An Electronically Tunable Reflectarray
Using Varactor Diode-Tuned Elements
Sean V. Hum*tt Michal Okoniewskit
tTRLabs tDept. of Electrical and Computer Engineering
Calgary, AB, Canada, T2L 2K7 University of Calgary, Calgary, AB, Canada T2N IN4
This paper proposes a reconfigurable reflectarray antenna using electronically-tunable el-
ements. The design of the unit cell is presented along with simulated and measured results
confirming a large degree of phase agility. A fixed-angle reflectarray based on the proposed
unit cell is also presented and assessed both theoretically and experimentally. The goal of this
research is to construct and demonstrate a large electronically reconfigurable reflectmay that
provides dynamic beam control for multi-beam applications.
In recent years, the reflectarray antenna [l] has evolved into an attractive candidate for appli-
cations requiring high-gain, low-profile reflector antennas. A basic reflectarray antenna consists
of a planar array of microstrip elements illuminated by a spatial feed, as shown in Figure 1.
Phasing of the scattered field to form the desired beam pattem is achieved by modifying the
characteristics of the individual elements composing the array. The most popular approaches
include varying the size of the elements , loading patches with variable-length stubs 131, and
varying the rotational angle of circularly-polarized stub-loaded microstrip patches . These
approaches have met with much success in the design of fixed reflectarrays. However, the one
untapped capability of the reflectanay is the possibility of making the beam pattem dynamic
using reconfigurable elements. This requires elements whose scattered field phase can be actively
adjusted over a broad range. Only a handful of such elements have been proposed. In , the
authors proposed that element could he rotated using miniature motom to produce the required
phase shift. In , the phase shift was produced through movement of a dielectric rod beneath
a dipole. Most recently, a promising approach using electronic control of the element has been
demonstrated whereby the radiating edge of a patch antenna was loaded with a varactor diode .
Unfortunately only a 180" phase range was achieved with this element, allowing for only limited
beam scanning and constraining its application to small reflectarrays.
11. PROPOSED REFLECTARRAY
Electronically tunable reflectarray elements borrow concepts from frequency agile antennas,
which were traditionally devised to allow a patch's input match frequency to be adjusted using
varactor diodes . Fixed reflectarray elements utilize changes in the electrical length of the
patch element to change the phase of the scattered field. When designing phase-agile reflectarray
elements, the patch must be chosen so that for a given size, the maximum range of phase angles is
achieved over the range of capacitances offered by the varactor diode. The choice of the patch
size also depends on the how the varactor diode is physically integrated with the patch. For
patches employing shunt-connected varactor diodes along the radiating edge, selecting a patch
that is too small will cause the phase range to be limited by the diode's maximum capacitance,
or conversely by the diode's minimum capacitance if the patch is chosen to be too large.
For the proposed design, a phase-agile reflectarray element was designed using two varactor
diodes to connect two halves of a patch as shown in Figure 2. The use of a serial connection
simplified the design since no vias were required, and for the patch size chosen, commercially
available packaged devices could be used. The operating frequency was chosen to be 5.5 GHz,
0-7803-8302-8/04/$20.00 02004 IEEE
and the element designed on a 1.524 mm substrate with E? = 3.02 and t a n 6 = 0.0016 using
the following dimensions: W = 14 mm, L = 19 mm, and g = 1 mm. An MCE Metelics
MGV-100-20 varactor diode was chosen for the design, which develops a capacitance of 1.80-
0.12 pF between 0-20 V of reverse bias voltage. For validation purposes the unit cell was also
assessed by replacing the varactor diode with various fixed packaged capacitors covering the
diode's tuning range.
The reflectarray element was designed using a custom in-house finite-difference time-domain
(FDTD) code. Equivalent circuit models of the capacitors and varactor diodes were used in
the simulations. To determine the scattered field from the array element in the simulation, it
was placed at the end of an ideal parallel-plate waveguide (PPWG) with the element polarized
with incident field. This technique can be used to simulate normal incidence on the patch in
an infinite-array scenario when the dimensions of the PPWG are chosen to be equal to that of
unit cell in the array [ 8 ] . By extracting the E M reflection coefficient from the structure, the
amplitude and phase of the scattered field from the patch can be determined.
To experimentally measure the scattered field from the element, a different setup was used, since
the simulated PPWG could not be practically realized. The element was placed at the end of
a section of rectangular waveguide (RWG) as shown in Figure 3. The TElo mode simulates
illumination at an angle of einc = sin-'($) radians from broadside in the H-plane, where k
is the wavenumber in the waveguide. Additionally, the waveguide approximates a semi-infinite
array where the elements are separated by a free-space distance of b due to image theory. WR-
187 waveguide (a= 47.55 mm, b = 22.15 mm) was used, which provides ern= 35" at 5.5 GHz.
While WR-187 does not simulate the element spacing of 0.55X0 = 30 mm used in the final
array, it is sufficient for comparing the scattering properties of the patch element with simulation
Graphs comparing the scattering behaviour of the unit cell utilizing fixed capacitors are shown
in Figure 4(a). The results compare FDTD simulations inside parallel-plate waveguide and
rectangular waveguide, and experimental results in the rectangular waveguide. There is good
correlation between all results. The minor deviation between the simulated RWG and PPWG
results is expected since the WR-187 approximates a much closer element spacing than that
used in the PPWG simulations. There is only a slight change in the scattered field amplitude
as the tuning capacitance is varied due to the low amount of loss in the capacitors. Measured
results for the varactor diode-based cell are shown in Figure 4(b). An excellent tuning range of
303" was observed for the unit cell at 5.5 GHz. This range was slightly lower than expected
(335") due to higher than anticipated inductance in the diode package, electrically lengthening
the patch. This effect can be compensated for by reducing the patch size. At 5.25 GHz, 328"
of phase range was achieved, and the phase remained within f22.5' of the centre of the tuning
range over a 50 MHz (1%) band.
As shown in the lower plot, the varactor diode-based cell suffered from considerable amplitude
fluctuations about the centre of the tuning range. This was caused by the significant series
resistance of the varactor diode. The effect is more severe the closer the frequency is to the
resonant frequency of the patch, suggesting that significant power dissipation in the varactor
diode is occurring at this point. This effect can be addressed by using lower resistance diodes,
or perhaps through adjusting the position of the varactor diodes away from the current maximum
of the patch. As an experiment, additional diodes were added in parallel with the existing diodes
to reduce their effective series resistance. The scattered amplitude was improved by more than
4.3 dB at 5 GHz, with the amount of improvement declining at higher frequencies.
BASEDON PROPOSED UNIT CELL
As a first step toward constructing and testing an electronically tunable reflectarray, a 10 x 7 array
was constructed from the unit cells employing fixed capacitors. The array was oriented according
to the spherical coordinate system shown in Figure 1. The array was designed at 5.5 GHz with
an element spacing of d = 30 nun. Array theory was used to determine the required phasing
of the array, and the required element phases discretized according to the available capacitor
values. The effect of amplitude variations between elements was also included in the simulation.
To confirm the proper operation of the array without a close proximity feed hom, the performance
of the array was studied when it was illuminated from a large distance. This is not the normal
manner in which reflectarrays are fed but provided the best insight into the performance of
the array. The array’s elements were chosen such that the array would reflect a plane wave
incident broadside to the array in the direction 6 = log”, 9 = -24”. Illuminating the array in
an anechoic chamber confirmed that the reflection was within 2’ of the expected illumination
angle when the array was illuminated at broadside. The slight beam pointing error is attributable
to the tolerance of the capacitors used to realize the elements.
A more detailed analysis was performed by scanning the array and examining its monostatic
radar cross section (RCS) in the anechoic chamber. The maximum RCS was found at 0 =
100.5”, 4 = -12.0°, where the antenna essential acts as a retrodirective reflector. This compares
favourably with the expected values of 6 = 99.O0,@= -10.5”. Plots of the measured and
theoretical monostatic RCS for two different scans of the array are shown in Figure 5. The
overall correlation between the simulated and measured results is very good. Discrepancies,
most apparent in the elevation scan, were likely caused by reflections off of a large boom that
was used to support the antenna. The boom’s cross-section changed most significantly during
elevation angle scans, leading to large errors when the array was scanned more than 30” from
The maximum monostatic RCS of the array was compared with the measured monostatic RCS
of a rectangular metallic plate of the same dimensions as the array. The maximum RCS of the
array was found to be approximately 1.7 dB lower than that of the plate. More than 0.6 dB of
this is directly attributable to the element factor associated with the microstrip patch. Simulations
of the array also demonstrated an additional 0.7 dB drop in peak monostatic RCS when element
amplitude variation was introduced. Hence, the loss is within theoretical expectations.
Iv. CONCLUSIONS A N D FUTURE
Experiments with the reflectarray and its components thus far are very encouraging. A phase-
agile reflectamay element has been developed that meets the requirements for an electronically
tunable reflectarray. Experiments with a reflectarray employing fixed versions of the element
further confirm that the element topology is sound. There are immediate plans to replace the
unit cells with their tunable equivalents, and to add a proximal feed hom to the array, We are
very excited about the work ahead on what promises to be a highly versatility antenna platform.
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Fig. 1. Generic reflectarray Fig. 2. Proposed reflectarray element Fig. 3. RWG measurement setup
2., . .
... . . . ...
(a) Fixed capacitors
Fig. 4. Simulated and measured scallering properties of the reflectarray element