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Experiment and observations: Geometry of the A/4 shorted
microstrip antenna is shown in Fig. 1. The substrate material
has the following constants measured at 100 MHz:
In eqn. 3 the slope parameter •/ takes the values
7 = 2-23 at Fucino
-/ = 2-13 at Lario
•/ = 2-50 at Spino d'Adda
and, assuming the typical probability level p0 = 10~ 2 o o , the
corresponding values of Ao are given by
Ao = 4-8 at Fucino
AQ = 80 at Lario
Ao = 6-8 at Spino d'Adda
M. MAURI
24th April 1981
Relative permittivity £r = 15
Relative permeability \iT = 1258
Loss tangent (dielectric) = 00476(tan Sd)
Loss tangent (magnetic) = 00062(tan 6m)
Height h of substrate = 0-2 cm
The antenna, 4 cm in length, 112 cm in width and fed at 1 cm
off the open end, resonates and radiates at 112, 168, 219, 285,
325, 375, 405, 430 and 500 MHz in the TM 0 1 mode. The vertical field EQ variation over the ground plane ((p = n 2) has been
studied and the normalised radiation pattern E^, against 6 has
been plotted. Figs. 2-4 show some of these observed patterns.
0
Centro di Studio per le Telecomunicazioni Spaziali del CNR
do Istituto di Elettrotecnica ed Elettronica del Politecnico di Milano
Milano, Italy
A. PARABONI
Istituto di Elettrotecnica ed Elettronica del Politecnico di Milano
Milano, Italy
References
1
2
'Alta frequenza', special issue on the SIRIO programme, 1978,
XLVII, (4)
HODGE, D.: 'Frequency scaling of rain attenuation 1 , IEEE Trans.,
1977, AP-25, pp. 446-447
-n/2
Fig. 2 Antenna radiation pattern
—•
—O—O— /, = 112 MHz
0
0013-5194/81/130440-02$!. 50/0
s~
QUARTERWAVE MICROSTRIP ANTENNA
ON A FERRIMAGNETIC SUBSTRATE
Indexing terms: Antennas, Microstrip, Ferrites
Experiment conducted on a microstrip quarterwave antenna
structure on a typical ferrite substrate reveals reduction in
size, interesting radiation characteristics and a broadband
nature over a wide range of frequencies in the lower UHF
region.
Introduction: Microstrip antennas have been built on dielectric
substrates to operate in the GHz range and reportedly 1 4 exhibit a narrow bandwidth. At lower (MHz) frequency range,
microstrip resonating structures on purely dielectric substrates
are not practicable due to large size considerations. By using a
ferrite substrate, however, the resonant length could appreciably be reduced 5 6 due to its high value of effective permeability at the desired range of operating frequency. Besides,
permeability being frequency-dependent, interesting radiation
properties and improved bandwidth 6 criterion could be
expected.
An experiment has been conducted on a quarterwave microstrip resonating structure built on a typical ferrite substrate
Niio62Co 0 o2F e i 948^4 to analyse the findings in the above
line.
1
1'
j
-ff/2
#— fr = 168 MHz
j
\
1
1
1
1_1_J
E<t>-e
1
1
1
1_
+TT/2
[284/3]
Fig. 3 Antenna radiation pattern E^ — 0
—•
— O — O — fr = 219 MHz
# — fr = 285 MHz
0
-ff/2
Fig. 4 Antenna radiation pattern E^ — 9
— O — O — X = 325 MHz
—•
• — fr = 375 MHz
7811
75<
4>=0
ferrite
substrate
quarter-wave
shorted strip
0=0
180
\7bU\\
Fig. 1 Geometry and co-ordinate system of Xj4 microstrip antenna
ELECTRONICS LETTERS 25th June 1981
Vol.17
200
220
240
frequency.MHz
260
fllZTn
Fig. 5 Variation of antenna input impedance with frequency
No. 13
441
Fig. 5 shows the observed variation of input impedance of
the antenna with frequency around a resonating frequency of
219 MHz as obtained from the Smith chart plot.
PUCEL, A., and MASSE, D. J. : 'Microstrip propagation on magnetic
substrates—Part I: Design theory', ibid., 1972, MTT-20, pp.
304-308
DAS, N., and CHOWDHURY, s. K. : 'Microstrip rectangular resonators
on ferrimagnetic substrates', Electron. Lett., 1980, 16, pp. 817-818
Analysis:
(a) Resonant length and frequency: The microstrip guide
wavelength is given as5
0013-5194/81/130441-02S1.50/0
(1)
where
Ao = freespace wavelength
THREE PERIOD (A1, Ga)As/GaAs
HETEROSTRUCTURES WITH EXTREMELY
HIGH MOBILITIES
eeff = effective relative permittivity
Heff = effective relative permeability
In the present case, Ag/4 = 4 cm and seff = 12188 for w/h
(width to height ratio) = 5-6.
For various resonating frequencies/,., values of fieff could be
calculated using eqn. 1:
Indexing terms: Semiconductor devices and materials, Electron
mobility
Selectively doped three period Al 02 Ga 0 . 8 As/GaAs structures
have been grown by molecular beam epitaxy and characterised by Hall effect over a temperature range of 10 K-300 K.
Electron mobilities as high as 211000, 95 800 and 6700
cm 2 V" 1 s" 1 have been obtained at 10 K, 78 K and 300 K,
respectively. The total charge concentration in all the structures was about 2-25 x 1012 cm" 2 . These extremely high electron mobilities are a result of separating the donors and the
electrons appreciably, and very-high-quality interfaces. To
date, the figures at 78 K are the highest reported, while the
10 K figures are about twice the best previously reported
results.
Heff = 23-01 for/ r = 112 MHz
Heff = 10-23 for/, = 168 MHz
Heff = 602 for/r = 219 MHz
Although the exact nature of the variation of fxeff with
frequency is not known, it is presumed (from the occurrence of
resonances in the TM 01 mode) to vary with frequency in such a
way as to pass through discrete points of resonance (i.e. 112,
168, 219, ..., 500 MHz), which lie on a curve \iei{.
fr = constant.
The size of an identical microstrip resonator on a purely
dielectric substrate (er = 9, eeff = 7-4) at 112 MHz (Ao = 267-8
cm) would be by eqn. 1, 24-61 cm and 12-58 cm at 219 MHz
(Ao = 136-98 cm). So, by using a ferrite substrate, reduction in
resonant length is attained by factors 615 and 3-14, respectively, at 112 and 219 MHz.
(b) Bandwidth: The 3 dB bandwidth is found to be 3-19% from
Fig. 5.
Conclusions: The value of fieJJ adjusts at discrete points of the
frequency spectrum ranging from 100 to 500 MHz to give
multiple resonant frequencies at a single mode, which could be
gainfully utilised. The bandwidth achieved is distinctly higher
than that obtained by conventional dielectric microstrip
resonators.
The efficiency of the antenna could be enhanced with reduction of inherent loss of the substrate material, which is being
investigated. The efficiency of the antenna could also be enhanced by choosing a proper value of w/h for which fieff
assumes a high value6—of course, at the cost of large bandwidth attained.
The size of the antenna could appreciably be reduced and
thus it can be used at a lower frequency range. This is a great
advantage of a microstrip antenna on a ferrite substrate, as
experimentally established.
N. DAS
J. S. CHATTERJEE
24th April 1981
Computer Centre,
Department of Electronics & Telecommunication Engineering
Jadavpur University, Calcutta 700032, India
References
1
2
3
4
MUNSON, E.: 'Conformal microstrip antenna and microstrip phased
arrays', IEEE Trans., 1974, AP-22, pp. 74-78
HOWELL, J. Q.: 'Microstrip antennas', ibid., 1975, AP-23, pp. 90-93
DERNERYD, A. c : 'Linearly polarised microstrip antennas', ibid..
1976, AP-24, pp. 846-851
DERNERYD, A. G., and LIND, A. G. : 'Extended analysis of rectangular
microstrip resonator antennas', ibid., 1979, AP-27, pp. 846-849
442
Introduction: Selectively doped (Al, Ga)As/GaAs heterostructures called 'modulation doped' have recently received a
great deal of attention because of their potential application to
high-speed devices. In these structures the GaAs is intentionally undoped while the (Al, Ga)As is doped with a donor
impurity. Since the electrons have a tendency to occupy the
lower energy states they transfer into the GaAs leaving their
parent donors behind. Dingle et al.1 and Stormer et al.2
reported
on
multiple period modulation
doped
(Al, Ga)As/GaAs superlattices which demonstrated the
absence of ionised impurity scattering and led to the achievement of high electron mobilities compared to bulk GaAs of
equivalent donor density. An undoped region of 50 A of
(Al, Ga)As was incorporated at the interface to act as a buffer
against any possible donor diffusion into the GaAs layers.
In a series of articles,3 5 we have reported the electron mobilities of single period modulation doped structures with particular emphasis on the effect of further spatial separation of
donors and electrons. Using this approach, extremely high
electron mobilities at 300 K and at lower temperatures were
obtained.56 Stormer et al.,1 utilising the same notion, also
demonstrated the advantages of further separation of donors
and electrons. Theoretical investigation of the transport
properties of these structures has been undertaken by several
groups.8 10 The low temperature mobility, limited by only the
spatially separated remote donors, is estimated to be over
300000 cm2 V" 1 s~\ which is a factor of three or more higher
than the best reported values. In a recent manuscript we have
attributed this limitation of the mobility to interface states and
interface roughness.11 Efforts were then directed toward improving the metallurgical quality of the interfaces as well as the
electronic quality of the layers. As a result, very high electron
mobilities were obtained which are the subject of this letter.
Experimental procedure and results: The structures reported
here were all grown with molecular beam epitaxy at a substrate
surface temperature of 600°C and at a GaAs growth rate of 0-7
fim/h. The details of the growth procedure have been reported
elsewhere4 and will not be repeated here. The surface morphology of the grown films was excellent and the observable defect
density was less than 500 cm" 2 as determined by a Nomarski
metallurgical microscope. The interfaces were optically very
smooth as evidenced by the Bragg diffraction of light which
ELECTRONICS LETTERS 25th June 1981
Vol.17
No. 13
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