Mechanically Steerable Antennas Using Dielectric Phase Shifters

Yasuo Kuga, Junho Cha, and James A. Ritcey

Department of Electrical Engineering
University of Washington, Box 352500
Seattle, WA 98195

Jim Kajiya
Microsoft Corporation
Redmond, WA 98052

Abstract
A mechanically steerable antenna was designed using an adjustable phase shifter which employs a
dielectric slab placed close to a coplanar TL. Numerical simulations using Ansoft HFSS were
conducted at 6 GHz, and a 4-element antenna was tested. A similar design can be used for a digital
phase shifter with a matched impedance at the designed frequency.

I. Introduction
A low-cost steerable antenna is one of the missing links of the future flexible wireless
communication systems. For example, the most flexible satellite to ground/airplane
communication systems are based on the phased-array antenna technology. Unfortunately, the
cost of phased array antennas is related to the number of active elements, and the present systems
are too expensive for many commercial/military applications. The antenna beam steering can also
be done by mechanically moving the reflector. Although the mechanically steerable antennas can
be inexpensive, current antennas which use the eletro-mechanical actuators are usually bulky and
prone to mechanical failure.

In this paper, we show that a movable dielectric slab placed close to a coplanar waveguide (CPW)
can be used as a phase shifter. The effective dielectric constant is calculated as a function of slab
height and the characteristics of the basic 4-element array antenna (shown in Fig. 1) is simulated.

Patch Antennas

Microstrip TL Delay lines

Coplanar TL Phase Shifters

Zo=95 ohm

Microstrip TL 8/4 transformer

Zo=35 ohm

Zo=50 ohm

Input
Fig.1: Block diagram of a 4-element steerable array antenna.
II. Phase Shifter Based on a Movable Dielectric Slab
The basic concept is shown in Fig. 2. The movable high-dielectric constant slab is inserted into the
gap of CPW. The effective dielectric constant will be a function of d for a given structure. In our
design, CPW for d=5mm (effectively d = ∞ ) has the characteristic impedance Z o = 98 Ω and the
effective dielectric constant of εeffective=1.27. As the distance d decreases, εeffective increases and Zo
decreases. The HFSS simulations were conducted for height d from d=0 to d=5 mm. The effective
dielectric constant was estimated from S21 data.

A. Analysis
The effective dielectric constant can be calculated using the transmission coefficient (T) of a
layered structure where Γ1 is the reflection coefficient at the boundary and θ is the phase shift due
to a change in the effective dielectric constant. Assuming that the reflection is small ( Γ1 <0.1),
2

we can approximate T as

T=
(1 − Γ ) e 2
1
− jθ

≈ e − jθ (1)
2 −2 jθ
1− Γ e 1
The phase change at the slab height d is with respect to that without a dielectric slab ( d = ∞ ), and
we can express it as
θ d = k o Ld ( ε eff _ d − ε eff _ d =∞ ) (2)

where k0 is wavenumber in free space, Ld is the slab length, ε eff _ d =∞ represents the effective
dielectric constant when the dielectric material is far enough away from the substrate, and ε eff _ d
represents the effective dielectric constant at the slab height is d.

εr εr
εr

(a) (b) (c)

Fig. 2: Phase Shifting Method. 1 (a): frontal view of a ground-signal-ground (G-S-G) CPW when
the dielectric material is attached to the substrate. (b): frontal view of G-S-G CPW when the
dielectric material is far enough away from the substrate. (c): top view of G-S-G CPW. The width
of signal trace is 2 mm and the width of ground trace is 10 mm. The gap between the ground and
the signal is 1 mm. Substrate thickness is 1.6 mm and the height of dielectric material is 5 mm.
The length of the dielectric material is 10 mm, the dielectric material is alumina ε r = 10 , and the
substrate has εsub = 2.2 (Duroid 5880).

B. Numerical Results
We conducted preliminary numerical simulations of two different configurations shown in Fig. 2.
A G-S-G TL with the center gap was found to provide a reasonable amount of phase shift with
ε r = 10 . Table 1 presents numerical results for the G-S-G TL with the center gap for cases with
and without the dielectric material (alumina).

As Table 1 demonstrates, the characteristic impedance and the effective dielectric constant are
changed depending on the distance of the dielectric material from the substrate. Fig. 3 shows the
effective dielectric constant as a function of distance d. Beyond d>2 mm, the slab is totally out of
CPW and there is no change in εeffective. The effective dielectric constant gradually changes from
d=2 mm to d=0.25 mm and then increases rapidly at d=0 mm.
 Parameters d=5 d=0
|S21| 0.99 0.96
Characteristic impedance ( Ω ) 98 51
Effective dielectric constant 1.23 7.75

Table 1: Simulated results at d=5 mm and d=0 mm.

Effective dielectric constant ( e e)

10

6
ee

0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
distance( mm ) from the substrate

Fig. 3: Effective dielectric constant as a factor of distance from the substrate.

III. Design and Fabrication of Phase Shifter

Fig. 4 shows the layout of the test phase shift circuit. The CPW with gap ( Z o = 98 Ω ) is matched to
the microstrip TL with Z o = 50 Ω for testing purposes. The dielectric slab will be machined using
alumina. The desired amount of phase shift can be obtained by adjusting the slab length Ld and
slab height d. Fig. 5 shows the radiation patterns for two dielectric slab positions based on the
antenna configuration shown in Fig. 1. Although we have an impedance mismatch at the phase
shifter, the adverse effects on the radiation patterns are not significant.

Fig. 4: A top view of the test circuit layout which includes the CPW and microstrip TL. This
circuit has a 360 degree phase shift at d=0 mm. The bottom layer of the CPW section does not
have a ground plane and via is used for connecting the ground.

Fig. 5: Radiation patterns of a 4-element array antenna shown in Fig. 1. The initial phase is
shifted to create -90o phase difference at d=0 mm (left figure). At d=5 mm, the phase difference
becomes +90o (right figure).
IV. Impedance Mismatch and Possible Solutions
If we change the slab height continuously, we can adjust the phase shift. Unfortunately, this also
changes the characteristic impedance of the CPW section and introduces reflection. However, we
can eliminate the impedance mismatch problem by using two positions ( d = 0 and d = ∞ ). To
minimize reflection when the material is added to TL, we will set the length of the modified
section to be λ/2 (or mλ/2 where m is an integer). Suppose we want a 3-bit phase shifter as shown
below. The phase shift is given by 8 states (0, 45, 90, 135, 180, 225, 270, and 315 degrees).
l1 l2 l3

Z o, nb Z 1, n 1 Z 2, n2 Z 3, n 3 Z o, n b

The non-enhanced section is given by Zo and nb. The center sections will be enhanced, and they
are given by (Z1, n1), (Z2, n2) and (Z3, n3) where Z and n are characteristic impedance and the
index of refraction of each section. We want to create impedance matching for all states. This can
be done by satisfying the following conditions:
Phase shift requirements:
Section 1: ( n1 − nb ) β b l1 = π / 4 (45o, bit 0)
Section 2: ( n2 − nb ) β b l2 = π / 2 (90 o, bit 1)
Section 3: (n3 − nb ) β b l 3 = π (180 o, bit 2)

Matched impedance requirements: m1, m2, and m3 are integers.

n1 β b l1 = m1π , n 2 β b l1 = m2π , and n3 β b l1 = m3π
Then, if we use n1 = n2 = n3 = 4 / 3 nb , m1=1, m2=2, and m3=4, we find
For π/4 section: l1 = 3/ 8( λo / nb )
For π/2 section: l2 = 3/ 4( λo / nb )
For π section: l3 = 3/ 2( λo / nb )
If we satisfy these conditions, the reflection due to dielectric slabs can be eliminated. The details
of this approach will be discussed in another paper [1].

V. Conclusions
A novel design was proposed for a low-cost mechanically steerable array antenna. The phase
shifter is based on a movable dielectric slab placed close to CPW. The impedance mismatch can
be avoided by choosing the slab dielectric constant and length carefully. A mechanical actuator is
required to move the dielectric slab in our configuration. One idea is the use of an electro-active
polymer (EAP) to move a small slab which may be suited to high MMW frequencies [2].

References
1. Y. Kuga, J-H Cha, A. Ishimaru, and S-I Lee, "Mechanically controllable microwave phase
shifter," to be submitted to IEEE Trans. Antenna and Propagation.
2. M. Le Guilly, C. Xu, V. Cheng, M. Taya, L. Opperman, and Y. Kuga, "Flemion based
actuator for mechanically controlled microwave switch," SPIE Meeting, Smart Structure and
Materials, San Diego, CA, March 2-6, 2003.