Universidad Nacional de Chimborazo
NOVASINERGIA, 2018, Vol. 1, No. 2, junio-noviembre, (30-37)
ISSN: 2631-2654
https://doi.org/10.37135/unach.ns.001.02.03
Research Article
Cross polarization in microwave antennas: Case study of a circular
waveguide
Polarizaci
´
on cruzada en antenas de microondas: Caso de estudio de una gu
´
ıa de
onda circular
Daniel Santill
´
an-Haro
1,2
*, Eva Antonino-Daviu
2
, Miguel Ferrando Bataller
2
, Daniel
S
´
anchez-Escuderos
2
, Diana Navarro-M
´
endez
3
, Fernando Carrera-Su
´
arez
3
1
Facultad de Ingenier
´
ıa, Universidad Nacional de Chimborazo, Riobamba, Ecuador
2
Instituto de Telecomunicaciones y Aplicaciones Multimedia, Universitat Polit`ecnica de Val `encia, Valencia, Spain;
evanda@upvnet.upv.es; mferrand@dcom.upv.es; dasanes1@gmail.com
3
Escuela Polit
´
ecnica Nacional, Quito, Ecuador; veronica.navarro@epn.edu.ec; fernando.carrera@epn.edu.ec
* Correspondence: dsantillan@unach.edu.ec
Recibido 15 octubre 2018; Aceptado 30 noviembre 2018; Publicado 10 diciembre 2018
Abstract:
During the last decade, the subject of antenna polarization attracts great interest. The
improve of the cross polarization generates a distribution of a highly symmetric field in
the aperture of the antenna, however, it is difficult to make low cross polarization cir-
cular sources to operate over a wide bandwidth. In this paper, a study case is presented
to improve the cross polarization of a circular aperture. As the circular waveguide has
a poor cross polarization, three possible solutions have been considered to improve the
cross polarization in previous works. The first solution is based on a metallic disk,
which after an optimization process obtains an external diameter of 90.42mm. The sec-
ond solution is based on a metallic ring around the circular waveguide, the optimized
dimension of the external diameter ring is 90.42mm with a thickness of 1.63mm. The
third solution is based on two rings around the circular waveguide, which optimized
dimensions of the rings are an outer diameter of 122.48mm, and an inner diameter of
90.42mm. This knowledge is valuable to explain the improvement of matching level of
a patch antenna with a ring around 5.8 GHz.
Keywords:
Circular polarization, cross polarization, metallic rings, numerical analysis, patch
antenna.
Resumen:
Durante la
´
ultima d
´
ecada, el tema de la polarizaci
´
on de la antena atrae gran inter
´
es.
La mejora de la polarizaci
´
on cruzada genera una distribuci
´
on de un campo altamente
sim
´
etrico en la apertura de una antena, sin embargo, es dif
´
ıcil hacer fuentes circulares
de baja polarizaci
´
on cruzada que operen en un ancho de banda amplio. En este art
´
ıculo
se presenta un caso de estudio para mejorar la polarizaci
´
on cruzada de una apertura
circular. Como la gu
´
ıa de onda circular tiene una polarizaci
´
on cruzada pobre, se han
considerado tres soluciones posibles para mejorar la polarizaci
´
on cruzada, que ha sido
presentada en trabajos anteriores. La primera soluci
´
on se basa en un disco met
´
alico
que, despu
´
es de un proceso de optimizaci
´
on, se obtiene un di
´
ametro externo optimizado
de 90.42 mm. La segunda soluci
´
on se basa en un anillo met
´
alico alrededor de la gu
´
ıa
de onda circular, la dimensi
´
on optimizada del anillo de di
´
ametro externo es de 90.42
mm con un grosor de 1.63 mm. La tercera soluci
´
on se basa en dos anillos alrededor de
la gu
´
ıa de onda circular, cuyas dimensiones optimizadas son un di
´
ametro exterior de
122.48 mm y un di
´
ametro interno de 90.42 mm. El conocimiento obtenido es valioso
para explicar la mejora del nivel de adaptaci
´
on de una antena de parche con un anillo
de alrededor a 5.8 GHz.
Palabras clave:
Polarizaci
´
on circular, polarizaci
´
on cruzada, anillos met
´
alicos, an
´
alisis num
´
erico,
antena de parche.
http://novasinergia.unach.edu.ec
1 Introduction
For more than a decade, the subject of antenna po-
larization has generated great interest. The defi-
nition can be complex as radiating and receiving
structures respond differently, both in frequency
and angle between the incident and transmitted
wave (Thornton & Huang, 2013). When antenna
patterns are taken in the usual way, the Ludwig
definition is commonly used. In (Ludwig, 1973)
the definitions of co-polarization (reference polar-
ization), and cross polarization applied to linearly
polarized antennas are presented. An antenna radia-
tion in a specified polarization, which is called refer-
ence polarization; whereas radiation in the orthogo-
nal polarization is known as cross-polarization (Az-
nar et al., 2004). In addition, Ludwig’s defini-
tions are clarified using different polarized refer-
ence sources located linearly in different orienta-
tions (Aboserwal et al., 2018). The low cross po-
larization generates a distribution of a highly sym-
metric field in the aperture, however, it is difficult
to achieve a reduction of cross polarization over a
wide bandwidth.
Alternatively, a wideband approach for significantly
reducing the cross-polarization levels of standard-
gain horns, for an X-band standard from 6.5 GHz
to 18 GHz, applying filter screens has been pro-
posed (Kuloglu & Chen, 2013). In addition, to im-
prove the cross-polarization performance, two pairs
of arc-shaped slots have been added to the ring slot
antenna, for a circular aperture antenna with dif-
ferential feeding (Chen et al., 2017). Also, a de-
fected ground structure with integrated square patch
has been proposed (Kumar et al., 2015). The de-
fect is deployed surrounding the element maintain-
ing a considerable spacing from the patch boundary.
Moreover, the undesired cross polarization of the re-
flector antennas, can be reduced with a circular-rim
(Pour & Shafai, 2012). In this paper, we propose
the analysis and simulation of a circular waveguide
with the addition of metallic rings as elements to
improve the radiation pattern in the 3−7 GHz band.
The simulation of the antenna has been done with a
CST frequency domain solver (Dassault Systemes,
2018). Finally, to validate the concept, metallic
square rings are added in the design of a patch an-
tenna at 5.8 GHz.
2 Modeling a circular waveguide
with metallic rings
Figure 1: Geometry of the circular waveguide.
2.1 Circular waveguide
The main parameters of a circular waveguide are the
length of the enclosing metallic wall (L), and the
inner diameter of the circular waveguide (D
i
). The
thickness of the waveguide (t) does not affect the
bandwidth, therefore a standard waveguide WR-229
can be used, which thickness is 1.63 mm.
The cutoff frequency of the circular waveguide is
determined by the equations (1) y (2) (Balanis,
2012), where ρ is the radius of the waveguide, χ
mn
are the roots of Bessel functions and χ
0
mn
are the
roots of the derivative of the Bessel functions.
f
c
|T E
mn
=
1
2π
√
µε
χ
0
mn
ρ
(1)
f
c
|T M
mn
=
1
2π
√
µε
χ
mn
ρ
(2)
Taking into account that the waveguide is mod-
eled as PEC (perfectly electrical conductor), the
dominant circular waveguide mode is T E
11
(Pozar,
2009), and the following mode is T M
01
. The cut-
off frequencies of these modes can be obtained with
equations (1) and (2).
For a cutoff frequency of 3.25 GHz for mode T E
11
,
it can be deduced from (1) that the radius of the
waveguide must be ρ = 27.03 mm. Table 1, shows
the values of the cutoff frequencies for the different
modes of the circular waveguide with this radius.
The geometry of the circular waveguide is depicted
in figure 1, with an inner diameter D
i
= 54.06 mm
(0.65 λ), an external diameter D
o
= 57.32 mm
(0.69 λ), and a length L = 25 mm, where λ is
the free-space wavelength at the design frequency
( f = 3.625 GHz).
To validate the performance of the circular waveg-
uide, the S
11
parameter is simulated. As can be ob-
http://novasinergia.unach.edu.ec 31
Table 1: Modes of a circular waveguide.
Modo f/ f
c
|T E
11
F(GHz)
T E
11
1 3.25
T M
01
1.3 4.245
T E
21
1.66 5.395
T E
01
2.08 6.76
T M
11
2.08 6.76
T E
31
2.28 7.41
T M
21
2.79 9.07
3 3.5 4 4.5 5 5.5 6 6.5 7
−35
−30
−25
−20
−15
−10
−5
0
Frequency (GHz)
S
11
(dB)
Circular waveguide
Figure 2: Simulated S
11
parameter of the circular waveg-
uide.
served in figure 2, the result show a good matching
(S
11
< −10 dB) from 3.5 to 7 GHz.
2.2 Analysis with metallic rings
As the circular waveguide has a poor cross polariza-
tion, three possible solutions have been considered
to improve the cross polarization (Santillan, 2017).
The first solution is to add a metallic disk as shown
in figure 3(a). After an optimization process the
value of the external diameter is D
1
= 90.42 mm.
The second solution is to place a metallic ring
around the circular waveguide (see figure 3(b)). The
optimized dimension of the external diameter ring is
D
1
= 90.42 mm with a thickness of 1.63 mm.
A third solution is to place 2 rings around the cir-
cular waveguide as indicated in figure 3(c). The
optimized dimensions of the rings are: an outer di-
ameter of D
2
= 122.48 mm, and an inner Diameter
D
1
= 90.42 mm. Note that the thickness of the rings
has the same value as the second solution.
(a)
(b)
(c)
Figure 3: Proposed solutions to improve cross polariza-
tion in a circular waveguide. (a) Metallic disk. (b) Metal-
lic ring. (c) Two metallic rings.
http://novasinergia.unach.edu.ec 32
(a) (b)
(c)
(d)
Figure 4: Surface current distribution at the design fre-
quency ( f = 3.625 GHz). (a) Circular waveguide. (b)
Metallic disk. (c) Metallic ring. (d) Two metallic rings.
3 Simulated results and validation
with a patch antenna
Figure 4 shows the surface current distributions of
the proposed solutions. It is clear that the currents
are concentrated around the internal diameter D
i
of the metallic disk (see figure 4(b)). Figs. 4(c)
and 4(d) show the total current in one and two rings,
respectively. Observe that the distribution of the
current in the circular waveguide is the same as in
the metallic ring, while in the configuration of two
rings, the current densities have the same direction
in the internal radius, and in the outer radius the
(a) (b)
(c)
Figure 5: Electric Field distribution at the design fre-
quency ( f = 3.625 GHz) of the circular waveguide. (a)
Metallic disk. (b) Metallic ring. (c) Two metallic rings.
Figure 6: Simulation of the radiation pattern of the pro-
posed configuration at 3.625 GHz.
currents are opposite and small. In figure 5, the
simulated E-field distribution of the circular waveg-
uide with metallic rings is presented. As can be ob-
served, the electric field distribution in the metallic
rings is symmetric, which gives an improvement in
the radiation diagram (figure 6).
http://novasinergia.unach.edu.ec 33
−180−150−120 −90 −60 −30 0 30 60 90 120 150 180
−150
−140
−130
−120
−110
−100
−90
−80
−70
−60
−50
−40
−30
−20
−10
0
θ (deg)
|E| (dB)
Cop.WGwith1ring Cop. WGwithdisk
Cop. WGwith2rings
Cop.WG
Crossp.WGwith1ring Crossp.WGwithdisk
Crossp.WGwith2rings
Crossp.WG
Figure 7: Copolar (Cop.) and cross-polar (Crossp) components of the radiation pattern in the xz plane at 3.625 GHz for the
proposed configurations.
Figure 8: Path antenna. (a) Simulated with HFSS. (b) Large antenna built (M
´
endez, 2015).
Figure 9: Modified path antenna. (a) Scheme. (b) S
11
parameter (M
´
endez, 2015).
http://novasinergia.unach.edu.ec 34
Figure 10: Prototype of the patch antenna with a square metallic ring around it (M
´
endez, 2015).
Figure 11: Electrical dimensions of the patch antenna with two metallic rings (M
´
endez et al., 2013).
Figure 12: Picture of the antenna with two square metallic rings (M
´
endez et al., 2013).
http://novasinergia.unach.edu.ec 35
Figure 7 shows the copolar and cross−polar com-
ponents of the radiation pattern on the xz−plane
at 3.625 GHz. As it can be seen, the cross-polar
component is 50 dB below the copolar component
within the main beam.
3.1 Patch antenna with a surrounding
metallic ring
A patch antenna type is the thinnest element for cir-
cular polarization (M
´
endez, 2015). For implemen-
tation it is considered an antenna with dimensions
as shown in figure 8.
The results of the large antenna have a great sim-
ilarity but increasing the size adversely affects the
behavior of the diagram and polarization, so it is
necessary to redesign.
3.1.1 Design modifications in the path antenna
Considering that electromagnetic band gap (EBG)
structures are used to suppress the effects of surface
waves (Cho & Lee, 2010), a metallic ring surround-
ing the original small ground plane is inserted, as
can be seen in figure 9. The behavior of this square
ring is similar to the one presented previously for
circular rings. By varying the width of the metallic
ring, and the distance from the plane, a good match-
ing level is obtained (see figure 9 (b)).
A prototype of the path antenna with a surround-
ing metallic ring was fabricated and measured. A
picture of the fabricated patch antenna is shown in
figure 10.
As a second improvement, the ring thickness of fig-
ure 10 is replaced by another 2 thinner rings. The
new dimensions are shown in figure 11. With the
variables S
1
and S
2
we can tune the resonance fre-
quency. If the spacing between the inner ring and
dipoles is less than 0.20λ there is a frequency shift
of 1% (approximately). For values of S
1
of 0.40 λ
the bandwidth is increased by about 10% compared
to the bandwidth of the dipoles without rings.
To improve the matching level, a circular groove
(3 mm of diameter and 0.2 mm of width) located
in the ground plane around the inner conductor can
be introduced. The manufactured antenna with two
surrounding metallic rings is shown in figure 12 (a).
By replacing the large ring by 2 thinner rings, a
bandwidth of 7.6% was obtained. In addition, a
good matching level (S
11
<-20dB) in the whole
5.5 − 6.10 GHz band (see figure 12 (b)) was ob-
tained.
4 Conclusion
In this work, the improvement of cross polarization
were obtained through metallic rings around a mi-
crowave antenna. An open-ended circular waveg-
uide was used as the primary feed for the metal-
lic rings. The simulated structures provide a good
cross-polar level (better than -50 dB) with a good
matching level (S
11
< −14 dB) from 3.5 to 7 GHz.
To verify the design, a patch antenna with surround-
ing metallic rings was presented. Measured results
show an optimum coupling, within a wide range of
frequencies from 5.5 to 6.10 GHz.
Interest Conflict
The authors declare that they have no conflicts of
interest.
Acknowledgment
This work has been supported by the Spanish
Ministry of Science, Innovation and Universities
(Ministerio de Ciencia, Innovaci
´
on y Universi-
dades) under the projects TEC2016-79700-C2-1-
R, TEC2016-78028-C3-3-P and college scholarship
graduate of the National University of Chimborazo.
References
Aboserwal, N. A., Salazar, J. L., Ortiz, J. A., D
´
ıaz, J. D.,
Fulton, C., & Palmer, R. D. (2018). Source current
polarization impact on the cross polarization defini-
tion of practical antenna elements: Theory and appli-
cations. IEEE Transactions on Antennas and Propaga-
tion, 66(9), 4391–4406.
Aznar,
´
A., Robert, J., Casals, J., Roca, L., Boris, S.,
& Bataller, M. (2004). Antenas. Spain: Uni-
versidad Polit
´
ecnica de Catalunya. Iniciativa Digital
Polit
´
ecnica.
Balanis, C. A. (2012). Advanced engineering electromag-
netics. New York: John Wiley & Sons.
Chen, C., Li, C., Zhu, Z., & Wu, W. (2017). Wideband
and low cross polarization planar annual ring slot an-
tenna. IEEE Antennas and Wireless Propagation Let-
ters, 16, 3009–3013.
Cho, T. & Lee, H. (2010). Dual-band surface wave sup-
pression using soft surface structure. Paper presented
at Antennas and Propagation (EuCAP), 2010 Proceed-
ings of the Fourth European Conference, Retrieved
from https://ieeexplore.ieee.org/.
http://novasinergia.unach.edu.ec 36
Dassault Systemes (2018). CST Microwave Studio.
https://www.cst.com.
Kuloglu, M. & Chen, C.-C. (2013). Ultra-wideband
electromagnetic-polarization filter applications to
conventional horn antennas for substantial cross-
polarization level reductions. IEEE Antennas and
Propagation Magazine, 55(2), 280–288.
Kumar, C., Pasha, M. I., & Guha, D. (2015). Microstrip
patch with nonproximal symmetric defected ground
structure (dgs) for improved cross-polarization prop-
erties over principal radiation planes. IEEE Antennas
and Wireless Propagation Letters, 14, 1412–1414.
Ludwig, A. (1973). The definition of cross polariza-
tion. IEEE Transactions on Antennas and Propagation,
21(1), 116–119.
M
´
endez, D. V. N. (2015). Nuevos sistemas radiantes real-
izados con tecnolog
´
ıas impresas. PhD thesis, Editorial
Universitat Polit `ecnica de Val `encia.
M
´
endez, D. V. N., Su
´
arez, L. F. C., & Escudero, M. B.
Circular polarization patch antenna with low axial ra-
tio in a large beamwidth. In Antennas and Propaga-
tion (EuCAP), 2013 7th European Conference on, pp.
3330–3333. IEEE.
Pour, Z. A. & Shafai, L. (2012). A simplified feed
model for investigating the cross polarization reduc-
tion in circular-and elliptical-rim offset reflector anten-
nas. IEEE Transactions on Antennas and Propagation,
60(3), 1261–1268.
Pozar, D. M. (2009). Microwave Engineering. New York:
John Wiley & Sons.
Santillan, D. (2017). Theses: Dise
˜
no de antenas directi-
vas de banda ancha a frecuencias de microondas. Uni-
versitat Polit `ecnica de Val `encia.
Thornton, J. & Huang, K.-C. (2013). Modern lens an-
tennas for communications engineering, volume 39.
USA: John Wiley & Sons.
http://novasinergia.unach.edu.ec 37
