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THE DESIGN OF A DUAL polarized vivaldi antenna

THE DESIGN OF A DUAL polarized vivaldi antenna
THE DESIGN OF A DUAL polarized vivaldi antenna

it has been shown to exhibit the desired char-acteristics: a broadband pattern, broadband impedance and high cross-polarization isola-tion.2,3

THE VIVALDI ELEMENT

The operation of the Vivaldi antenna itself is not yet fully understood.4Interested readers are referred to the book by Lee and Chen,4which provides an overview of the tapered slot antenna (TSA) and its many variations. For the purpose of this article, only a qualitative description of the traveling wave mode Vivaldi antenna is attempted.

Briefly, the traveling wave mode Vivaldi an-tenna provides a smooth transition between the guided wave traveling in the slot transmis-sion line (slotline) and the plane wave, which is radiated.4This transition is achieved by a gradual tapering of the slotline. Since the slot-line is a balanced transmission line, a wide-band balun is an important component in the antenna design. A description of a printed

A DRIAN S UTINJO AND E DWIN T UNG

Murandi Communications Ltd.Calgary, Canada

T

he design for the Vivaldi antenna is mo-tivated by the need for a broadband measurement antenna for an antenna test range (ATR). The ATR is capable of a dual axis rotation to create 3-D plots and to calcu-late the total radiated power (TRP).1The TRP calculation involves the summation of the measured radiated power due to E φand E θ.Therefore, measurement of each polarization is required.

Because of the need for quick characteriza-tion of antennas in the ATR, it is desirable to have a receiving antenna that can be electroni-cally switched to receive E φor E θ. Otherwise,the antenna under test (AUT) would need to be rotated for each polarization, making the test twice as long.

Currently, dual polarized dipoles are used to achieve fast characterization. Cross-polar-ization isolation in excess of 20 dB can be achieved with these dual dipoles. However, as the frequency gets higher (a few gigahertz),the dipoles become more tedious to construct due to their small size. Also, since the dipoles are inherently narrow band, it is quite desir-able to replace them with a broadband dual polarized antenna. The Vivaldi (end fire expo-nentially tapered slot) antenna array is a promising candidate for this application since

T HE D ESIGN OF A D UAL P OLARIZED V IVALDI A RRAY

In this article, the design of a dual polarized Vivaldi antenna array is described.The Vivaldi elements are fed by broadband microstrip-to-slotline baluns. A 2:1bandwidth (3.4 to 7 GHz) was achieved for VSWR, gain and cross-polarization isolation. The design iteration, simulations and measured results are presented.

Reprinted with permission of MICROWAVE JOURNAL ?from the September 2004 issue.

?

2004 Horizon House Publications, Inc.

Vivaldi with a microstrip feed is pro-vided in Figure 1. The microstrip line is printed on a substrate and the tapered slotline is etched on the ground plane below the microstrip.A few parameters are considered to be of great importance for satisfac-tory wideband performance:

?The length and the width of the tapered slotline: to achieve the travel-ing wave mode of radiation, the slot-line length and width generally needs to be greater than λo and λo /2, re-spectively.4

?The opening rate of the tapered slotline: the Vivaldi antenna employs an exponential taper.4The coordinates of the tapered slot are defined by:3

where

x C e C Rz =+12

1()

?The dimensions of the microstrip-to-slotline (M-S) transition: To achieve a broadband transition, the microstrip open stub and the slotline short stub are to present a virtual short and a virtual open at the point of transition, respectively. To that end, the radius of the radial mi-crostrip stub (R rad ) and the diameter of the circular slot stub (D s ) may be approximated by λm /4 and λs /4, re-spectively. The λm is the effective wavelength of the microstrip and λs is the effective wavelength in the slot-line. In-depth discussions on the M-S transitions are given in references 5,6,7.

THE DUAL POLARIZED ARRAY To achieve the dual polarization,two coplanar horizontal and two coplanar vertical Vivaldi elements are arranged into an array. The pair of ac-tive co-polarized elements is driven with equal phase and equal ampli-tude excitation to achieve a broadside pattern. The cross-polarized elements are switched off using an electronic switch.

Figure 2. A few ad-3?Broadside cross-polarization isola-tion greater than 10 dB from 3.4 to 6GHz.

From experience, it is felt that the following iterations yield a reasonably good design in an efficient manner:?Determine the antenna width based on the array’s spacing require-ment: For a broadside array, the ele-ment spacing (d) must be less than λo at the highest frequency to avoid grating lobes.

?Determine the antenna length and width based on the traveling wave de-sign requirements: Recall that the slotline length and width generally needs to be greater than λo and λo /2at the lowest frequency, respectively.?Select a board material: A treat-ment of the effect of the dielectric on the performance of the Vivaldi anten-na is given in Kasturi, et al.8

?Design the microstrip-to-slotline transition for the required frequency range with S 11less than –15 dB. The characteristic impedance Z o of the slotline and the port impedance may be varied for best S 11.

?Connect the M-S transition to the tapered slotline: Vary the opening rate until the VSWR, gain and cross-polarization specifications are met.Re-optimize the M-S transition if necessary.

?Design a microstrip tapered line to match the Z o of the microstrip to 50?.

?Ensure that the coaxial connector to the microstrip transition is accept-able.

?Arrange the elements in the dual polarized array and verify that the VSWR, gain and polarization goals are met. Re-optimize if necessary.DESIGN PROCESS AND SIMULATION RESULTS

Following the design procedure,the following dimensions were deter-mined:

?Array spacing, d =1 ? 30 cm/6GHz =5 cm.

?Flared slotline length, f l =30 cm/3.4 GHz =8.8 cm.

?Flared slotline width, f w =0.5 ? 30cm/3.4 GHz =4.4 cm. However, due to the d requirement and the need for spacing between the antenna edges, the antenna width (a w ) was set at 4 cm. The f w was set at 3.6 cm such that the ends of the taper are 0.2 cm away from the top and bottom edges

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