EXPERIMENTAL INVESTIGATIONS OF FERRITE PHASE SHIFTER

MAGNETIZED WITH ROTARY FOUR-POLE MAGNETIC FIELDS

Mateusz Mazur*, Edward Sędek

*, Jerzy Mazur

**,

Ryszard Frender*, Dariusz Wiśniewski

*

Abstract: The practical ferrite rotary field phase changer is designed and fabricated for low frequency by

using a composite ferrite dielectric rod that allows to reduce diameter of a circular waveguide of phase

shift section. Experimental results at 1.3 GHz agree well with ones predicted theoretically confirming the

developed design procedure.

Introduction The ferrite phase shifter based on a section of cylindrical waveguide, in which high dielectric constant material

and ferrite filled section alternate, was designed, fabricated and measured. The differential phase shift between

orthogonally polarized fundamental modes is obtained by applying a transverse rotary four-pole magnetic field.

This structure proposed recently by Boyd [1-4] has been used to design a rotary field phase shifter for 1-2 GHz

range where design of the conventional rotary field phase shifter require a ferrite rod of large diameter.

Although operation principle, design procedures and constructions of such devices are known in literature [1-6]

the scattering models for their satisfactory optimization had not been achieved. The coupled mode method [7,8]

was implemented to analyze the propagation of em. wave in cylindrical waveguide fully filled with ferrite

magnetized by an electrically rotary four - pole magnetic field. This model describes wave propagation in terms

of the gyromagnetic interaction between orthogonally polarized TE11 modes propagated in dielectric cylindrical

waveguide. From this approach the multimodal scattering matrix of the ferrite- dielectric rotary field sections

was simulated and the results were used for phase shifter design for use at the center frequency of 1.3GHz. The

material presented below describe the basic configuration of the rotary field phase shifter using composite

ferrite-dielectric rod. The design procedure is discussed and experimental results show the feasibility of the

fabricated model.

Design of the rotary-field phase shifter The practical realization of the rotary field phase

shifter using 180o differential phase section (half -

wave plate) designed with alternating ferrite

dielectric rod is shown in Fig.1. The rod is biased

with transverse four-pole rotatable magnetic field.

In order to electrically rotated field pattern the rod

is biased by yoke fitted by two interlaced windings.

By proper setting of the currents in these windings

the resulting four pole field is rotated to any angle.

The central rotatable half -wave plane is connected

at each end to 90o differential phase section

(quarter-wave plate polarizer ) where the linearly

polarized field is changed to circularly polarized

wave and vice versa.. These polarizers couple to

cicular waveguide of transitions to coaxial ports.

For damping cross polarized error waves a

polarization filters are realized at each end of the

device using film load located in the waveguide.

Transition

Ferrite-dielectric rod

Polarizer

Polarization filter Magnesing coils

Fig.1. Ferrite phase shifter

Periodically loading a ferrite rod with low-loss dielectric of high dielectric constant consists of alternating ferrite

and dielectric discs stuck together. Ferrite disks are made from material of yttrium-gadolinium aluminum doped

garnet. For this material the saturation magnetization Ms = 20 [kA/m] ;and electric permittivity εf = 14.2 ± 2% .The barium titanium doped with lanthanide Sm or Nd ions is used to make a dielectric discs of electric

permitivity ε = 90.

* Authors are with Telecommunication Research Institute, Warsaw and Gdańsk Division, email:mefis@pit.gda.pl ** is also with Technical University of Gdańsk, faculty of ETI

The computational model allows accurate prediction of the scattering matrix S for the rotary field section . For

the half -wave plate the scattering matrix is defined in relation to two incident and reflected, left (L) and right

(R) circularly polarized waves at each port of this section. These waves are defined by two linearly polarized

TE11 fields. Using in optimization process S- matrix of the composite ferrite –dielectric type of this device is

calculated as cascade of the ferrite and dielectric sections. From optimization of the transmission of (L) to

(R) circularly polarized wave or vice versa the dimensions of the circular waveguide filled with composite

ferrite –dielectric rod are found. This type of the rod design for use at the center frequency of 1.3 GHz have

diameter of 37.2mm and length of 260mm. This model fits reasonable well parameters for magnetization M

approximately equal 0.8 Ms. Fig.2 shows optimal operation of the structure at the frequency 1.3 GHz over

bandwidth up to about ten percent. The phase shift of the model change as a function of rotation angle of the

bias magnetic field pattern as predicted in Fig. 3. Note that the phase characteristic is linear and it assure proper

operation of the designed rotary field phase shifter.

f[GHz]

. θ[°]

Fig.2. Theoretical scattering characteristics for

designed ferrite-dielectric rode versus frequency

Fig.3. Phase characteristics. Change of transmission

signal phase versus angle θ defining l rotation of magnetizing field pattern.

The quarter-wave plate circular polarizer located at the each end of the half wave plate phase changer is

a dielectric slab at 45o to the plane of the linearly polarized wave at the input. This polarizer was designed using

QW-3D professional software and fabricated with dielectric slab of ε =90 located in circular waveguide filled

with dielectric of ε = 25. The coaxial to circular waveguide transitions are followed by a polarized absorber to

absorb any cross polarized mode generated in the phase changer. This type of absorber is similar to the

quarter-wave plate configuration where dielectric slab is replaced by thin plate of epifer being well absorbing

material at L band.

Experimental results

Fabricated model of rotary-field phase shifter is presented in fig 4. The bias magnetic four pole magnetic field

pattern must be rotated to effect phase shift. The rotation angle is steered by currents flowing in two windings

of the external magnetic circuit. The ratio of the windings currents defines electrical angle θ that simulate

mechanical rotation angle φ =θ/2. Because phase shift is proportional to twice mechanical angle, it well means

that the phase change should be equal electrical angle θ. Fig .5 shows characteristics of a phase change versus

electrical angle θ measured at 1.3GHz on the L-band model operating at a different bias level defined by the

battery current Im supplying two windings. For this measurement the linear phase shift characteristics are

obtained when Im > 0.5 A (Vm=1V) and a proper characteristics are observed for Im ˜ 0.75 A(Vm=1.5V) . For

this value of I the M/Ms ˜ 0.8 .

Fig.6. presents an effect of hysteresis observed for changes of electrical angle θ in the opposite directions when supplied windings current is equal Im =0.5 A. Note that the phase error can achieve values up to 15 degree. The

error depends on the variation of the magnetic field pattern in the ferrite. This effect is not really surprising since

the ferrite hysteresis and no ideal construction of the magnetic bias circuit can disturb the configuration of the

four-pole magnetic field when its pattern is turned electrically.

])[arg( 21

oRLs

L

L

LL

a

bs

1

221 || =

L

R

RL

a

bs

1

111 || =

L

L

LL

a

bs

1

111 || =

L

R

RL

a

bs

1

221 || =

Fig.7. shows the differences in the phase characteristics measured for two opposite direction of propagations.

Although the structure is reciprocal the small differences in phase are seen when the bias magnetic field is

turned electronically. It is worth to note that this effect decrease when the current Im = 0.5A i.e. optimal feed

conditions for measured model of the phase changer are satisfied.

Fig.4. Model of a phase shifter

Fig. 5. Phase shift versus electrical angle θ. For different Supplying voltages from 0.4V up to 1.5V

-360

-270

-180

-90

0 -15 30 75 120 165 210 255 300 345

electrical angle θ

arg(S21)

Positive increment

Negative increment

Phase error

Fig. 6. Phase shift versus electrical angle θ for positive and negative increment of θ. Supplying

voltage Vmax=1V,

-180

-135

-90

-45

0

45

90

135

180

-90 -60 -30 0 30 60 90

electrical angle θ

arg(S21)-arg(S12)

0.9V

1.05V

1.2V

1.35V

1.5V

Fig. 7. Phase difference arg(S21) - arg(S21) [°] versus angle θ [°] for different Vmax, (f=1.3 GHz)

Figures 8 and 9 give insertion losses and return loss frequency characteristics measured for different values of

electrical angle θ. It is worth to note small variations in patterns of transmission and reflection characteristics

when the bias magnetic field is turned. However the insertion losses of the phase changer measured at the central

frequency 1.3 GHz are -3.5 dB while the reflection losses are below -20dB. In order to define the insertion

losses of the designed half –wave plate section the configuration of the phase changer without this section was

measured.

For this configuration the insertion loss not exceed 0.5 dB at the measured range. Hence we can deduce that

the insertion losses of the fabricated half-wave plate section are about 3 dB. Therefore the additional

investigations are needed concerning the technology of the composite ferrite- dielectric rod and improvements

of the magnetic bias circuit.

-360,00

-270,00

-180,00

-90,00

0,00

90,00

180,00

0,00 45,00 90,00 135,00 180,00

kąt obrotu układu magnesującego

arg(S21) 0.4V 0.6V 0.8V 0.95V 1V 1.20V

0.9V

1.50V

electrical angle θ

a) b)

Fig 8. |S21| versus frequency for different values of θ [°] a)0°, b) 180°

a) b)

Fig 9. |S11| versus frequency for different values of θ [°] a)0°, b) 180°

Conclusion

Rotary field phase changer laboratory model was designed , fabricated and measured at low frequency. At this

stage of our work no attempt was made to optimize the construction and technology of the fabricated model.

However good agreement between measured data and theoretical results proves the validity of the scattering

matrix design procedure developed on the base of coupled - mode solution of the considered problem.

Literature[1] A.G. Fox, „An Adjustable Waveguide Phase Changer," Proc. IRE, Vol. 35, December 1947, pp. 1489-1498.

[2] C.R.Boyd,JR, “Design of Ferrite Differential Phase Shift Section” IEEE MTT-S Int. Symposium Digest,

1975, pp.240-242.

[3] W.E. Hord, C.R. Boyd, Jr., D. Diaz, „A New Type of Fast-Switching Dual-Mode Ferrite Phase Shifter,"

IEEE Trans, on Microwave Theory and Tech., Vol. MTT-35, December 1987, pp. 1219-1225.

[4] C. R. Boyd, Jr., „Design Considerations For Rotary-Field Ferrite Phase Shifters," Microwave J., Vol. 31,

November 1988, pp 105-115.

[5] C.R. Boyd, Jr. and C. M. Oness, „Ferrite Rotary-Field Phase Shifters With Reduced Cross-Section," IEEE

International Microwave Symposium Digest, May 1990, pp. 1003-1006.

[6] C. R. Boyd, Jr., „Microwave Phase Shift Using Ferrite-Filled Waveguide Below Cutoff," IEEE Trans, on

Microwave Theory and Tech., Vol. MTT-45, December 1997, pp. 2402-2407.

[7] I. Awai and T.Itoh, “Coupled –Mode Theory Analysis of Distributed Nonreciprocal Structures” vol MTT-

29,October. 1981, pp.1077-1086.

[8] Marcuse D. “Coupled-mode theory for anisotropic optical guide”, Bell Syst. Tech. Journal, Vol.54 No. 5.

May 1973 pp. 985-995