Improved Vlasov antenna with curved cuts for high power microwaves

Abstract

This paper presents an improved Vlasov antenna with a novel cut shape suitable for high power microwave applications. The curved shape of the proposed cut totally eliminates the sharp edges and angles present in Vlasov antennas with Step and Bevel cuts. A Vlasov antenna, designed for operation at 3 GHz, is used to compare the three cut types. In addition to being better suitable for high power microwave applications, the proposed cut results in increased antenna gain and in good performance in terms of sidelobe level and half-power beamwidth.

Full text

Improved Vlasov Antenna with Curved Cuts for
High Power Microwaves
H. M. El Misilmani1,2, M. Al-Husseini2, K. Y. Kabalan1, and A. El-Hajj1
1ECE Department, American University of Beirut, Beirut, Lebanon
2Beirut Research and Innovation Center, Lebanese Center for Studies and Research, Beirut, Lebanon
hme36@aub.edu.lb, husseini@ieee.org, kabalan@aub.edu.lb, elhajj@aub.edu.lb
Abstract—This paper presents an improved Vlasov antenna
with a novel cut shape suitable for high power microwave applica-
tions. The curved shape of the proposed cut totally eliminates the
sharp edges and angles present in Vlasov antennas with Step and
Bevel cuts. A Vlasov antenna, designed for operation at 3 GHz, is
used to compare the three cut types. In addition to being better
suitable for high power microwave applications, the proposed
cut results in increased antenna gain and in good performance
in terms of sidelobe level and half-power beamwidth.
Keywords—Mode converters, high power microwaves, Vlasov
antenna, bevel cut, step cut.
I. INTRODUCTION
High Power Microwave (HPM) sources, such as the
backward-wave Oscillator (BWO), the gyrotron, and the virca-
tor (virtual cathode oscillator), generate power in cylindrically
symmetric transverse electric (TE0n) or transverse magnetic
(TM0n) modes. The side lobe generation, gain reduction, and
inefficient power loading on the antenna aperture, make these
modes unsuitable for driving conventional antennas. This gave
the idea of using mode converters at the output of these sources
to convert these modes into a plane-parallel linearly polarized
beam. A Vlasov antenna [1], which is one of the most known
mode converters used, is composed of a cylindrical waveguide
with a shaped end, which can directly radiate energy from
cylindrically symmetric modes in circular waveguides, without
the need for an additional mode converter.
The two well-known Vlasov antenna types come with a step
cut and with a beveled cut [2]. The first type, a waveguide
aperture with a step cut, originally suggested by Vlasov, has
sharp edges and therefore may suffer from electrical break-
down when radiating HPM. The beveled cut, was suggested
by Nakajima to avoid the sharp points of the step cut, leading
to a more suitable shape for usage in HPM applications. This
observation has been reported in [3].
Several studies have been made to increase the gain of the
bevel-cut and step-cut Vlasov antennas. In [4], a bevel-cut
Vlasov antenna with a reflector is proposed to better direct
the main beam and increase the gain. In [5], two methods
are used for increasing the gain of a bevel-cut Vlasov, one is
based on a parabolic cylinder reflector, and the second on a
horn on the aperture. In [6], the step cut has been studied in
the presence of a parabolic reflector.
In this paper, we first design a bevel-cut Vlasov operating
at 3 GHz with the aim of obtaining maximum gain. Next,
a step-cut version is designed to have radiation in the same
direction as the bevel-cut counterpart, and a comparison of the
performance of both conducted. Later, a new cut shape, better
suitable for the use of Vlasov antennas in HPM applications
is presented and its advantages are reported.
II. VL AS OV ANT EN NAS
Both step- and bevel-cut Vlasov antennas are the result of
shaping the end part of a circular waveguide. For operation at
3 GHz, the used circular waveguide has a radius of 45 mm
and a length of 300 mm.
A. Vlasov with Bevel Cut
A Vlasov antenna with a beveled cut is shown in Fig.
1(a). The cut angle αis the single parameter available for
optimization, and it has the main effect on the gain and
radiation patterns of the antenna. The angle that maximizes
the gain of the antenna is given by [2]:
α=sin−1((ρ0nλ)/(2πa)),(1)
(a) Bevel-cut design
(b) Step-cut design
Figure 1. Configurations and parameters of bevel- and step-cut Vlasov
antennas

where ρ0nis the n-th root of the equation J0(ρ0n) = 0,λ
is the wavelength, ais the inner radius of the waveguide,
and J0is the Bessel function of the first kind and zeroth order.
For the T M01 circular waveguide designed for 3 GHz, a=
4.5cm and λ= 10cm. Also, ρ01 = 2.405, so the bevel cut
will be calculated as follows:
α=sin−1((2.405 ×10)/(2π×4.5)) = 58.32 ◦.(2)
The highest gain according to the equation is obtained
at a cut angle of 58.32 ◦. This result has been verified by
simulations using ANSYS HFSS [7]. For this angle, the
resulting peak gain is 10.9 dB. The gain patterns, computed
in CST MWS [8] for both φ= 0◦and φ= 90◦planes
are shown in Fig. 2. Maximum radiation is obtained in the
θ= 32◦, φ = 90◦direction.
(a) φ= 0◦plane
(b) φ= 90◦plane
Figure 2. Gain patterns of bevel-Cut design
B. Vlasov with Step Cut
A Vlasov antenna with a beveled cut is shown in Fig. 1(b).
The step cut is determined by the two parameters, A and B, as
indicated. The value of A is fixed at 148.5 mm, which is the
same value obtained with the bevel cut after finding the angle
α. For comparison purposes, the step-cut Vlasov is designed so
that it has the same direction of maximum radiation obtained
with the beveled cut. For this purpose, via CST simulations,
B is found to be 25 mm. It was noticed that, as the value of
B decreases, the angle of maximum radiation increases. The
gain patterns of the step-cut Vlasov antenna, in the φ= 0◦and
φ= 90◦planes, are shown in Fig. 3(a) and 3(b) respectively.
As is the design aim, the main lobe peaks at θ= 32◦.
(a) φ= 0◦plane
(b) φ= 90◦plane
Figure 3. Gain patterns of step-Cut design
Comparing the two Vlasov antenna types, it is verified that
the step cut gives better performance in term of maximum gain
and half-power beamwidth (HPBW). However, the beveled cut
results in lower side lobes, and this comes in addition to its
suitability for HPM applications.
III. IMP ROVE D CUT
The beveled cut was introduced to avoid the sharp edges
present in the step cut, but it results in decreased gain and
increased HPBW of the antenna. A new cut shape, shown in
Fig. 4, is proposed here. This shape, totally based on curves,
goes ahead of the beveled cut in removing the sharp edges
and corners, and is as a result better suitable for applications
involving HPM.
In this design, we have the flexibility to optimize several
parameters to reach the desired gain, HPBW, and direction of
maximum radiation. These are the radius of Curve 1 (R1), the

(a) 3D view
(b) Parametrized dimensions
Figure 4. Vlasov antenna with proposed cut
radius of Curve 2 (R2), the radius of Curve 3 (R3), and the
separation between Curves 2 and 3, noted by L on Fig. 4.
To compare it to the step- and bevel-cut versions, a Vlasov
antenna based on the same waveguide and on the proposed
cut is designed so that it has the maximum radiation in the
same θ= 32◦, φ = 90◦direction. Table I lists the obtained
parameters of the new design. The resulting patterns are shown
in Fig. 5.
(a) φ= 0◦plane
(b) φ= 90◦plane
Figure 5. Gain patterns of the proposed Vlasov antenna
The gain patterns resulting from the step, the beveled and
TABLE I
PARA MET ER S OF TH E PRO PO SED VL AS OV ANT EN NA
Parameter Value (mm) Parameter Value (mm)
Waveguide Radius 45 R2 24.5
Waveguide Length 300 R3 41
R1 24.5 L 65
the proposed Vlasov cuts are compared in Fig. 6. The obtained
peak gain values are given in Table II. Both CST and HFSS
reveal that the proposed Vlasov antenna has a higher peak
gain, which overcomes the decreased gain issue the bevel cut
has.
-25
-20
-15
-10
-5
0
5
10
-150 -100 -50 0 50 100 150
Absolute Gain [dB]
Theta [deg]
Step Cut
Bevel Cut
New Cut
(a) φ= 0◦plane
-25
-20
-15
-10
-5
0
5
10
-150 -100 -50 0 50 100 150
Absolute Gain [dB]
Theta [deg]
Step Cut
Bevel Cut
New Cut
(b) φ= 90◦plane
Figure 6. Comparison of the gain patterns of the step, the beveled and the
proposed Vlasov cuts
TABLE II
COMPARISON OF THE PEAK GAIN
Antenna 3D Gain in CST (dB) 3D Gain in HFSS (dB)
Bevel Cut 10.91 10.30
Step Cut 11.33 10.64
Proposed Cut 11.40 10.74
The compared HPBW and sidelobe level ratio (SLR) results,
computed in CST, are listed in Table III. In addition to
totally eliminating the sharp edges and corners that limit the
performance of the antenna at high powers and providing
higher peak gains, the results have shown that the proposed
cut gives a smaller HPBW and a better SLR in the φ= 90◦
plane, a better SLR compared to the step cut in the φ= 0◦

plane, and a slightly narrower beam in the φ= 0◦plane when
compared to the bevel cut. These observations are also verified
using HFSS simulations.
TABLE III
COMPARED HPBW AND SI DE LOB E LEV EL R ATIO
φ= 0◦plane
Antenna Main lobe level (dB) HPBW◦SLR (dB)
Bevel 10.9 58.4 24.7
Step 11.3 55 22.4
Proposed Cut 11.4 58 23.5
φ= 90◦plane
Antenna Main Lobe (dB) HPBW◦Side Lobes (dB)
Bevel 10.9 44.5 17.2
Step 11.3 40.1 17.5
Proposed Cut 11.4 39 18.6
The reflection coefficient plots, for the three cut types, are
given in Fig. 7. The three antennas operate at and around 3
GHz with very low S11 values.
-40
-35
-30
-25
-20
-15
-10
2.4 2.6 2.8 3 3.2 3.4 3.6
Reflection Coefficient [dB]
Frequency [GHz]
Step Cut
Bevel Cut
Proposed Cut
(a) S11 using CST
-40
-35
-30
-25
-20
-15
-10
2.4 2.6 2.8 3 3.2 3.4 3.6
Reflection Coefficient [dB]
Frequency [GHz]
Step Cut
Bevel Cut
Proposed Cut
(b) S11 using HFSS
Figure 7. Comparison of the reflection coefficient, using both CST and HFSS
IV. CONCLUSION
The Vlasov antenna, originally designed with a step cut
made to one end of a circular waveguide, has been improved
by Nakajima who implemented it using a beveled cut. The
latter cut gets rid of the sharp corners present in the former
one, which makes the antenna supportive of higher microwave
powers, but this comes at the cost of reduced antenna gain and
broader beam widths. The all-curved cut proposed in this paper
is suitable for even higher microwave powers and provides an
increased gain compared to both the step and beveled cuts.
Furthermore, a reduced HPBW and a better SLR are obtained
in one major plane. In the second major plane, the proposed
design is a compromise between the step and beveled cut
cases.
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