In Proceed. of URSI/IEEE XXVII Convention on Radio Svience,
pp. 84-87, Espoo, Finland, Oct.2002.
WIDEBAND VIVALDI FEED FOR A REFLECTOR RADIO TELESCOPE
Golovkov A.A.*, Khaikin V.B.**, Golubeva E.Yu.*,
Kalinikos D.A.*, Kiselev
B.A.*, Sugak M.I.*
*The St.Petersburg Electro-Technical University
**The Special Astrophysical Observatory of RAS
ABSTRACT
Dual-polarized (X/Y) feed from Vivaldi radiators for a reflector radio telescope is offered. Proposed solution provides growth of an effective aperture of the feed at longer wavelengths and stability of beam pattern in the band. Exception of frequency dependent junction to a strip line increased the bandwidth of antenna. Good matching in the prototype and low enough insertion losses (0.5 dB) with FLAN-2.8 substrate material were reached. Measured co- and cross-polarized characteristics of the first Vivaldi feed prototype in the range of 0.5-5.0 GHz are presented.
Introduction
A primary feed of RATAN-600 radio telescope is a high
effective, good matched and low sidelobe feed
traditionally used for a wide-angle parabolic reflector. The operating frequency
range of the radio telescope is 0.5 GHz -30 GHz. It is not possible to cover
this range by one antenna with low enough noise temperature. The most of
RATAN-600 receivers work with pseudo-scalar feeds (20% band) placed
along the focal line of a secondary mirror that occupies focal volume and gives
unavoidable aberrations when a feed is moved rather far from the radio
telescope axis. A circular polarization (L/R) ring feed [1] with the common
phase center covering a few octaves is also used but this feed has high enough noise
temperature and is difficult to repeat, manufacture and tune. A similar type feed developed
for RADIOASTRON project had satisfied noise temperature [2] but it is still
difficult to make it.
The serious problems arise at manufacturing of
a wideband antenna at the bottom of an operating range at frequencies below 1
GHz. One of the most effective elements for construction of wideband antennas
is the printed Vivaldi – type radiator(fig.1)),
which can provide a working frequency band up to some octaves [3,4]. The
detailed research of the
Vivaldi – type antenna’s design and
characteristics is carried out in [3-6]. It is shown, that the single Vivaldi antenna can effectively work in a frequency band,
in condition when electric length of the exponential transformer and its
aperture width are close to half of wave length of mean square operating range
frequency [3-5]. The shape
of a slot and the type of an aperture excitation
essentially influence the frequency band of the antenna and its losses. The radiators with slotted cavities
in the shape of a circle and rectangle at excitation by using microstrip line – slot junction were investigated in [3-6].
Procedure of research
The radiator topology optimization was carried
out by a mathematical simulation analysis in Agilent
HFSS software. In this program the finite element method is realized.
The antenna described in [4] was chosen as the
starting point for simulation analysis. The aperture (the exponential
transformer) of this antenna has the satisfactory characteristics in a
frequency band 1-6 GHz. For acceleration of optimization process only
dependencies of antenna impedance characteristics and antenna efficiency on the
shape of a radiator were examined. The field characteristics of the antenna
were analyzed only on a final stage of design. The relative permittivity of a
substrate in all cases was 2.8, thickness - 2 mm (FLAN).
When designing a receive antenna of a radio
telescope different variants of printed Vivaldi – type radiator were examined.
The slotted cavities in the shape of a circle, ellipse, radial sector, “drop”,
and also the case of the degenerate ellipse, which has the best frequency
characteristics, were tested. Besides last variant of the antenna was simulated
by a method of the moments in spectral domain and was experimentally tested.
Development of feed
for RATAN-600 radio telescope
At the first stage the study of currents
distribution on a radiator’s surface was carried out. The study has shown that
the maximum frequency band and minimum of losses is achieved by using of a
slotted cavity as two ellipses (case of a degenerate-shape cavity), as shown in
a fig.1b. Such shape provided operating frequency range 0.5-4 GHz with minimum
losses. Essential advantage of such construction is the possibility of antenna
excitation by the strip balun transformer. It allows to except microstrip line – slot junction, traditionally
used in classical variant of the antenna Vivaldi. Exception of this frequency
dependent junction increases the bandwidth of antenna. Moreover the
characteristic impedance of strip balun transformer
is equal to slotted line’s 100 Ohms characteristic impedance. It makes
assembling radiators into array very simple.
a) b)
Fig. 1. A Vivaldi – type radiator with the
degenerate-shape slotted cavity for RATAN-600 radio telescope a) ‑ feeding system of
a Vivaldi radiator b) ‑ aperture
of a radiator
The radiator was made on a material FLAN-2.8
with 2-mm thickness with a unilateral metallization. The homogeneous section of
a slotted line in the bottom of a radiator has characteristic impedance close
to 100 Ohms practically on all operating frequencies. The antenna was fed by a
flexible piece of the symmetrical microstrip line with characteristic impedance
100 Ohms (material Roger3003, h=0.25 mm). The slotted line has strong frequency
dispersion, especially on frequencies of 1GHz and lower. For the optimum shape
of a radiator aperture selection the simulation analysis of the slotted
transformer was carried out. This procedure provides matching of a homogeneous
slot line section with characteristic impedance of 100 Ohms and characteristic
impedance of free space 377 Ohms on frequencies of 0.5, 0.65, 0.8, 1.0, 2.0
GHz. The length of the transformer in all cases was fixed. It was 150mm that in
shortening in dielectric conditions makes approximately half of wavelength of
mean square operating range frequency. At longer exponential transformer the
loss in metal of a radiator aperture is magnified. On the other hand it is
known from the theory of exponential transformers, when its length is less than
λ/2 the matching characteristics are degraded. The best ratio between
frequency band and losses in a radiator was obtained at a variant of geometry
corresponding to the frequency of 0.65GHz. This geometry was used in antenna.
The calculated and measured radiator scattering
parameter (|S11|) are shown in a fig. 2. Figure 2 shows that the radiator has a good matching level in a
frequency band 0.5-4GHz.
Fig. 2 |S11| versus
frequency of Vivaldi – type radiator for RATAN-600 radio telescope.
Simulation shows that at a frequency change
around three octaves the shape of a radiator’s farfield pattern varied
insignificantly, and the «forward – backward" ratio was not worse 16dB
even without of the lower screening surface.
Co- and cross- polarized amplitude and phase characteristics of the first Vivaldi feed prototype in the range of 0.5-5 GHz were measured in HUT anechoic chamber (fig.3)[7]. The measured results (fig.4, 5) are closed to expected.
Fig. 3. A transmitting diagonal wideband radiator (left) and
single Vivaldi radiator under test (right) in HUT anechoic chamber
Figure 4 shows co-polarized amplitude
characteristics in working frequency band. Only five dependences represents on
fig.4 to prevent the stodgy graph. The measuring was done with a 100 MHz step
and there are not abnormal dependences in test data.
a) b)
Fig. 4. Co-polarized amplitude characteristics of the radiator, a) – pattern in
a Å-plane, b) – pattern in a H-plane
Figure 4 shows normal decreasing of beamwidth under frequency growth but ripple of main lobe of farfield pattern is significant at lowest frequencies in E-plane and at most of frequencies in H-plane. Farfield pattern width in H-plane is much wider in a single radiator case. It is possible to reach acceptable beam pattern symmetry in E- and H- planes by using array from two pyramidal-located Vivaldi radiators. In this case the radiators fed points are located near to each other. The angle between radiators must provide a distance between effective apertures of radiators close to aperture width at all working frequencies. Figure 5 shows cross-polarized amplitude characteristics of the first prototype at boundary frequencies of working band .
a) b)
Fig. 5.
Co- and cross- polarized amplitude characteristics of the radiator, a) –
pattern in a Å-plane, b) – pattern in a H-plane
Array from several Vivaldi
radiators
Two-element array from Vivaldi radiators allows
us to reach acceptable beam pattern symmetry in E- and H- planes. Offered
“corner” and “pyramidal” designs for one (X or Y) and dual (X/Y) polarized feed has
some advantages over the “cubic” one [4]. This solution provides the growth of
an effective aperture at longer wavelengths, stability of beam pattern in the
band and less ripple of the main lobe at low frequencies. Tests of the “corner”
design confirmed simulation results. Array from two pyramidal-located Vivaldi radiators with proposed feeding system has 50 Ohms
impedance without additional matching elements. This improves frequency
characteristics of an array. The above and other solutions are being developed
now.
Conclusion
For improving frequency properties of the
Vivaldi – type antenna it is rational to refuse a traditional radiator
construction. The microstrip line – slot junction, its
matching with regular slotted line and aperture increase the loss in the
system. The use of the geometry with degenerate slotted cavity at
excitation with symmetrical microstrip line allows realizing an antenna with
VSWR better than 2 in three-octave frequency interval. Two-element Vivaldi array of each polarization allows to reach
acceptable beam pattern symmetry in E- and H- planes. A final design of dual polarized
(X/Y) wideband Vivaldi feed is being considered.
REFERENCES
[1] V.N.Dikiy, D.V.Dikiy, V.K.Nuzhin. Proceedings of JINA 94, Nice, 1994, pp.548-549.
[2] V.N.Dikiy, O.A.Kuz’min, K.vant’Klooster.The
multi frequency-band capability of the Radio-Astron antenna”, Proceedings of
JINA-92,
[3] J. Shin, D.H. Schaubert, “A Parameter Study of Stripline-Fed Vivaldi Notch-Antenna Arrays” IEEE Trans. on Antennas and Propagation, vol. 47, No. 5, May 1999, pp. 879-886.
[4] Tan-Huat Chio, D.H. Schaubert, “Parameter Study and Design of Wide-Band Widescan Dual-Polarized Tapered Slot Antenna Arrays” IEEE
Trans. on Antennas and Propagation, vol. 48, No. 6, June 2000, pp. 879-886.
[5]
Henrik Holter, Tan-Huat, and Daniel H. Schaubert,
“Elimination of Impedance Anomalies in Single- and Dual-Polarized Endfire Tapered Solt Phased
Arrays” IEEE Trans. On Antennas and Propagation, vol. 48, No 1,January
2000, pp.122-124.
[6] R.Janaswamy, D.H. Schaubert,
“Analysis of th tapered slot antenna” IEEE rans. Antennas and Propagation, 1987, 35,
No9, pp.1058-1065.
[7] C.Icheln. Methods for measuring of RF
radiation properties of small antennas. PhD thesis.
HUT,