Download design and simulation of 505.8 MHz strip line directional

DESIGN AND SIMULATION OF 505.8 MHz STRIP LINE DIRECTIONAL
COUPLER*
M. Ahlawat# and R.S. Shinde, RRCAT Indore, INDIA
Abstract
A strip line directional coupler is designed and
simulated using finite difference time domain technique
(FDTD), for low insertion loss and high directivity of 40
dB, optimized up to 10 kW RF power at 505.8 MHz. This
will be be used for circulator testing, and other high
frequency applications for reflection measurement in
transmission lines.
INTRODUCTION
RF power reflected back from RF cavity is attenuated
using circulator (situated between RF source and cavity),
which may otherwise damage the RF source (or
amplifier). By means of this device, reflected RF power is
circulated to high power matched termination, and hence
protecting the RF source. For characterization of
circulator, which has been developed & is further being
improved for higher powers at Ferrite Lab, AMTD,
RRCAT, a high directivity (better than reflection from
circulator) coupler is required. It is also important for
taking proper safety measures for attenuation of any
reflection coming from the circulator itself. To meet the
requirement of circulator testing and mismatch
measurement transmission lines, a narrowband stripline
dual directional coupler with high directivity by utilizing
phase superimposition technique, has been designed.
THEORY AND DESIGN
Directional coupler, in general is a passive RF device
which couples a fraction of RF power flowing through its
main line in one direction only[6]. In dual directional
coupler power flowing in one direction is coupled to one
port (of the auxiliary line), and in opposite direction to
another port. Stripline, microstrip and waveguide
technologies are used depending on frequency of
application and RF power level. Considering its size
constraints, strip line geometry has been implemented for
Indus 2 RF frequency of 505.8 MHz. Parallel line coupled
stripline directional couplers are wideband type and have
low directivity[1][5]. To meet the requirement of the high
directivity, phase superimposition technique has been
implemented for constructive and destructive interference
at coupled and at isolated port respectively.
First of all stripline geometry including stripline
conductor width and thickness, dielectric medium and its
thickness are optimized to achieve the transmission line
system of 50  characteristic impedance, which is given
by equation Eq-1[2]. Characteristic impedance is
proportional to spacing between ground planes, and
*Work supported by RRCAT (DAE) Indore
#[email protected]
inversely proportional to strip line conductor width &
square root of dielectric constant of the medium.
Z0 =
30𝜋 𝑤𝑒
(
√𝜀𝑟 𝑏
−1
+ 0.441)
where
𝑤𝑒
𝑏
=
𝑤
𝑏
0
-{
𝑤 2
(0.35 − )
𝑏
𝑓𝑜𝑟
𝑓𝑜𝑟
𝑤
𝑏
𝑤
𝑏
> 0.35
< 0.35
}
(1)
where ε𝑟 relative permittivity, 𝑤𝑒 effective strip line
conductor width, w width of strip line conductor, b
spacing between two ground planes.
Dielectric thickness in combination with strip line
width and thickness plays an important role in impedance
matching and determining the breakdown voltage, peak
power & the power handling capacity of the directional
coupler. In addition to optimizing these parameters,
Impedance matching techniques like quarter wave (λ/4)
transformer are used, if heavy mismatch between systems
characteristic (around 50 Ω) impedance and typical
characteristic impedance of the strip line structure, which
may be case at very high powers. Dielectric thickness
may be allowed to be within a range of values, as strip
line conductor width and thickness can be adjusted for
achieving impedance matching. Increased dielectric
medium thickness gives higher peak and average power
handling capability but the reduced equivalent
transmission line capacitance and hence higher the
characteristic impedance (as characteristic impedance
inversely proportional to the square root of capacitance,
Z0 = √𝐿⁄𝐶 ). Increased the strip line conductor width
reduces the inductance per unit length due to increased
magnetic field path length, and increases capacitance pet
unit length which results in the reduced characteristic
impedance. Increased strip line conductor width and
thickness provides the higher average power handling
capacity.
Secondly and most critical, it is the schematic design
for the phase superimposition of the two RF components
coupled from the main line. It is having 3 types of lines main line, auxiliary line and coupling lines. Distance of
open end half guide wave length λg/2 coupling lines from
the main line, and the dielectric thickness determines the
amount of RF coupling. Constructive and destructive
interferences are decided by length of the path travelled
by coupled RF signal before superimposition at the
auxiliary line. However design is based on the stripline
geometry, but works like wave guide directional coupler.
RF power is coupled to the coupling lines only at two
points separated by half the guide wave length[4], rather
than along whole the line length as in the side-coupled
strip line couplers. Length of the coupling lines as in fig-1
is also taken as half the guide wavelength g/2, so that it
will not affect the auxiliary line impedance. Analysis is
done by the phase calculations as in waveguide
directional coupler, and not the "even and odd coupling
mode" method used for stripline couplers[3]. In this way,
by considering main power from port 1 to port 2, coupled
RF signals are in same phase in forward direction to port
4 (coupled), and out of phase in backward direction to the
port 3 (isolated).
ELECTROMAGNETIC MODELING
Electromagnetic modelling and the thermal analysis of
the designed circulator has been done using CST
Microwave studio utilizing FDTD technique[7], with
transient & thermal solver. Symmetrical stripline
geometry with 140 mm long, 10 mm wide & 0.2 mm
thick stripline centre conductor, and 320 mm long, 10 mm
thick, 150 width two dielectric slabs of alumina (and
epoxy) has been optimized for the 50  characteristic
impedance. Two coupling lines separated by g/2 70 mm
are used to tap the RF power from the main line through
its open ends situated at a distance of 10 mm. Coupling of
RF power takes place by means of the electrical field
between centre conductor and ground planes. Schematic
of the stripline dual directional coupler is as shown in
Fig-1. Full geometry having 50,000 mesh cells, air as
background material, and open boundary conditions is
excited using discrete ports.
Figure 2: Smith chart of the S11-reflection from the
port-1 of directional coupler, showing good impedance
matching throughout the geometry.
S parameter observed for the optimized directional
coupler are as in figure 3, showing insertion loss,
reflection, isolation, coupling, and hence giving
directivity better than 40 dB .
Figure 3: S-Parameters i.e. reflection S11, Insertion loss
S21, coupling S41 and isolation S31, observed for the strip
line directional coupler.
Figure 1: Schematic diagram of the stripline dual
directional coupler.
Impedance matching in the whole geometry is
important criteria for smooth operation of the directional
coupler. Matching condition i.e. reflection parameter S11
is observed using the smith chart shown in fig-2. As
apparent from the smith chart, all ports are having good
matching (49.36 Ω port impedance against 50 Ω
excitation source) and hence negligible reflection.
Whole strip line geometry is enclosed in the copper
casing to avoid the leakage of RF signal to surrounding.
Electrical &Thermal analysis
Port 1 has been excited using high voltage port up to
700 V giving out ~ 10 kW RF power. As in fig - 4,
maximum electrical stress observed in the strip line
geometry is observed is 704 V/mm, which is far below
breakdown voltage of alumina 30 kV/mm, hence
electrically very stable. However maximum voltage
gradient applied across the dielectric medium is limited
by the surrounding air (3 kV/mm) and humidity contents,
but still can perform stably up to higher power by taking
proper safety measures.
RESULTS AND DISCUSSION
Scattering parameters for the optimized strip line
directional couplers are insertion loss S21 ~ -0.1 dB,
reflection S11 ~ 27 dB , coupling S41 ~ 27 db, and isolation
S31 ~ 65 dB, so directivity ~ 40 dB are observed, which
are satisfactory enough for the measurement of reflection
from the circulator (~ 30 dB). High directivity of the
narrowband directional coupler is achieved by frequency
selective phaser sum of the two coupled RF signal
components. Since internal dielectric material is able to
support higher peak breakdown voltage, so by selecting
proper port termination, and taking high voltages
measures, safety factor (typically 2), and cooling
arrangement for the centre stripline conductor, directional
coupler may be further upgraded for higher power levels.
ACKNOWLEDGEMENT
Figure 4: Vector electrical field profile of the
directional coupler.
As is evident from the thermal profile, maximum of 13
°C temperature rise has been observed (306 K). Up to this
temperature there is no significant change in dielectric
properties of the mediums used copper and alumina, so
can perform thermally stable. Thermal profile of the
directional coupler at 10 kW input power is observed as in
fig-5.
Figure 5: Thermal profile across the geometry of
directional coupler.
Thermal simulation has been carried out by using
forced cooling of outer casing to ambient temperature of
293 K. As evident from the temperature profile of strip
line directional coupler maximum rise is observed nearby
the strip line conductor due to higher electric field (fig. 4).
Stripline conductor cooling is mainly controlled by the
heat transfer through the dielectric medium. Due to
additionally cooling through port contacts, actually
temperature rise will be much lesser than the simulated.
We are indebted to Dr P.D. Gupta, director RRCAT for
constant encouragement for development of ferrite
circulator and associated components .
REFERENCES
[1] Howe. Harlan Jr, Stripline circuit design, 2nd edition,
1974. Artech House.
[2] Pozar D.M., Microwave Engineering (2011). John
Wiley and sons.
[3] Experiment
5
Coupler
design.
http://www.hit.ac.il/.upload/engineering/microwave_
-_experiment_5_-_coupler_design.pdf
[4] George
Kennedy, Bernard
Davis,
Electronic
communication systems, (2005), Tata Mc Graw-Hill.
[5] Directional
coupler,
Microwave
101.com.
http://www.microwaves101.com/encyclopedia/directi
onalcouplers.cfm.
[6] Power divider and directional coupler, Wikipedia.
http://en.wikipedia.org/wiki/Power_dividers_and_dir
ectional_couplers.
[7] CST (computer simulation technology) Website.
http://www.cst.com/Content/Events/workshop_docu
ments/2012/1116-PR-Microwave-Circuits-andComponents.pdf