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INFLUENCE OF DIELECTRIC AND MAGNETIC
PROPERTIES OF FERRITES UPON EMBEDDED
ANTENNAS RADIATION
G. Dubost, A. Bizouard
To cite this version:
JOURNAL DE PHYSIQUE Colloqrre Cl, supplhment au no 4, Tome 38, A v r i l 1977, page Cl-275
INFLUENCE OF DIELECTRIC AND MAGNETIC PROPERTIES
OF FERRITES UPON EMBEDDED ANTENNAS RADIATION
G. DUBOST (*), A. B l Z O U A R D (**)
R6sum6.
-
Nous montrons d'abord I'influence d'une gaine cylindrique d'un materiau ferrite sur les diminutions de la frkquence de rbonance, de la resistance de rayonnement et de la bande passante d'une antenne A ondes statiomaires.Nous avons optimist le materiau ferrite pour realiser un compromis entre les differentes caracteristiques de rayonnement d'un doublet.
Pour connaitre ensuite le facteur de reduction en dimensions d'une antenne spirale a large bande, nous avons calcule les courbes de dispersion d'une onde guidk le long d'une ligne a bande immergk dans un materiau ferrite sans perte. Le meilleur est un matkriau ferrite au Ni avec des substitutions de 0. Zn et 0. Co. L'anteme immergee, qui fonctionne dans un octave, est une double spirale d7Archimede plane disposke devant un contrepoids. Compare A un doublet demi-onde classique, le facteur de reduction est de 5 environ. Le gain de I'antenne est mauvais en raison des pertes dans le matkriau et de I'existence de modes superieurs.
Abstract.
-
For a stationary wave antenna we show the influence of a cylindrical ferrite sheath upon the resonant frequency, radiating resistance and bandwidth reductions. To realize a compro- mise between various radiating characteristics of a dipole we have to choose the suitable ferrite material. To know the size reduction factor of a large bandwidth spiral antenna, we have computed dispersion curves of a guided-wave along a strip-line embedded into a lossless ferrite material. The best is a Nickel ferrite with ZnO and COO substitution. The embedded antenna, which works in an octave, is a plane archimedian 2 arm spiral placed in front of a counterpoise. The size reduction factor is about 5 as compared to classical dipole. Antenna gain is poor because of propagation of upper modes and material losses.1. Introduction. - T h e use of isotropic linear ferrite material in which electromagnetic waves radia- tors are embedded enables t o make these latter work at lower frequencies. Generally a n arrangement between certain radiation characteristics of antenna is possible if the ferrite material is suitably chosen. Thus in the case of stationary wave antennas, the decrease of the resonant frequency is translated into a serious reduction of bandwidth which shall be atte- nuated by a judicious choice of ferrite. The thickness influence of a lossless ferrite sheath o n resonant fre- quency, radiation resistance a n d the bandwidth of a cylindrical dipole is shown in a n earlier study [I]. We have shown that the difference between the theore- tical and experimental results are due t o the magnetic and electric losses defined respectively by their loss angles :
tg 6, = ,u:'/p: and tg 6, = E:/E:
where
pr = pi - ip: and E, = E ;
-
i ~ : )are the complex relative permeability and permittivity of material. The latter, whose average values a r e :
(*) Universite de Rennes, Laboratoire : c( Antennes &
Rayonnement )), Avenue du General Leclerc, 35031 Rennes
Cedex France.
E: = 10 and p: = 7.5 is used between 200 and 400 MHz.
These are the frequencies for which tg 6, remains comprised between 4 X 10-2 a n d 6 X 10-2 a n d t g 6,
close t o 2 X 10d2. The antenna yield is poor because
of high losses [l].
2. Influence of ferrite medium on a stationary wave antenna.
-
The antenna is a half dipole embedded in a cylindrical ferrite sheath. The latter is responsible for a decrease of resonant frequency, radiation resis- tance, and antenna bandwidth. A recent study on the approximate theory of the functioning of such an antenna, supposed t o be thin [S], confirms the results of a rigorous study [ l ] and enables to interpret physically the role of ferrite material. Figure 1 gives the antenna definition.counterpoise , ferrite sheath
E i
---.
coaxial lifie2 h
(**) Thomson-C. S. F., Groupement A. V. S., 178 Bd Gabriel- FIG.
,.
-
Definition of the antenna.= lo&
Peri, 92240 Malakoff France. a
Cl-276 G. DUBOST AND A. BIZOUARD
The phase velocity v, along the dipole, divided by the velocity c of electromagnetic waves in vacuum is given by the expression (1) :
( u / c ) ~ = [Q - 2.5
+
2(1/4 - l ) log, ( l+
ela)] X X[L?
- 2.5+
2(,4 - 1) log, ( l+
ela)] -'
.
( 1 )B is the Hallen expansion parameter of dipole without ferrite ( [ 2 ] p. 69) :
l2 = 2 log,(2 hla)
.
The approximation of thin antenna corresponds to B 2 6 .
The radiation resistance R, of the antenna at first resonance divided by the wave resistance of vacuum R, is written as :
&,/R,
= ( V / C ) ~ . ( ~ 1 3 2 ).
(2)The Q-factor of antenna equivalent circuit at first resonant frequency f, such that l / Q = Aflf, where Afis the bandwidth, may be written as :
Q = ( 4 1 ~ ) [S2 - 2,5
+
2 0 :-
1) log, (1+
ela)] 3/2 X X [Q-
2;5+
2 ( 1 / 4-
1) log, (1+
ela)] - ' l 2.
(4)The loss resistance R, of the antenna at resonance, which is added to the radiation resistance, is given by the expression :
The dipole will effectively work like a stationary wave and transversal radiation antenna if no guided mode propagates on the structure, that is, if
Finally, the antenna efficiency at the first resonant frequency is given by the relation [l] :
Taking into account (2) and (5) we have :
q = L1
+
0.8 tg G,(c/v).(pC1: - 1) log, (1+
ela)].
(8)The formulas (l), (2), (4), (8) and the validity condition
(6) enable to know the influence of ferrite medium on the antenna radiation.
For a given reduction factor, one may search for the conditions to obtain the largest possible bandwidth and hence the weakest possible Q factor. From (1) and (4), we have :
For a given ulc, the smaller the /A:, the larger will
be the bandwidth (see Fig. 3). But the decrease in p:
and the corresponding increase in E: are limited by
condition (6). It is for this reason that the curves
augmented (Fig. 2). However, the decrease in the reduction factor u/c is rapidly limited. It is therefore necessary, if it is desired to still decrease u/c, to use a ferrite material (Fig. 3) by admitting a more important reduction of the antenna bandwidth.
FIG. 2. - Influence of the relative permittivity Er on the reduc-
tion factor v/c and the Q factor of a Half dipole at resonance.
E: = F(p:, v / c ) of figure 3 are limited toward the top.
FIG. 3. - Influence of the characteristics ,m and 8; of the ferrite
DIELECTRIC AND MAGNETIC PROPERTIES OF FERRITES UPON EMBEDDED ANTENNAS RADIATION Cl-277
For a given reduction ratio vlc, the bandwidth will always be greater if a dipole is used whose Hallen expansion parameter Q is the weakest possible. Finally, by increasing p:, E: varying between l and an upper
limit related to the non propagation of guided modes so as to decrease vlc, the antenna yield is damaged [see (g)].
The approximate theory, [5] allows therefore to find, following the set problem, a compromise between different antenna characteristics : geometry, band- width, yield, ferrite material technology, realizable volume, p:, E:, tg hm, etc
...
3. Influence of a ferrite medium on a large bandwidth, progressive wave antenna.
-
To know the reduction factor in terms of dimensions of a large bandwidth antenna, of equiangular plane spiral type with circular polarization, which is made unidirectional by a reflec- tor and embedded in a ferrite material, we have first studied the guided propagation along a strip line embedded in a material [3]. In order to solve the equa- tion of propagation and determine the dispersion curves, the method of finite differences is used, taking into account the limiting conditions. The problem is reduced to the study of spectrum of proper values of an elliptical type partial derivative equation.The experimental verification proves the accuracy of the method of calculation.
he
measured characte- ristics of the ferrite material used are given in table I. The quality of the material is much improved as compared to the previous one [l].To determine the phase velocity v for the fundamen- tal mode along the microstrip disposed on, or in the ferrite substrate, we measure the successive resonant frequenciesf, of the ring resonator made up of such a line.
21 = Lf,/n, where L is the geometric length of the line.
FIG. 4.
-
Phase velocity for three microstrips : Theoretical ; J dispersion from experiments.transmission line does not radiate and the phase velocity is more easily reduced than in the case of dipole (paragraph 2). The attenuation constant per unit length is given by the expression.
Or in MKSA system :
Expression (12) shows that for the reduction factor vlc Figure 4 shows the experimental and theoretical of the order of 0.2 (Fig. 4) the attenuation a is of the results for different lines [3]. They are in perfect order fo 10 dB per meter. This is excessive. It is agreement. The dispersion between the experimental therefore necessary to further decrease the loss angles results is due to the inhomogeneity of substrate. The of ferrite material. The ferrite material, meant for dimensions of the line and the screening, taking into progressive wave antenna, must finally respond to the account the ferrite material, are such that the upper
- -
following conditions :Cl-278 G . DUBOST AND A. BIZOUARD
1-- V p:/&: nearly 1 for obtaining characteristic
impedance of strip line close to those used in the case of air,
- weak temperature coefficient,
- low loss material,
- sufficient density to allow a good metallization of spirals.
Starting with a Niferrite, different substitutions were made (Pb, Cu, AI, CO) for rejecting the resonant frequency of wall towards high frequencies. Finally
The studied progressive wave antenna is a plane archimedian two self-complementary arm spiral ([2], p. 161) which has been embedded into 1107 fernilite ferrite material square plates of 120 mm X 120 mm.
Following are its characteristics : spiral constant K = 0.452 mm (rd)-', radius of the feeding zone
r, = 4.4 mm, width of radiating wire W = 0.71 mm and spiral length = 4 m. It can be admitted that the radiation zone of the antenna is defined by :
1
r = - =
(f)
.
(
!
l
)
2 n
where r is the magnitude of the radius vector measured from the expansion pole. The spiral length between the latter and the radiation zone is fairly equal to :
The attenuation sustained by the wave between the feeding zone and the radiation zone, taking into account (1 l), (13), (14) is given by the expression (in Neper) :
The decrease in the value of v (which necessitates an increase in the volume of ferrite) is restricted by the appearance of upper order modes (or of resonance cavity when the antenna is embedded) which would modify the characteristics of the antenna radiation. As a matter of fact, for a ferrite plate of thickness d,
the frequency at which appear the TE type surface waves is given by the expression ([l], p. 62) :
the best material obtained is an alloy of Ni and the substitutions of ZnO (zero to 10
%)
and of O . C o (zero to 7.5%).
This material is the fernilite 1107, studied and manufactured by the Company L. T. T. (France). Its high annealed temperature assures it 2 "/,porosity. When the functioning temperature of the ferrite increases, the resonant frequency of the wall slides towards low frequencies. The variation of tempe- rature coefficient A
J z / a
between 10 and 35 OC is+
8%.
The measured electrical characteris- tics of the material are given in the following table 11 :Taking f, as the limit of antenna bandwidth i. e. 500 MHz and for the ferrite material, d must remain less than 20 mm aproximately. This small thickness does not favour the antenna radiation because d/ic remains close to 0.06 a t 250 MHz. T o aIIow an effective
radiation we will haye to admit the existence of upper modes by adopting a greater thickness of ferrite material.
For different antennas realized with 1 107 ferrite material and for the reduction ratios comprised between 3 and 5, the attenuation given by (15) between 200 and 500 MHz is still comprised between 5 and 6 dB
to which is added the attenuation due to upper modes. However, taking into account the limited dimensions of ferrite plates, for low frequencies, it was necessary to load the spirals with resistance to absorb non- radiated energy, at the loss of antenna yield.
Nevertheless, inspite of poor antenna yield, the radiation characteristics (directivity, back radiation, polarisation rate ...) remain acceptable in a band of frequencies of the order of an octave. The maximum dimensions of the antenna are less than 1,/10 where
A,
is the longest wavelength of bandwith.4. Conclusion. - For a stationary wave antenna,
the presence of a ferrite material allows a lowering of resonant frequency, at the expense of a reduction in the
bandwidth and the antenna yield. In the case of a progressive wave antenna of large bandwidth, like plane spiral type, the yield is strictly limited by the appearance of upper modes. The antenna gain is
DIELECTRIC AND MAGNETIC PROPERTIES OF FERRITES UPON EMBEDDED ANTENNAS RADlATlON Cl-279
References
[l] MADANI, A., LE REST, D., DUBOST, G., Ann. des Te1t;commun.
27 (1972).
[2] DURXT, G., ZISLER, S., Antennes 2 large bande. Thborie et applicalions (Masson & Cie) 1976, 352 pages, 222 fig. [3] LE REST, D., DUROST, G., MADANI, A., Ann. des Tilicomtnun.
29 (1974).
[4] DANIEL, J. P., DUROST, G., ROSPARS, J., Elect. ~ I I . 11 (1975). [5] DUBOST, G., c( Contribution h I'etude d'un demi-doublet mince immerge dans unc gaine cylindrique de ferrite D.
