NINETEENTH NATIONAL RADIO SCIENCE CONFERENCE, ALEXANDRIA, March, 19-21,2002

MODIFIED BATWING BROADCASTING ANTENNA ELEMENT

Hassan M. Elkamchouchi Department of Electrical Engineering

Faculty of Engineering, Alexandria

Email: helkamchouchi@ ieee.org Senior meniber IEEE

Mona N. Abd El-Salam Convm"cations Sector

Student nieniber IEEE entail: nznabil @ ieee.org

Telecom. Egypt

Abstract

Modified batwing antenna element is a nzodijied form of the conventional batwing array element used in broadcasting. The introduced design is based on the real shape of the wing bones, various trials have been carried out to get an approximate wing dimensions. The input characteristics and gain showed an enhaced response for the niodified batwing to the conventional one with less dirnensions. Tiie method of rnornents is used to aquire all nurnerical analysis, the numerical electronzagnetic code (NEC) is used to get numerical results. Radiation patterns for single, double, and quadruple element are shown for both antennas.

I. Introduction

The mother nature have been and will always be a depletionless resource to feed inspiring minds with creative and progressive ideas. One of the fields that had benefited a lot from nature is antenna design. Many insects have different shapes of instincts that act as antenna to help them get their proper direction. Such phenomena helped designers to start thinking with a look like shapes. For example, the widely known V-shaped antenna is an easy resemblance of the cockroach instincts. Not only insects but also some mammals such as bats are great specimens to scientists to test and study their unique ultrasonic systems that guide them in the darkness. In this paper an antenna design is chosen based on a wire analogy to the anatomy of the batwing.

Wire antennas is chosen because they have long been taken a superior role as cheap, simple and versatile antennas in a very wide range of applications. Various attempts have been made to acquire a better characteristics of wire antennas, such as changing the shape of the straight wire by adding multiple wires joined together to form a new shape with which we can get a better and enhanced antenna patterns. In this paper a modified shape to the previously introduced batwing antenna used in TV broadcasting as shown in figure l.a,l.b

Fig.1.a Picture of a horizontall$ mounted Batwing antenna with eight element array[2]

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Fig.1.b Picture of a vertic :ally moiirited Batwing antertna with t n ienty- four element array[2]

11. A Batwing Antenna Design Challenge

The Batwing antenna is a top-mounted, omni-directional, horizontally polarized antenna for VHF/UHF television broadcasting. Each bay is comprised of an array of radiating elements fabricated of steel rods. The radiators are mounted at 90" with respect to one another. The top and bottom of each radiator are solidly connected to the support mast, thus providing excellent protection against lightning discharges, and complete rigidity against winds and other weather effects.

The conventional 'design of the batwing antenna element is shown in figure 2.a, the dimensions where taken from [I]. These dimensions have been used as a base to start building the modified antenna element, which resembles more the natural batwing shape. The challenge here was to select the proper dimensions and angles of the wing bones and bending. Finally these presented dimensions were based on trial calculations to reach a better antenna pattern than the conventional batwing element with a smaller overall size.

111. A Modified Batwing Antenna Element Design

The bones of the bat wing are elongated finger bones with a layer of skin stretched on either side of the finger bones and forearm and attached to the sides of the body and the rear limbs, forming a strong flying surface. The length of the finger bones determines the shape of the wing. For example, some bats have long narrow wings which enable them to fly quickly and for great distances while some species have short broad wings, enabling them to hover while feeding. In between these two extremes are many differently shaped wings. In this paper a general form of the wing will be used, which consists of a main feeding dipole that represents the body of the bat, and multiple wires connected via specific junctions to keep the batwing shape of the antenna element. Figure 3.a, 3.b show the natural anatomy of the batwing and the skeleton of the bat. While figure 4 identify the main wires that are used to construct the antenna model.[3]

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17.66

Single Bahvlng Element

Fig.2.a Geometry of a conventional single batwing antenna element (dim. In inch)

/ wires for support

Fig.2.b The modified batwing antenna element (drawing to scale)

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--

Fig.4.a the 30 radiation pattern of the conventional batwing antenna element

Fig.5.a the 3 0 radiation pattern of the modified batwing antenna element rotated by A 0 degrees.

Fig.6.a The 30 radiation pattern of the two conventional batwing antenna element

Fig.4.b the 30 radiation pattern of the modified batwing antenna element

Fig.5.b the 30 radiation pattern of the modified batwing antenna element Rotated by 4 0 degrees then inverted

Fig.6.b The 3 0 radiation pattern of the two modified batwing antenna element

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NINETEENTH NATIONAL RADIO SCIENCE ]~1011 CONFERENCE, ALEXANDRIA, March, 19-21,2002

Fig.7.a The 3 0 radiation pattern of the conventional four batwing antenna elements

Fig.7.b The 3 0 radiation pattern of the four modified batwing antenna elements

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Input Impedance vs Frequency Input Impedance vs Frequency

600 500

500 0

Y E 400 400

m 2 -1000 a 1 300 -1500 5 -2000 4..

1 2 200

-2500 a - = l o o , !q I ! ! ! ! I 3 0 0 0 7

I I I I I I I

0 350 400 410 420 430 440 450 460 470 480 490 600 400 410 420 430 440 450 460 470 480 490 600

Frequency (MHz) Frequency (MHz) -- Fa IhemenlWbd(wbq - FwIhe&Iiedbalwlng -- For the c w v e n l d M w h g - For ths modifi bslwhg

(4 (6) Fig. 8. The variation of the input impedance vs. frequency for a single batwing Antenna element

(a) Input Resistance

Input Impedance vs Frequency

(b) Input reactance

Input Impedance vs Freauencv

60

6 0

Frequehcy (MHz) -- F u I h , c ~ b n s l t w o b ~ w h g - F u l h e ~ l k d t w o b s l w h g

(4

Frequency (MHz)

(6) F a the m v e r d m d Iwo Mwhg - Fn 111.3 modiflcd I- b d w

. . Fig. 9. The variution of the input impedance vs. frequency for two batwing Antenna elernerrts

(a) Input Resistance (b) Input reactance

Input Impedance vs Frequency Input Impedance vs Frequency

400 410 420 430 440 450 460 470 480 490 500 Frequency (MHz)

- - F a Ihe menlional b.sMag - For the modfiid balH/ns

500

5 250 0

i o - s Y

-250 -

400 400 410 420 430 440 450 460 470 480 490 500

Frequency (MHz) For We m v e m b d m g - Fw the d f k d bdwing --

(4 (b) Fig. IO. The variation of the input impedance vs. frequency for four batwing Antenna elements

(a) Input Resistance (b) Input reactance

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IV. Numerical Analysis

The numerical simulations of the antenna system are carried out via the method of moments. Numerical modeling commercial software (NEC)[S] is used in all simulations. The modeling process is simply done by dividing all straight wires into short segments where the current in one segment is considered constant along the length of the short segment, but a segment length is restricted to the diameter of the wire.

The collocation (point-matching) method is a numerical technique whose solutions satisfy the electromagnetic boundary conditions (e.g. vanishing tangential electric fields on the surface of an electric conductor) only at discrete points between these points the boundary conditions may not be satisfied, and we define the deviation as a residual [residual = BE 1 f 0 on the surface of an electric conductor]. To minimize the residual in such a way that its overall average over the entire structure approaches zero, the method of weighted residuals is utilized in conjunction with the inner product of the following equation,.

E (scattered) I t;m+E (incident) I

Where the w’s are the weighting (testing) functions and S is the surface of the structure being analyzed. This technique, does not lead to a vanishing residual at every point on the surface of a conductor, but it

forces the boundary conditions to be satisfied in an average sense over the entire surface. To accomplish this, we define a set of N weighting (or testing) functions (w,,,] = w1,w2,.. .,wN in the domain of the operator F. from the following equation,

N

h = c a” F( g ,I 1 “ = I

Where, F is referred to as a linear integral operator, g, represents the response of a function, and h is the known excitation function. Forming the inner product results in,

N

( W , , , J 4 = ~ ~ , l ( W , , # 7 % N rn = 1,2, ..., N ,,=I

This set of N equations may be written in matrix form as, [htl = [Ftrznl [an1

Which is solved by L-U decomposition to obtain the coefficient F,,,,. Once the current distribution is obtained, the other properties (e.g. input impedance, gain, the electric and magnetic fields distribution) could be determined. The choice of weighting functions is important in that the elements of (w, ] must be linearly independent. Further, it will generally be advantageous to choose weighting functions that minimize the computations required to evaluate the inner product. The condition of linear independence between elements and the advantage of computational simplicity are also important characteristics of basis functions. Because of this, similar types of functions are often used for both weighting and expansion. This technique is known as Galerkin’s method.

All conductors are assumed perfect. In NEC, the basis and weight functions are different, wi being chosen as a set of delta functions wi (7) = 6(7 - 6 ) with 5 a set of points on the conducting surface. Short straight segments model wires in NEC with the current on each segment represented by three terms a constant, a sine, and a cosine. This expansion provides rapid solution convergence. It has the added advantage that the fields of sinusoidal currents are easily evaluated in closed form. The amplitudes of the constant, sine, cosine terms are related such that their sum satisfies physical conditions on the local behavior of current and charge at the segment ends. Small flat patches model surfaces on which the magnetic field integral equation (MFIE) is used. The surface current on each patch is expanded in a set of pulse functions except in the region of wire connection. The pulse function expansion for N, patches is

j=l

Where,

;IJ = ;I (‘1)

;zj = ?*I (‘1 1 r, = position of the center of patch number j vJ (r) = 1 for r on patch j and 0 otherwise.

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V.

VI.

The constants Jlj and JZj, representing average surface current density over the patch, are determined by the solution of the linear system of equations derived from the integral equations. The integrals for fields due to the pulse basis function are evaluated numerically in a single step so that for integration, the pulses could be reduced to delta functions at the patch centers.[6]

Numerical Results

The first set of calculations is done on only one batwing element. The physical structure of the modified batwing element is shown in figure 2.b, the dimensions are taken to nearly simulate a natural batwing. The 3D-radiation pattern of both the conventional and the modified antenna elements are shown in figure 4.a and 4.b respectively. In order to change the direction of the pattern of the modified batwing the wire elements that represent the real wing were rotated by - 40 degrees which results in the new radiation pattern that is shown in figures 5.a,b. The gain of both elements is nearly the same and equal to 2.5-3 dB. There is no great difference in the input impedance value for both the modified element and the conventional one, and both can be easily matched. Usually these type of antenna is used in an array with each element is inverted and two double elements are mounted together on a mast or a pipe support as shown in figure 1.a and 1.b. So an inverted and doubled element is analyzed using the same technique. The 3D-radiation pattern of the modified batwing element has a reduced back loop and a broader front loop to provide more coverage area, but the conventional one will have a large amount of lost power as it will radiate upwardly which is not a desired direction. The four wing model of the batwing element shows that when the modified element is mounted horizontally it will provide a good pattern that will have an omni-directional coverage in the horizontal plan, and the same radiation downwardly but it will reduce the radiation upwards, which is not a desired one. Figure 7.b shows the 3D- radiation pattern of the pre-described antenna.

Both, the modified batwing antenna and the conventional batwing elements inherit a good input impedance-frequency relation over. a wide frequency range, that covers part of the VHF and UHF frequency bands. The input characteristics versus frequency relations are shown for each element in figures 8 to 10.

Conclusions

When the final element of both the conventional and the modified batwing antenna is analyzed, a superior pattern is obtained for the modified batwing antenna element. This pattern provides a complete coverage in the horizontal plane and a directed coverage downwardly with a total gain of 5.25dB. That makes the modified batwing antenna element a good broadcasting antenna when an array of elements are mounted horizontally. Or even a single element may be used as a receiving antenna.

Using these elements to form an array antenna will be continued in a future work. Also the thickness of the wires used and the material of the supporting mast may be considered in order to get a better characteristics using smaller and less number of elements.

References

[ 11 J.D. Kraus, “Antennas”, W8JK, Second Edition [2] The KNTV Antenna Crash, http://www.announcetech.com/kntv.html [3] htttx//www.roni.on.ca/nctivities/bats/ [4] Wing Anatomy, Department of Ecology and Evolutionary Biology: SWARTZ [ 5 ] NEC-WIN Professional v l . la, Nittany-Scientific Inc. 1997 [6] G. J. Burke and A. J. Poggio “Numerical Electromagnetic Code (NEC)- Program description”, January198 1 Lawrence Livermore Laboratory.

LAB. www.brown.edu