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Optimized Reflector Position for Vlasov Antennas

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  • Beirut Research & Innovation Center and American University of Beirut
Conference Paper

Optimized Reflector Position for Vlasov Antennas

Abstract and Figures

This paper presents a Vlasov antenna with optimized reflector position and angle suitable for high power microwave applications. With the proposed configuration, the reflector is directly attached to the waveguide, which is an advantage and makes it simpler to radiate in the direction of the axis of the waveguide. Bevel-cut and Step-cut Vlasov antennas, designed for operation at 3 GHz, are used to validate the effect of the reflector. In addition to proper radiation of the direction of maximum radiation, the optimized reflector results in increased antenna gain and reduced half-power beamwidth.
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Progress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug. 12-15, 2013 139
Optimized Reflector Position for Vlasov Antennas
H. M. El Misilmani1,2,M. Al-Husseini2,K. Y. Kabalan1,and A. El-Hajj1
1American University of Beirut, Beirut 1107 2020, Lebanon
2Lebanese Center for Studies and Research, Beirut 2030 8303, Lebanon
AbstractThis paper presents a Vlasov antenna with optimized reflector position and angle
suitable for high power microwave applications. With the proposed configuration, the reflector
is directly attached to the waveguide, which is an advantage and makes it simpler to radiate in
the direction of the axis of the waveguide. Bevel-cut and Step-cut Vlasov antennas, designed for
operation at 3 GHz, are used to validate the effect of the reflector. In addition to proper radiation
of the direction of maximum radiation, the optimized reflector results in increased antenna gain
and reduced half-power beamwidth.
1. INTRODUCTION
High Power Microwave (HPM) sources, such as the Backward-Wave Oscillator (BWO), the gy-
rotron, and the vircator (virtual cathode oscillator), generate power in cylindrically symmetric
transverse electric TE0nor transverse magnetic TM0nmodes. The side lobe generation, gain re-
duction, 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. Vlasov antenna is one
of the most known mode converters used. The well-known Vlasov types are the Step Cut and the
Bevel Cut antennas [1]. The step cut, originally suggested by Vlasov, has sharp edges and there-
fore may suffer from electrical breakdown when radiating HPM. The bevel cut, which was later
suggested by Nakajima, avoids the sharp points of step cut, and as a result has a more suitable
shape for HPM applications. However, a Vlasov antenna with either the bevel or step cut has its
maximum radiation shifted by some angle with respect to the axis of the waveguide.
In [2], a comparison of the performance of bevel-cut and step-cut Vlasov antennas in HPM is
conducted, concluding that the bevel cut has better performance in such applications. Other studies
focused on increasing the gain of bevel-cut and step-cut Vlasov antennas. In [3], a reflector is added
to a bevel-cut Vlasov antenna to increase its gain and to obtain more directive radiation. In [4], two
methods are proposed for increasing the gain of a bevel-cut antenna, one using a parabolic cylinder
reflector, and the second using a horn. In [5], the step cut is studied in the presence of a parabolic
reflector. However, none of these studies considered bringing back the maximum radiation along
the axis of the waveguide.
In this paper, an optimized reflector position for Vlasov antennas is presented, which will help,
with the proper rotation angle, to orient the generated waves along the +Zdirection, which is
the axis of the waveguide in our case. In addition, with our proposed configuration, the reflector
is directly attached to the waveguide structure, decreasing the size of the usual Vlasov antennas
with reflectors, and eliminating the need of extra components to hold the waveguide and reflector
together. The proposed reflector is applied to a bevel-cut and a step-cut Vlasov antennas to evaluate
its performance. It could also be applied to the cut proposed in [6], where the same results will
hold.
2. BEVEL-CUT VLASOV ANTENNA
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 45mm and a
length of 300 mm.
2.1. Bevel-cut Vlasov Antenna without Reflector
A Vlasov antenna with a beveled cut is shown in Figure 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 [1]:
α= sin1((ρ0nλ)/(2πa)) (1)
140 PIERS Proceedings, Stockholm, Sweden, Aug. 12–15, 2013
(a) Bevel-cut Vlasov antenna (b) Bevel-cut Vlasov antenna with reflector
Figure 1: Bevel-cut Vlasov antenna: (a) without reflector and (b) with reflector.
where ρ0nis the n-th root of the equation J0(ρ0n) = 0, λis 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= 4.5 cm and λ= 10 cm. Also, ρ01 =
2.405, so the bevel cut will be calculated as follows:
α= sin1((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], are shown in Figure 3. Maximum radiation
is obtained in the shifted direction corresponding to θ=θm= 28and φ= 90, computed using
HFSS.
2.2. Bevel-cut Vlasov Antenna with Reflector
A reflector having the shape of a half hollow cylinder is attached to the bevel-cut Vlasov antenna
as shown in Figure 1(b). The added reflector has the optimized values of 60 mm for the cylinder
radius and a length of 200 mm (height of the cut cylinder). Upon rotating the reflector by a specific
angle, it is seen that as the angle increases the shift angle approaches to origin. The initial bevel-cut
Vlasov antenna gives a maximum computed gain of 10.3 dB, with the maximum radiation along
the θ= 28and φ= 90direction. The optimized reflector angle for perfect direction along the
+Zaxis is seen at angle of 17.5. For this angle, the maximum radiation is back along the axis of
the waveguide, i.e., θ= 0and φ= 90.
This bevel-cut antenna operates at 3 GHz as shown in the reflection coefficient plot (S11) shown
in Figure 2. It has a gain of 10.9 dB and a reduced HPBW, as indicated in Figures 3(a) and 3(b).
It is shown that, for the case with the reflector, the maximum radiation is redirected along the axis
of the waveguide.
Figure 2: Reflection coefficient computed using HFSS, with no-reflector case shown in red, and with reflector
in blue.
2.3. Verification of the Results Using CST
The results in Section 2.2 have been verified using CST. The gain patterns in the two cases (without
and with reflector) are shown in Figures 4 and 5. As can be seen, the maximum gain of the antenna
Progress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug. 12-15, 2013 141
(a) (b)
Figure 3: Simulated gain patterns computed using HFSS, with initial bevel-cut results shown in red and
proposed design results in blue. (a) Red in the plane formed by the X-axis and the point of maximum
radiation (θ=θm, φ = 90), blue in φ= 0plane. (b) φ= 90plane.
(a) (b)
Figure 4: Bevel cut simulated gain patterns using CST without adding the reflector. (a) In the plane formed
by the X-axis and the point of maximum radiation (θ=θm, φ = 90). (b) φ= 90plane.
(a) (b)
Figure 5: Bevel cut simulated gain patterns using CST after adding the reflector. (a) φ= 0plane.
(b) φ= 90plane.
is redirected along the axis of the waveguide. The 3D gain patterns comparing the two cases, are
shown in Figure 6. Furthermore, the peak gain has increased and HPBW has decreased, as listed
in Table 1.
3. STEP-CUT VLASOV ANTENNA
The step cut is determined by two parameters, Aand B, as indicated in Figure 7(a). The value
of Ais fixed at 148.5 mm, which is the same value obtained with the bevel cut after finding the
142 PIERS Proceedings, Stockholm, Sweden, Aug. 12–15, 2013
(a) Without reflector (b) With reflector
Figure 6: Bevel cut 3D gain patterns using CST: (a) without reflector and (b) with reflector.
Table 1: Comparison of the radiation characteristics of the bevel-cut antenna having and without having an
added reflector computed using CST.
With/Without Reflector HPBWat φ= 0plane HPBWat φ= 90plane 3D Gain (dB)
Without Reflector 58.4 44.5 10.88
With Reflector 41.7 42.9 12
(a) Step-cut Vlasov antenna (b) Step-cut Vlasov antenna with reflector
Figure 7: Step-cut Vlasov antenna: (a) without reflector and (b) with reflector.
angle α. For comparison purposes, the step-cut Vlasov is designed so that it has the same angle
of maximum radiation obtained with the bevel cut, which is 28. For this purpose, Bis found
to be 35 mm. The gain patterns of the step-cut Vlasov antenna, computed using HFSS in the
θ=θm= 28and φ= 90planes, are shown in Figures 8(a) and 8(b) respectively.
The same reflector used in Section 2.2 is then attached to the step-cut antenna as shown in
Figure 7(b). Here, Lis the distance between the waveguide port and the start of the reflector. By
inspecting Figure 8(b), the concept of rotating the reflector is validated and maximum radiation
(a) (b)
Figure 8: Step cut simulated gain patterns, with initial step-cut results shown in red and proposed design
results in blue. (a) Red in the plane formed by the X-axis and the point of maximum radiation (θ=θm, φ =
90), blue in φ= 0plane. (b) φ= 90plane.
Progress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug. 12-15, 2013 143
is obtained along the waveguide axis for a rotation angle of 17.5similar to the one used for the
bevel-cut case.
4. CONCLUSION
In Vlasov antennas with step cuts or bevel cuts, the maximum radiation is shifted by some angle
with respect to the axis of the waveguide. In previous work, reflectors have been used to focus the
beam in some direction. In this paper, a reflector attached to the waveguide structure was proposed,
and its rotation angle was optimized to obtain maximum radiation along the axis of the waveguide.
The advantages of this reflector structure are the smaller overall size of the antenna, its simpler
design in terms of attaching the reflector to the waveguide, an increased gain and a decreased
HPBW. The proposed reflector was tested with both bevel-cut and step-cut Vlasov antennas.
REFERENCES
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3. Dahlstrom, R. K., L. J. Hadwin, B. G. Ruth, and L. F. Libelo, “Reflector design for an X-
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6. El Misilmani, H. M., M. Al-Husseini, K. Y. Kabalan, and A. El-Hajj, “Improved Vlasov
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