CC BY
73
K band Antennas Conjugated with a Metal
Waveguide
Maksym Khruslov, Member, IEEE
Abstract—Two planar spiral antennas conjugated with the standard metal waveguide and operating in the millimeter range are proposed. The first antenna has the bandwidth of 6.12GHz and second 5.93GHz. The axial ration of both antennas does not exceed -3dB in the frequency band 32.4 - 37 GHz. The elevation angle of peak directivity close the zenith for single planar spiral antenna, is oriented and at 0=20° for the double planar spiral antenna. The radiation pattern shape is practically the same within the operational frequency band for both antennas. The nature of axial ratio changing is explained from the analysis of near-field distributions at two orthogonal polarizations. The proposed antennas can be used for different practical applications in the millimeter range.
Index Terms—spiral antenna, millimeter range, impedance transformer, near-field distribution, axial ratio
I. Introduction
' I 'HE different types of planar spiral antennas are usually excited by a co-axial feed line with various balun circuits [1-4]. Moving towards the higher frequencies the radiating area of the spiral antenna is reduced. However, the disadvantages of such designs include the fact that in the millimeter range they will have the great losses due to the long feed path. The original planar single-arm fractional spiral antenna with the aforementioned exciting method was reported in [5]. A simple antenna design and the good expected performance seem to be very attractive for its millimeter wave applications by taking into account a possibility to be easily integrated with RF components or MMICs on the same printed circuit board. For good matching of feeding 50-Ohm coaxial cable with the antenna in the wide frequency band the different approaches are used [4, 5]. At the same time, the variety of practical applications of planar spiral antennas sometimes has specific requirements to the feeding or receiving networks, in particular, when the antenna should be loaded with the standard waveguide [6].
In this article, we present the two types of antennas with circular polarization and different radiation patterns: first antenna produce the radiation pattern with maximum radiation close to the zenith, and second antenna with omniderectional radiation pattern (minima radiation to the zenith) for the wireless applications in the millimeter range.
Manuscript received December 21, 2013.
Khruslov M.M. is with the Usikov Institute for Radiophysics and Electronics of the National Academy of Sciences of Ukraine, 12 Ak. Proskura St., Kharkov, 61085, Ukraine, tel. +38 (057) 7203594, fax. +38 (057) 3152105 (e-mail: ireburan@yahoo.com)..
II. Antenna Design
Figure 1 shows the antennas geometry. Both spiral antennas 1 are printed on the low losses dielectric substrate 2. First planar spiral (called single planar spiral) (Fig 1a) is described by two equations (1):
x(t)=aebtcos(t) y(t)=aebtsin(t) (1)
Second planar spiral (called double planar spiral) is consist of two spirals noted above (Fig. 1b). Parameters a and b are changed in the simulation process in such limits: 0.0005 <aj<0.0015, 0.1 <bi=<0.2; 0.001<a2<0.002, 0.1 <b2<0.25, 0<t<2n.
Planar spiral is located on the output aperture of the conical wave impedance transformer 3, which is similar to that described in [7] (Fig. 1c). It is a truncated hollow metal cone filled with a low-loss dielectric. The spiral antenna is excited by a co-axial line and coupled with the standard rectangular waveguide by means of the coaxial-to-waveguide transition 4. In this case the adjustment of the coupling coefficient is implemented by a shorting piston 5. In simulations the height H2 of the cone, the diameters Dj of the cone, and the relative dielectric permittivity of the transformer filling e are varied within the following limits: 4mm<H2<16mm;
6mm<Di<16mm; 1.07<et<3.8. Cross-section of the smaller cone base has the diameter D2 and corresponds to the crosssection of the feeding coaxial cable.
Fig. 1. Geometry of the proposed antennas: single planar spiral (a); double planar spiral (b); antenna with waveguide (c)
Ill. RESULTS AND DISCUSSIONS
In simulations the antenna characteristics such as: input reflection coefficient Sn, radiation patterns, axial ratio, and near-field distributions were studied. As can be seen from the Figure 2a, for first antenna the -10dB return loss bandwidth ranges from 31 to 37.5 GHz excluding the frequency band 32<f<34GHz. Two bandwidth for the double planar spiral antenna is observed 30.05-31.8 GHz and 32.46-36.64 GHz (Fig. 2b).
b
Fig. 2. Simulated return loss of two antennas: single planar spiral antenna (a) double planar spiral antenna (b)
The simulated radiation patterns of single spiral antenna for two basic planes are virtually the same within the operational frequency band. At that, the elevation angle of peak directivity is oriented close the zenith and slightly changed within the working band. For example, the radiation pattern at 35GHz is shown in the Figure 3a. The beamwidths for both basic plane are A0=29° . For the double planar spiral antenna the elevation angle of peak directivity is oriented at 0max=±220 and 0max=±250 for two basic planes (Fig 3b).
Following from simulations the axial ratio is less than 3dB in the frequency band 34.2<f<37GHz for both antennas (Fig. 4).
The influence of edge effects as well as impedance matching of the antenna input with the feeding line on the antenna performance can be studied by applying near-field technology [8]. Therefore, in order to explain the axial ratio behavior within the operational frequency band the simulations of near-field distributions for both orthogonal components have been carried out. The similar near-field distributions on both orthogonal components and the location of areas with the high intensity of the electromagnetic field close the spiral arm guarantee the circular polarization of the antenna radiation. Indeed, in the frequency band 34.2<f<37GHz the near-fields have the spatial distributions shown as the pictures are presented in the Figure 4.
a
b
Fig. 4. Near-field distributions on the Ex (a) and Ey (b) components for single planar spiral antenna at f=35GHz
We are noted, that maxsima intensity of the near field distributions for both components of electromagnetic fields and for total field is situated on the spiral surface (Fig. 5).
c
Fig. 5. Near-field distributions on the Ex (a) and Ey (b) components and E-total (c), for double planar spiral antenna f=35GHz
Fig. 3. Simulated radiation patterns at the frequency f=35GHz for two
antennas: single planar spiral antenna (a), double planar spiral antenna
iv. Experimental Results
Both antenna prototypes were manufactured and tested. In Fig. 6 the antennas are presented.
Fig.6 Two antenna prototypes: single planar spiral antenna (a); double (b)
Input reflection coefficient Sii, radiation patterns, axial ratio were measured. in the case of single planar spiral antenna the -10dB return loss bandwidth ranges from 32.88GHz to 39GHz (Fig. 7a). The maximum efficiency is observed at the f=35.2GHz (-27dB). In case of double planar spiral antenna the -10dB return loss bandwidth ranges is f=31.37GHz -v f=36.6GHz, the maximum efficiency is observed at the f=34.9GHz (-49dB)
a
Fig. 7 Measured return loss of two antennas: single planar spiral antenna (a) double planar spiral antenna (b)
In case of single planar spiral antenna the monobeam radiation pattern is observed in all working frequency band. The radiation pattern at the f = 35.2GHz for the both of measured components (Ex and Ey) characterized by one beam slightly shifted from the zenith direction (Fig. 8a). Here the axial ratio is changed from 9dB at f=27GHz where polarization is linear to 3dB in operating frequency band f=32GHz -v f=38.8GHz (Fig. 9a). Qualitative agreement between the experimental near-field (Fig. i0) and simulation results (Fig. 4) confirm the presence of circular polarization of the under test antenna.
The elevation angle of peak directivity for double planar spiral antenna is oriented at 0max=±2O0 for Ex and Ey components (Fig. 8b). The circular polarization have to observe within working frequency band, but in experiment the axial ration less than 3dB from 32.4 - 37.1GHz (Fig. 9b).
Fig. 8. Measured radiation patterns of two antennas for Ex and Ey components; single planar spiral antenna f=35.2GHz (a) and double planar spiral antenna f=34.9GHz (b)
Fig. 9. Experimental axial ration of the single spiral antenna (a), double spiral antenna (b)
a b
Fig. 10. Measured near-field distributions on the Ex (a) and Ey (b) components for single planar spiral antenna f=35.2GHz
Near field distribution for the double spiral antenna were measured by waveguide probe in the plane XY, when the vector E of the probe was in the plane Y = X (Fig. 11a) and Y =-X (Fig. 11b). The obtained pattern in the near field zone exhibit substantially uniform distribution of intensity (Fig. 11). Distribution data confirm the numerical simulation result (Fig. 5). Presented distributions confirm the veracity of the experimental axial ratio. The obtained distributions show that the double spiral antenna has a circular polarization.
a
b
Fig. 11. Measured vector E near-field distributions in the plane Y = X (a) and in the plane Y = -X (b) components for double planar spiral antenna
v. Conclusions
Two planar spiral antennas conjugated with the standard metal waveguide and operating in the millimeter range have been proposed. Following the results of simulations, the optimal physical and geometric parameters of both antennas have been determined. The prototypes have been manufactured and tested. As the result, the measured return
loss coefficients point out the -10dB impedance bandwidth of 6.12GHz and 5.93GHz for the single and double planar spiral antennas respectively.
The radiation pattern of the single spiral antenna is characterized by the wide lobe with the elevation angle of peak directivity near the antenna axis. For the double spiral antenna we obtained the omnidirectional radiation pattern in the azimuth plane with the elevation angle of peak directivity oriented at 0max=±200 and with the minimum intensity in the antenna axis.
The axial ration does not exceed -3dB in the frequency band f=32GHz 38.8GHz for the single spiral antenna and f=32.4GHZ 37.1GHz for the double spiral antenna. The
different values of the axial ration and the nature of their changing in the impedance bandwidth of both antennas become apparent following the analysis of measured nearfield distributions.
The proposed antenna can be used for different practical applications in the millimeter range.
References
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[6] Roman Chernobrovkin, Christophe Granet, Vladimir Khaikin, and Nina Popenko. Compact Efficient Feed-Horn at 30-38 GHz for a Multi-beam Radio Telescope. - J Infrared Milli Terahz Waves DOI10.1007/s10762-010-9652-x , Springer, May 2010, - 12pages.
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Maksym Khruslov was born in Kharkov, Ukraine, in 1982. He received the M.S. degree in radiophysics and electronics from Karazin Kharkov National University (Ukraine) in 2004. Since 2004, he works in the Institute for Radiophysics and Electronics of the National Academy of Sciences of Ukraine, Kharkov, Ukraine. He received the Ph.D degrees in Radiophysics in 2012. From 2013 he is a Researcher with the Department of Radiospectroscopy in the IRE NASU. with the Radiospectroscopy Department. His research interest includes the near-field technology, computational modeling of microwave antennas. Mr. Maksym Khruslov is a Member of IEEE, and a Member of EuMa.
