of absorption performance between these two models. The tangential magnetic field increases with a reduced cross-sectional area of space in waveguide when wave gets into the zone of absorber [Fig. 4.2(a)]. Therefore this leads to more power being absorbed. For satisfying boundary conditions of electromagnetic fields, the electric field going into absorber can produce a current near the surface of the conductor, which causes the same magnetic field to resist the entrance of the magnetic field outside the surface. However, the effect doesn’t appear in wedge-shaped absorber-2 though cross-sectional area also reduces
Secondarily, based on Fig. 4.1 and 4.2(a), the directions of TE10 mode electric field are closely parallel to the absorbing surfaces of wedge-shaped absorber-1 (parallel polarization). It is easier to form the electric force lines than the other case of wedge-shaped absorber, wedge-shaped absorber-2 (perpendicular polarization), for
whic superior per
rber-2 [Fig.
Hence, tha istent with llel polariza
The simulati in Fig. 4.7, e combinati omparison a rption perfo The tip of a
than that m nitude of th gure 4.7
crowaves.
e difficult t rformance o
. 4.6].
at the absor the theoret ation is alwa
ion results , identically ion of wedg among Fig.
ormance, w a pyramidal made by a lin he power r
The return
to do that. T of absorption
rption perfo ical calcula ays less than
of the third y, with diffe ge-shaped a
4.5-7, the p which can b
absorber is ne, the tip o eflected is n loss of pyr
This is one n in compa
ormance in ation in Sec n under perp
d model, py erent length absorber-1 a pyramidal a
e attributed s a dot and of a wedge-less when ramidal abs
reason wed rison with t
case (a) is c. 3.2, the r pendicular p
ramidal abs s. Geometri and absorbe absorber con d to the stru the impeda shaped abso
electromag sorber [Fig.
dge-shaped the result of
better than eflection co polarization
sorbers [Fig ically, a pyr er-2. But, a nspicuously uctural char ance made b
orber, whic gnetic wave
4.2(c)] for
d absorber-1 f wedge-sha
n in case (b oefficient u n.
g. 4.2(c)], c ramidal abs as a consequ y has the op
racteristic o by a dot is ch means th e contacts a r 22 to 28 G sorber uence ptimal on the much at the a dot.
GHz
Exce of absorbe owaves. Th lly used i tromagnetic
cially the w midal absor
Another po easing the l
decrease in itionally, th ch means th nite. And a eguide is sim
gure 4.8