Leccture--2
5.3 Exccitation tecchniques:
Microstrip
p antennas haave radiating elements on one side of a dielectric suubstrate, and thus the RF ppower
may be feed directly to the
t radiating patch using a connecting eelement such as microstripp line or
coaxial prrobe. Matchin
ng is usually
y required bettween the fedd line and thee antenna, beecause the anntenna
input imp
pedance diffeers from the customary 50 ohm line impedance. M
Matching maay be achieveed by
properly selecting
s
the location
l
of the feed line.
In an ano
other approach
h the microsttrip antenna can be exciteed by couplinng the electroomagnetic waave to
transfer power
p
betweeen the micrrostrip line an
nd the radiatting patch, w
without actuaally contactinng the
radiator. Such
S
techniqu
ues are the ap
perture coupliing and proxiimity couplingg techniques.
5.3.1 Miicrostrip Line
L
Feed
In this typ
pe of feeding
g technique, a conducting strip is connnected directlly to the edgee of the micrrostrip
patch as shown
s
in Figu
ure 5.5. The conducting
c
sttrip is smallerr in width as compared too the patch annd this
kind of feeed arrangemeent has the ad
dvantage that the feed can be etched on the same subbstrate to provvide a
planar stru
ucture.
Fig 5.5 Microstrip
M
Linee Feed
For imped
dance matchiing between the
t antenna input
i
point an
and feeding liine, sometimees a cut is maade at
the feedin
ng edge of thee antenna (see Fig. 5.9 a). The purposee of the inset cut in the patch is to matcch the
impedancce of the feed
d line to the patch
p
withou
ut the need foor any additioonal matchinng element. T
This is
achieved by properly controlling the inset possition. Hencee this is an easy feedingg scheme, sinnce it
e
of fabriccation and sim
mplicity in modelling as w
well as impedance matchinng. However as the
provides ease
thickness of the dielectric substrate being used, increases, surrface waves aand spurious feed radiationn also
increases,, which hamp
pers the bandw
width of the antenna. Thee feed radiatioon also leads to undesired cross
polarized radiation.
5.3.2 Co
oaxial Feed
d
The Coax
xial feed or prrobe feed is a very common technique used for feeding microstrrip patch anteennas.
As shown
n in Figure 5.6
6, the inner co
onductor of th
he coaxial coonnector extennds through thhe dielectric aand is
soldered to
t the radiatin
ng patch, whille the outer co
onductor is coonnected to thhe ground plaane.
Fig 5.6 Co
oaxial Fee Probe fed Rectaangular Micro
ostrip Patch A
Antenna
The main advantage of this type off feeding scheeme is that thhe feed can bee placed at anny desired loccation
on the pattch in order to
o match with its input impedance. This feed method is easy to fabbricate and haas low
spurious radiation.
r
However, its maajor disadvan
ntage is that itt provides naarrow bandwidth and is diffficult
to model since a hole has to be drrilled in the substrate
s
and the connectoor protrudes ooutside the ground
us not making
g it completely
y planar for th
hick substratees ( h > 0.02λλ0 ).
plane, thu
5.3.3 Ap
perture Coupled Feed
d
In this typ
pe of feeding
g technique, the
t radiating patch and thhe microstrip feed line aree separated bby the
ground pllane as shown
n in Figure 5.7
7. Coupling between
b
the ppatch and the feed line is m
made through a slot
or an aperrture in the grround plane.
Fig 5.7: Aperture-coup
A
pled Feed
The coup
pling aperturee is usually centred
c
under the patch, leading to loower cross ppolarization ddue to
symmetry
y of the configuration. Thee amount of coupling
c
from
m the feed linne to the patchh is determinned by
the shape, size and loccation of the aperture.
a
Sincce the groundd plane separaates the patchh and the feedd line,
spurious radiation
r
is minimimum.
m
Generally, a high dielectrric material iss used for the bottom subbstrate
and a thicck, low dielecctric constantt material is used
u
for the ttop substrate to optimize radiation from the
patch. Th
he major disaadvantage of this feed tech
hnique is thaat it is difficuult to fabricaate due to muultiple
layers, wh
hich also increeases the anteenna thicknesss. This feedinng scheme alsso provides narrow bandw
width.
5.3.4 Prroximity Co
oupled Feeed
This type of feed tech
hnique is also
o called as thee electromagnnetic couplinng scheme. A
As shown in F
Figure
bstrates are used
u
such th
hat the feed lline is betweeen the two substrates annd the
5.8, two dielectric sub
pper substrate. The main advantage oof this feed teechnique is tthat it
radiating patch is on top of the up
a provides very high ban
andwidth (as hhigh as 13%)), due to an ooverall
eliminatess spurious feeed radiation and
increase in
n the thickness of the micrrostrip patch antenna. Thiss scheme alsoo provides chhoices betweeen two
different dielectric media, one for the patch and one forr the feed liine to optim
mize the indivvidual
nces.
performan
Fig 5.8: Proximity-cou
P
upled Feed
Matching can be achieeved by conttrolling the leength of the feed line andd the width-tto-line ratio oof the
he major disad
dvantage of this
t
feed scheeme is that itt is difficult tto fabricate bbecause of thhe two
patch. Th
dielectric layers which
h need properr alignment. Also,
A
there iss an increasee in the overaall thickness oof the
antenna.
5.4 Critteria for Substrate
S
Selection:
S
While ch
hoosing substtrates for fab
bricating miccrostrip antennnas, followiing characterristics have to be
focussed on:
o
1) Surfacee-wave excitaation
2) Disperssion of the dieelectric consttant and loss tangent
t
of thee substrate
3) Copper loss
4) Anisotropy in the substrate
5) Effects of temperature, humidity, and aging
6) Mechanical requirements: conformability, machinability, solderability, weight, elasticity, etc.
7) Cost
The first 3 factors are of special concern in the millimeter-wave range (f> 30 GHz).
ELECTRICAL PROPERTIES OF COMMONLY USED SUBSTRATE MATERIALS FOR
MICROSTRIP ANTENNAS
Material
Dielectric
Constant
Unreinforced PTFE, Cuflon
2.1
Reinforced PTFE, RT Duroid 5880 2.20 (1.5%)
Fused Quartz
3.78
96% Alumina
9.40 (5%)
99.5% Alumina
9.80 (5%)
Sapphire
9.4, 1.6
Semi-Insulating GaAs
12.9
Loss
Tangent
0.0004
0.0009
0.0001
0.0010
0.0001
0.0001
0.0020
5.5 Radiation mechanism of microstrip antenna:
Radiation from microstrip antenna can be understood by considering the simple case of a rectangular
microstrip patch spaced a small fraction of a wavelength above the ground plane as shown in the
fig.5.9(a). Assuming no variation of the electric field along the width and the thickness of the microstrip
structure, the electric field configuration of the radiator can be represented as shown in the fig 5.9(b). The
field varies along the patch length which is about half a wavelength. Radiation may be ascribed mostly to
the fringing fields at the open circuited edge of the patch. The field at the end can be resolved into the
normal and tangential component with respect to the ground plane. The normal components are out of
phase because the patch line is λ/2 long. Therefore the far field produced by them cancel in the broadside
direction. The tangential components which are parallel to the ground plane are in phase and the resulting
fields combine to give maximum radiated field normal to the surface of the structure. Therefore the patch
may be represented by two slots λ/2 apart excited in phase and radiating in half space above the ground
plane.
Fig.5.9(a)) Rectangular microstrip paatch antenna
Fig.5.9(b)) Side view