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Hybrid waveguide/coaxial feeder for dual-pol
Fabry-Perot cavity antennas radiating
omnidirectional TM/TE waves
Davide Comite, Paolo De Santis, Paolo Burghignoli, Paolo
Baccarelli, and Alessandro Galli
A hybrid waveguide/coaxial TM/TE launcher is proposed as a feeder
for planar Fabry-Perot cavity antennas radiating omnidirectional beams.
The structure is based on a pair of nested coaxial cables. The inner
coax (TM launcher) operates on the standard TEM mode and feeds a
vertical probe protruding into the antenna cavity through the ground
plane. The outer coax (TE launcher) operates on the TE01 mode
excited by an aperture-coupled rectangular-waveguide feeding network.
Geometrical symmetry ensures high isolation between the TM and
TE input ports, which can be alternatively excited to achieve dual-
pol operation with either horizontal or vertical linearly-polarized far
field. Good impedance matching and omnidirectional dual-pol radiative
features are demonstrated for a Fabry-Perot cavity antenna operating at
16 GHz and based on a thin periodic metal partially-reflecting surface.
Introduction: Omnidirectional radiation with conical scannable patterns
is of interest in a variety of applications including, e.g., WLANs or
SATCOM scenarios [1]-[3]. It can be achieved in a simple but effective
way by means of Fabry-Perot (FP) cavity leaky-wave antennas (LWAs),
a class of two-dimensional radiators supporting outward propagating
cylindrical waves [4], [5]. FP cavity antennas are typically formed by
a planar cavity, bounded at the bottom by a metal ground plane and
on top by a partially-reflecting surface (PRS), the latter in the form
of either a uniform dielectric or a thin patterned metal screen [5]. By
exciting such a cavity through a single slot in the ground plane, a linearly-
polarized pencil beam at broadside can be achieved by suitably designing
the cavity thickness [6]. Dual or reconfigurable polarization can then
be achieved by using two mutually orthogonal slots with independent
complex excitations [7].
Omnidirectional scanned beams with conical pattern can also be
produced using the same crossed slots; however, this requires that
their complex amplitudes have the same absolute value and phases in
quadrature, thus leaving no further degrees of freedom for the control
of far-field polarization. This can instead be achieved by independently
exciting purely TM and TE fields, characterized by vertical or horizontal
linearly-polarized patterns, respectively, as shown by the authors in [8]
by means of non-optimized sources.
In this letter we focus on the design and optimization of an integrated
feeding structure with two input ports, capable of independently exciting
azimuthally invariant TM or TE fields inside the antenna cavity, thus
allowing for dual-polarized operation of such antennas (see Fig. 1). To
the best of the authors’ knowledge, no similar realistic feeding structure,
capable to simultaneously excite such invariant TM/TE fields with good
impedance matching, has been proposed so far.
Table 1: Parameters of the structure presented in Fig. 1, design at 16 GHz.
parameter symbol value
inner radius coax1 a10.6 mm
outer radius coax1 = inner radius coax2 b1=a21.425 mm
outer radius coax2 b213.075 mm
coax1 probe length dprobe 12.5 mm
relative permittivity inside coax1,2 εr1,21.06
width RWs aRW 11.0 mm
height RWs bRW 5.0 mm
distance shorted coax bottom/RW center dbottom 10.0 mm
distance antenna ground plane/RW center dtop 33.45 mm
cavity thickness h14.27 mm
relative permittivity inside cavity εrc 1.2
PRS: slot width s 25 µm
PRS: spatial period p3 mm
Structure description: The designed FP antenna cavity is a radial
parallel-plate waveguide whose upper plate is a thin patterned metal PRS
constituted by a 2-D periodic array of rectangular patches (see Fig. 1(d))
with a square unit cell. By operating in the large wavelength regime,
the PRS can be homogenized and represented in the spectral domain
through a surface impedance dyadic exhibiting rotational invariance and
negligible TM/TE coupling [9]. These properties allow us to model the
Fig. 1 (a) 3D view of proposed nested-coaxial structure feeding a Fabry-
Perot cavity antenna: the vertical probe protruding from the inner 50-Ωcoax
excites a TM field inside the cavity and hence produces vertically-polarized
radiation in the far-field region; the outer coax opening on the antenna
ground plane excites a TE field and hence produces horizontally-polarized
far-field radiation. The relevant zoom-in view represents the launcher under
the transparent PRS and ground plane. (b) 2D cross section of the entire
hybrid waveguide/coax feeder, including the relevant rectangular-waveguide
feeding network. (c) 3D perspective view of the designed dual-pol launcher.
(d) Detailed geometry of the homogenizable PRS.
Fig. 2 (a) Magnitude of the tangential field distribution (in dBV/m) on
the ground plane aperture generated by the designed Y-shaped TE-mode
converter. (b) Vector field distribution. The color map illustrates the electric
field of the outer-coaxial cross section on the antenna ground plane.
PRS using two single (generally different) scalar admittance values for
the TM and TE modes supported by the structure, both depending on
the radial wavenumber. (The simple Cartesian pattern used here can
be contrasted with the more complex pattern used in [8], [9], where a
polar arrangement was needed in order to equalize the TM/TE modal
wavenumbers.)
Once the modal spectrum of the FP structure has been described,
the radiative features of the antenna can be completely characterized
by means of the leaky-wave theory [5]. In particular, the design of the
feeding network requires to match the feeder geometry with the modal
field distribution inside the cavity. We consider here the excitation of
TM/TE leaky modes radiating omnidirectional conical beams at an angle
different from broadside: for the TM mode, this is easily accomplished by
means of a vertical coaxial probe penetrating the cavity, which provides
electromagnetic matching with the TM1field vertical profile inside the
structure.
Due to the horizontal distribution of the transverse electric field inside
the cavity, the TE mode excitation is much more challenging. Indeed, to
preserve the azimuthal symmetry, this would require to feed the cavity
with a circular electric ring (coupled with the TE1vertical magnetic
field), which in turn would be excited from the lateral side. Unfortunately,
this would spoil the radiation of the TM contribution, which couples with
the current that flows on the metalization of the horizontal transmission
line exciting the ring inside the cavity (e.g., a horizontal coaxial cable or
a microstrip). In addition, the presence of such a horizontal transmission
ELECTRONICS LETTERS xxth December 20xx Vol. 00 No. 00
15 15.5 16 16.5 17
f [GHz]
-30
-25
-20
-15
-10
-5
0
[dB]
S11 (TM)
S22 (TE)
Fig. 3 Magnitude of the input scattering parameters (in dB): (a) S11,
corresponding to TM excitation (inner coax port), for the optimum value of
dprobe of the inner coax probe; (b) S22, corresponding to TE excitation (RW
port).
line would perturb the azimuthal symmetry of the system. It should also
be mentioned that, if we penetrate the cavity from the bottom with an L-
shaped probe (coupled with the horizontal TE electric field), or from the
lateral side with a horizontal coaxial probe, we also break the azimuthal
symmetry, thus obtaining the dual-pol desired radiation on only one
azimuthal plane. Besides, the former solution would be not compatible
with the presence of the vertical probe exciting the TM field.
Here we propose, instead, a practical and effective solution to excite
TE fields inside the FP cavity, also perfectly integrated with the TM
feeder. We base part of our design on a sidewall TE01 -mode converter
proposed and experimentally validated for the Ka-band excitation of
gyrotron systems in [10]. The mode converter is represented in Figs.
1(a)-(c) and is essentially made by a proper arrangement of rectangular
waveguides (RWs) including a Y-shaped power divider. The TE10 mode
of the rectangular waveguide is injected into port 1 (see Fig. 1(b)) and is
split into four signals of equal amplitude in the power-dividing section.
These four signals, originally led into a circular waveguide [10], are now
driven into a second coaxial cable that contains the internal one exciting
the TM mode (see Fig. 1(b)-(c) for the relevant graphical representation).
The mode-converting section is thus able to generate a highly pure
TE01 higher-order mode in the outer coaxial opening on the ground
plane of the antenna (see Fig. 2), which will then excite the desired
TE azimuthally invariant field inside the FP cavity. We stress that, even
though the original Y-shaped mode converter was designed to excite the
TE01 mode of the circular waveguide, the proposed solution still works
for excitation of the TE01 mode supported by the coaxial cable, which
present a similar distribution of the azimuthal electric field going to zero
on the metalization of the external conductor of the internal coaxial cable.
Input scattering coefficients: The input matching of the hybrid launcher
converter has been properly optmized by operating on its main
geometrical parameters (synthesized in Table 1). The inductive nature of
the input impedance of the TM feeder has been compensated by tuning
its length, starting from an initial value equal to dprobe =λm/4, being
λmthe wavelength inside the medium filling the cavity at the central
operating frequency, assumed here at 16 GHz. Changing dprobe, the
distance between the probe and the top PRS changes accordingly. This
introduces a desired capacitive effect, whose exact value is selected by
means of a parametric analysis; the magnitude of the resulting input
scattering coefficient S11 at the coaxial port 1 is presented in Fig. 3,
showing very good performance around the frequency design. We stress
that this procedure has been developed in the presence of the TE launcher
to account for the mutual coupling, which is anyway minimized thanks to
the geometrical symmetry of the structure. The relevant S12 parameter,
equal to S21 thanks to reciprocity, is not reported in Fig. 3 being below
-80 dB inside all the considered frequency range.
Optimal operation for the TE-mode converter has been obtained by
properly tapering the rectangular sections of the Y-shaped power divider
and adjusting the distance between the bottom short-circuit and the side-
wall rectangular waveguides. The reflection coefficient S22 at the RW
port 2 is also presented in Fig. 3. Both the reflection coefficients are below
-10 dB around the working frequency, providing an operative fractional
bandwidth equal to about 3%.
Dual-polarized antenna operation: To assess the radiative features of
the proposed leaky-wave launcher we consider a FP cavity antenna
Fig. 4 Normalized polar radiation patterns in an arbitrary elevation plane:
(a) TM excitation (inner coax), vertically-polarized electric far field; (b) TE
excitation (outer coax), horizontally-polarized electric far field.
as described in the previous section (with cavity and PRS parameters
reported in Table 1). The modal properties of the considered TM and
TE leaky modes have been obtained by means of a flexible transverse
equivalent network (see, e.g., [8] and references therein). To radiate
the 90% of power provided by the source [5], we have considered the
smallest of the TM/TE attenuation constant values at 16 GHz, which
enforces an aperture radius for the FP antenna equal to 250 mm. In
Figs. 4(a), (b) the normalized TM and TE radiation patterns, respectively,
produced in any elevation plane by the hybrid TM/TE launcher, are
shown at the design frequency of 16 GHz. The maximum directivities for
the TM and TE conical beams are 10.5 and 11.3 dBi, respectively. The
expected dual-pol omnidirectional behavior of the FP antenna is validated
and in full agreement with the leaky-wave theory [5].
Conclusion: The design of an original integrated TM/TE feed for
launching omnidirectional conical beams in FP cavity structures, with
a dual reconfigurable polarization state, has been described in this letter.
Its performance has been tested in conjunction with an optimized Fabry-
Perot cavity antenna supporting a pair of azimuthally invariant TM and
TE cylindrical leaky modes. The effectiveness in terms of input matching
and dual-pol operation has been assessed through full-wave simulations.
The proposed antenna can be particularly useful for advanced indoor
WLAN communications devices and for surveillance as well as satellite
applications.
E-mail: davide.comite@uniroma1.it
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