20-08-2012, 04:04 PM
Coupling to DRAs
[attachment=32207]
Chapter 2 examined the basic DRAs and presented design equations for predicting
the resonant frequency and radiation Q-factor for the commonly used lower order
modes. The models for deriving these equations assumed the DRAs were in
isolation or mounted on an infinite perfect conducting ground plane and did not
account for the feeding mechanisms used to excite the DRAs. The selection of the
feed and that of its location both play an important role in determining which
modes are excited. This, in turn, will determine the input impedance and radiation
characteristics of the DRAs. The coupling mechanism can also have a significant
impact on the resonant frequency and Q-factor, which the previous equations fail
to predict. This chapter begins with a brief review of coupling theory and an
examination of the internal fields within rectangular and cylindrical DRAs. A
knowledge of the internal field configuration is essential for understanding how
the various feeds can excite different modes within the DRA. The more common
feeds are then surveyed and examples provided to highlight practical design
considerations.
COUPLING COEFFICIENTS
For most practical applications, power must be coupled into or out of the DRA
through one or more ports. (One notable exception is the DRA reflectarray, which
will be discussed in Chapter 9.) The type of port used and the location of the port
with respect to the DRA will determine which mode will be excited and how much
power will be coupled between the port and the antenna.
APERTURE COUPLING
One common method of exciting a DRA is through an aperture in the ground
plane upon which the DRA is placed. Figure 3.7 shows some of the aperture
shapes that have been used for exciting DRAs. The small rectangular slot is
probably the most widely used aperture [7-14]. By keeping the slot dimensions
electrically small, the amount of radiation spilling beneath the ground plane can be
minimized. Annular slots have also been used for exciting cylindrical DRAs [15],
while cross-shaped and C-shaped slots are used to excite circular polarization [16-
18]. The aperture can itself be fed by a transmission line (either microstrip or
coaxial) or a waveguide [19, 20], as shown in Figure 3.7. Aperture coupling offers
the advantage of having the feed network located below the ground plane,
isolating the radiating aperture from any unwanted coupling or spurious radiation
from the feed.
PROBE COUPLING
Another common method for coupling to DRAs is with a probe, as shown in
Figure 3.17 [22-28]. The probe usually consists of the center pin of a coaxial
transmission line that extends through the ground plane. The center pin can also be
soldered to a flat metal strip, that is placed adjacent to the DRA [29], whose length
and width can be adjusted to improve the impedance match. Instead of a coaxial
line, the flat metal strip can also be connected to a microstrip line [30-32]. For
coupling purposes, the probe can be considered as a vertical electric current, as
shown in Figure 3.17 and, from (3.1), it should be located in a region of the DRA
having high electric fields to achieve strong coupling.
MICROSTRIP LINE COUPLING
A common method for coupling to dielectric resonators in microwave circuits is
by proximity coupling to microstrip lines. Figure 3.22 shows this feeding
technique applied to DRAs [36]. Microstrip coupling can be used to excite the
TEx
δ11 mode of the rectangular DRA or the HE11δ mode of the cylindrical DRA, as
shown in Figure 3.23. This sketches the magnetic fields in the DRA and the
equivalent short horizontal magnetic dipole mode.
The Multisegment DRA
As mentioned in Section 3.5, to achieve strong coupling between the microstrip
line feed and the DRA, the dielectric constant of the DRA needs to be relatively
high (usually ε
r > 20). Since the radiation Q-factor is proportional to the dielectric
constant, the bandwidth of these DRAs is typically narrow. For wider-band
applications, DRAs with lower values of dielectric constant are required, but only
a small amount of coupling is achievable between the microstrip line and the
DRA, resulting in poor radiation efficiency. One solution to overcoming the weak
coupling of DRAs with lower dielectric constants is the multisegment DRA [39-
40]. The multisegment dielectric resonator antenna (MSDRA) consists of a
rectangular DRA of low permittivity under which one or more thin segments of
different dielectric constant substrates are inserted, as shown in the exploded view
of Figure 3.24. The inserts serve to transform the impedance of the DRA to that of
the microstrip line by concentrating the fields underneath the DRA; this
significantly improves the coupling performance. In general, more than one insert
can be added to obtain the required impedance match, but to reduce the
complexity of the fabrication process and ultimately the cost, it is desirable to use
only a single insert, as shown in Figure 3.25.
[attachment=32207]
Chapter 2 examined the basic DRAs and presented design equations for predicting
the resonant frequency and radiation Q-factor for the commonly used lower order
modes. The models for deriving these equations assumed the DRAs were in
isolation or mounted on an infinite perfect conducting ground plane and did not
account for the feeding mechanisms used to excite the DRAs. The selection of the
feed and that of its location both play an important role in determining which
modes are excited. This, in turn, will determine the input impedance and radiation
characteristics of the DRAs. The coupling mechanism can also have a significant
impact on the resonant frequency and Q-factor, which the previous equations fail
to predict. This chapter begins with a brief review of coupling theory and an
examination of the internal fields within rectangular and cylindrical DRAs. A
knowledge of the internal field configuration is essential for understanding how
the various feeds can excite different modes within the DRA. The more common
feeds are then surveyed and examples provided to highlight practical design
considerations.
COUPLING COEFFICIENTS
For most practical applications, power must be coupled into or out of the DRA
through one or more ports. (One notable exception is the DRA reflectarray, which
will be discussed in Chapter 9.) The type of port used and the location of the port
with respect to the DRA will determine which mode will be excited and how much
power will be coupled between the port and the antenna.
APERTURE COUPLING
One common method of exciting a DRA is through an aperture in the ground
plane upon which the DRA is placed. Figure 3.7 shows some of the aperture
shapes that have been used for exciting DRAs. The small rectangular slot is
probably the most widely used aperture [7-14]. By keeping the slot dimensions
electrically small, the amount of radiation spilling beneath the ground plane can be
minimized. Annular slots have also been used for exciting cylindrical DRAs [15],
while cross-shaped and C-shaped slots are used to excite circular polarization [16-
18]. The aperture can itself be fed by a transmission line (either microstrip or
coaxial) or a waveguide [19, 20], as shown in Figure 3.7. Aperture coupling offers
the advantage of having the feed network located below the ground plane,
isolating the radiating aperture from any unwanted coupling or spurious radiation
from the feed.
PROBE COUPLING
Another common method for coupling to DRAs is with a probe, as shown in
Figure 3.17 [22-28]. The probe usually consists of the center pin of a coaxial
transmission line that extends through the ground plane. The center pin can also be
soldered to a flat metal strip, that is placed adjacent to the DRA [29], whose length
and width can be adjusted to improve the impedance match. Instead of a coaxial
line, the flat metal strip can also be connected to a microstrip line [30-32]. For
coupling purposes, the probe can be considered as a vertical electric current, as
shown in Figure 3.17 and, from (3.1), it should be located in a region of the DRA
having high electric fields to achieve strong coupling.
MICROSTRIP LINE COUPLING
A common method for coupling to dielectric resonators in microwave circuits is
by proximity coupling to microstrip lines. Figure 3.22 shows this feeding
technique applied to DRAs [36]. Microstrip coupling can be used to excite the
TEx
δ11 mode of the rectangular DRA or the HE11δ mode of the cylindrical DRA, as
shown in Figure 3.23. This sketches the magnetic fields in the DRA and the
equivalent short horizontal magnetic dipole mode.
The Multisegment DRA
As mentioned in Section 3.5, to achieve strong coupling between the microstrip
line feed and the DRA, the dielectric constant of the DRA needs to be relatively
high (usually ε
r > 20). Since the radiation Q-factor is proportional to the dielectric
constant, the bandwidth of these DRAs is typically narrow. For wider-band
applications, DRAs with lower values of dielectric constant are required, but only
a small amount of coupling is achievable between the microstrip line and the
DRA, resulting in poor radiation efficiency. One solution to overcoming the weak
coupling of DRAs with lower dielectric constants is the multisegment DRA [39-
40]. The multisegment dielectric resonator antenna (MSDRA) consists of a
rectangular DRA of low permittivity under which one or more thin segments of
different dielectric constant substrates are inserted, as shown in the exploded view
of Figure 3.24. The inserts serve to transform the impedance of the DRA to that of
the microstrip line by concentrating the fields underneath the DRA; this
significantly improves the coupling performance. In general, more than one insert
can be added to obtain the required impedance match, but to reduce the
complexity of the fabrication process and ultimately the cost, it is desirable to use
only a single insert, as shown in Figure 3.25.