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In this paper the design equations for an array of circumferential slots on a cylindrical waveguide are obtained, following the procedure introduced by Elliott for slots on rectangular waveguides. The minimization of the error function is achieved for optimization of slot array parameters. The optimization of slot parameters is not goal of this paper but a numerical example are presented as illustrations of the proposed synthesis method. The results of array designs by the method of the least squares are verified by two computer simulation softwares, namely CST and HFSS.

Train communication systems have received a considerable interest over the years, with tunnel connectivity providing an ongoing challenge due to the hostile environmental characteristics. In recent times, security aspects have come to the forefront with high-definition closed-circuit television monitoring being considered, together with possible remote train control and passenger emergency assistance networks [

The first study of radiation from an aperture on an infinite metallic plane was reported by Silver and Saunders in 1950, who derived a formula for the generated external field [

The TM_{01} mode is assumed in the cylindrical waveguide, where the electric field is radial in its cross-section. Consequently, the radiation from the circumferential slots on its surface is omni-directional and independent of the azimuthal angle, which is desired for cylindrical slot arrays.

The configuration of circumferential rectangular slots on a cylindrical waveguide is shown in

The backward (B) and forward (C) scattered field amplitudes are given by [

B m n = ∫ slot E s ¯ × H ¯ t C ⋅ d s ¯ 2 ∫ S E ¯ a t × H ¯ a t ⋅ u ¯ z d s (1)

C m n = ∫ slot E s ¯ × H ¯ t B ⋅ d s ¯ 2 ∫ S E ¯ a t × H ¯ a t ⋅ u ¯ z d s (2)

where subscripts B and C represent the amplitudes of backward and forward waves, t indicates the tangential field in the cross-section, S indicates the cross- section of uniform waveguide, and “slot” shows the slot surface area.

The field components of TM_{01} mode in the cylindrical waveguide are:

E z = J 0 ( h ρ ) e ± j β 01 z (3)

E ρ = ± j β 01 h J ′ 0 ( h ρ ) e ± j β 01 z (4)

H φ = − j ω ϵ h J ′ 0 ( h ρ ) e ± j β 01 z (5)

E φ = H ρ = H z = 0 (6)

where β 01 is the phase constant and h = 2.405 a is cut-off number with a being

the radius of waveguide.

The tangential electric field on the n’th aperture is:

E z = V n W cos ( π ( φ − φ 0 ) α n ) { z n − W 2 < z < z n + W 2 φ 0 − α n 2 < φ < φ 0 + α n 2 W → 0 (7)

E ¯ s = E z u ¯ z , E φ = 0 (8)

where α n is the angle of n’th slot. The other field component are:

H ¯ t B = H φ u ¯ φ (9)

H ¯ t C = H φ u ¯ φ (10)

H ¯ a t = H φ u ¯ φ (11)

E ¯ a t = E ρ u ¯ ρ (12)

These field components are substituted in Equations (1) and (2) to obtain:

C 01 = B 01 = j V n h J 1 ( h a ) a β 01 π 2 α n π [ J 1 2 ( h a ) − J 0 ( h a ) J 2 ( h a ) ] (13)

Observe that the forward and backward traveling wave amplitudes are equal. Therefore the transmission line equivalent circuit consists of a parallel admittance.

The first design equation is then derived. The reflected power from the aperture is:

P r e f = 1 2 R e ∫ S ( E ¯ t × H ¯ t * ) ⋅ d s ¯ = 1 2 R e ∫ ρ = 0 a ∫ φ = 0 2π ( B 01 E 01 , t × B 01 * H 01 , t * ) ⋅ u z ¯ ρ ′ d ρ ′ d φ ′ = π β 01 ω ϵ h 2 a 2 2 [ J 1 2 ( h a ) − J 0 ( h a ) J 2 ( h a ) ] B 01 B 01 * (14)

where

E ¯ t = { B 01 E ¯ a t z < z 1 C 01 E ¯ a t z > z 2 (15)

The equations of the equivalent transmission line are:

{ V ( z ) = A e − j β z ︷ V + + B e j β z ︷ V − I ( z ) = A G 0 e − j β z ︸ I + − B G 0 e j β z ︸ I − (16)

which give the reflected power:

P r e f , T L = 1 2 V − ( z ) I − * ( z ) = − 1 2 B B * Z 0 (17)

The equality of reflected powers due to the scattered fields and the transmission line leads to the following relation:

− 1 2 B B * Z 0 = π β 01 ω ϵ 0 h 2 a 2 2 [ J 1 2 ( h a ) − J 0 ( h a ) J 2 ( h a ) ] B 01 B 01 * (18)

However, the scattered voltage amplitudes are [

B = C = − 1 2 Y n a G 0 V n (19)

The amplitudes B and B_{01} from Equations (19) and (13) are then substituted in (18) to obtain the first design equation:

Y n a G 0 = − j 4 ω ϵ 0 J 1 2 ( h a ) G 0 β 01 π 5 [ J 1 2 ( h a ) − J 0 ( h a ) J 2 ( h a ) ] V n s V n α n (20)

where the Bessel functions J 0 , J 1 and J 2 are calculated for ha = 2.405 for TM_{01} mode.

For the derivation of the second design equation, the procedure described by Elliott ( [

Y n a G 0 = η 2 2 G 0 | K | 2 α n 2 z n d , α (21)

where K = − j 4 ω ϵ 0 J 1 2 ( h a ) G 0 β π 5 [ J 1 2 ( h a ) − J 0 ( h a ) J 2 ( h a ) ] , η = 120 π is the intrinsic

impedance of medium and G 0 is characteristics admittance of cylindrical

waveguide. Equation (21) can be written as Y n a G 0 = K 1 α n 2 Z n d , a where K 1 = η 2 2 G 0 | K | 2 . Z n d , a is the active admittance of equivalent dipole that is assumed in the derivation of second design equation. we have Z n d , a = Z n n + Z n b where:

Z n n : Self impedance of circumferential slot, which is equal to K 1 α n 2 Y n s e l f / G 0

Z n b : Mutual impedance between circumferential slots on the cylindrical waveguide, which is equal to ∑ m = 1 m ≠ n N V m s V n s Z n m d .

Then

Z n d , a = Z n n + Z n b = K 1 α n 2 Y n s e l f / G 0 + ∑ m = 1 m ≠ n N V m s V n s Z n m d

Z n m d is mutual impedance between two dipole which may be obtained from the mutual admittance between two slots Y n m s by the Booker’s relation.

Z n m d = ( η 2 2 ) Y n m s (22)

The second design equation is then determined by these relations.

Consider the equivalent circuit of the linear traveling wave slot array as shown in

Y n G 0 = K 1 α n 2 Z n d , a + ( Y n − 1 / G 0 ) cos β 10 d n − 1 + j sin β 10 d n − 1 cos β 10 d n − 1 + j ( Y n − 1 / G 0 ) sin β 10 d n − 1 (23)

where the second design Equation (21) is used.

The mode voltages at successive junctions are then related by:

V n = V n − 1 cos β 01 d n − 1 + j I n − 1 Z 0 sin β 01 d n − 1 = V n − 1 [ cos β 01 d n − 1 + j Y n − 1 G 0 sin β 01 d n − 1 ] (24)

which may be written for V n V n − 1 . This ratio may also be obtained by Equation

(18) for n and n − 1. Equating these two ratios gives:

cos ( β 01 d n − 1 ) + j Y n − 1 G 0 sin ( β 01 d n − 1 ) = V n s V n − 1 s ⋅ α n α n − 1 ⋅ Y n − 1 a / G 0 Y n a / G 0 (25)

This expression is appropriate for the construction of an error function.

The error function consists of three terms:

ε ErrorFunction = ε Matching + ε DesignEqs . (26)

ε Matching = W 1 | Re ( Y n G 0 ) − 1 | 2 + W 2 | Im ( Y n G 0 ) | 2 (27)

ε DesignEqs . = W 3 | ∑ n = 2 N ( cos ( β d n − 1 ) + j Y n − 1 G 0 sin ( β d n − 1 ) ) − V n s V n − 1 s ⋅ α n α n − 1 ⋅ Y n − 1 d , a / G 0 Y n d , a / G 0 | 2 (28)

where

The error function depends on the slot spacings and angular dimensions and will be used for optimizing slot parameters by minimizer algorithms such as gradient conjugate or genetic algorithm.

Modified Taylor Pattern at 5.35 GHzThe cylindrical slot array is designed for 13 slots at the frequency 5.35 GHz. The design parameters of the array are given in

In this paper the design equations are developed for the traveling wave mode by

13 | Number of slots | Antenna parameter |
---|---|---|

5.60 mm | Slot width: W | |

0.20 mm | Thickness of waveguide wall: Thickness | |

22.43 mm | Radius of cylindrical waveguide: R | |

5.35 GHz | Operation frequency | |

15.70 mm | Slot length: Ra | Optimized parameters |

23.05 mm | Slot spacing: d | |

Modified Taylor pattern with SLL = −13 dB | Characteristics of desired pattern |

employing the equivalent circuits according to the Elliott’s method. The geometrical dimensions of the slot array on the cylindrical surface are determined by the minimization of the appropriate error functions. The proposed synthesis method of cylindrical slot array is demonstrated by one design example at 5.35 GHz frequency and is verified by simulation softwares of CST and HFSS. Such arrays are appropriate for various platforms of cylindrical shape, such broadcasting transmitter antennas (TV station).

Khayati, A., Amirabadi, A. and Rajabi, B. (2017) Presentation of Design Equations for Array of Circumferential Slot on Cylindrical Waveguide. Open Journal of Antennas and Propagation, 5, 180-187. https://doi.org/10.4236/ojapr.2017.54014