The Design of Reconfigurable Planar Log-periodic Dipole Array (LPDA) Using Switching Elements

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The Design of Reconfigurable Planar Log-Periodic DipoleArray (LPDA)

Using Switching Elements

Ahmad A. Gheethan*, and Dimitrios E. Anagnostou

South Dakota School ofMines and Technology, Rapid City, SD, 57701

E-mail: [email protected]

Introduction

This paper shows the design of a reconfigurable log periodic dipole array antenna

(LPDA). The design is based on utilizing switches and high resistive DC bias lines to

achieve the reconfigurable LPDA. The operating frequency and maximum directivity of

the design can be manipulated according to the switches positions and their state ('on' or

'off). The simulated and measured results of the return loss and gain responses are

shown. The results are promising for the implementation of such new reconfigurable

antenna designs.

Theory

The general schematic of the proposed reconfigurable design is illustrated in Fig. 1. The

antenna consists of N planar dipoles placed on the top and bottom of a substrate of

thickness t. The antenna's feed is similar to that in [1]. The length and width ratios of

each adjacent dipole are given by the geometry constant i . The spacing between the

dipoles is related to the spacing factor (1 . The spacing factor and geometry constant

determine the apex angle a, where [2]

l-rtan(a) = - (1)

40-

With these three factors one can specify the maximum directivity of the array. This is

also shown in the directivity contours in Fig. 2 [3].

A previous work [4] has illustrated a mechanically reconfigurable LPDA which is

achievable through the adjustment of its physical dimensions such as the dipoles length

and their separation. However, this is a complicated procedure to be done manually or by

using bulky mechanical equipment, and it can easily result in inaccurate LPDA structures

that do not match with the 'log-periodic' definition. In addition, this kind of

reconfigurable antenna is not applicable with printed LPDAs.

It is challenging, yet possible to change the design shown in Fig. 1 to an electrically or

micro-electrically-mechanically reconfigurable antenna. This can be accomplished by

adding switching elements such as PIN diodes or MEMS to each printed dipole. The

switching element on the largest dipole should be placed at a distance Xn measured fromthe antenna's boom. The remaining distances should be chosen relative to ireconf, where

ireconfis the geometry constant of the antenna when the switches are 'off given by:

Xn- 1T reconf =-- (2)

Xn

When the switches are in the 'off state, the antenna will exhibit a geometry constant

ireconf that may equal or not equal to i , which contributes to the antenna when the switches

are 'on'. However, the resultant apex angle will show a new value areconf, thus, a new

978-1-4244-3647-7/09/$25.00 ©2009 IEEE



value for the spacing factor called O"reconf. As a result, the same antenna will have different

directivities depending on the switches' state, according to Fig. 2.

For example, let us consider an LPDA antenna designed with a geometry constant r = 0.9and spacing factor 0" = 0.12 at its 'on' state. From Fig. 2, the expected maximum

directivity is 8dBi. For that antenna, when the switches tum 'off', the antenna exhibits

ratios of r = 0.88 and 0" = 0.11. Then, the anticipated directivity is 7.5dBi (a similarvalue), but most important the antenna's frequency of operation will double. This

illustration is marked by the squares in Fig. 2, and the operating frequency and bandwidth

are determined by the dipoles active length and number of elements respectively, at the

two different switch states.

The switches require a DC bias to alter their state. However, the DC bias requires highly

resistive bias lines that will not deteriorate the antenna's return loss and radiation pattern.

High resistive materials can be utilized to fabricate such bias lines, which reject the RF

and pass the DC current simultaneously [5]. Such lines are bandwidth independent and do

not require a ground plane underneath or next to the antenna.

Results and Discussion

To verify the concept in the proposed work, two hard-wired models of the reconfigurable

LPDA were designed and simulated using IE3D™ [6]. The design parameters for the 'on'

state are FO.9, 0-=0.085 and a=16.39°. In the 'off' state, the parameters change to FO.9,

0-=0.17 and a=8.37°. The reconfigurable design is marked in Fig. 2 with circles. The

antenna was designed to operate over the 1-2 GHz range in the 'on' state and 2-4 GHz in

the 'off' state. Two hard-wired LPDAs prototypes were fabricated on a R04003C

substrate and measured. One design represents the antenna with switches at the 'on' state

and the second design represents the antenna with switches at the 'off' state. The

simulated and measured return loss response of the designs is shown in Fig. 3, and the

measurements agree well with the simulations. When the switches are 'on', the antenna

operates over the 1-2 GHz range. Additionally, when the switches are set 'off', the

antenna works in the 2-4 GHz range. This way a 1-4GHz range can be covered, by using

an original antenna design from 1-2GHz. Therefore by reconfiguring the antenna

structure, its bandwidth has been doubled.

The 2D radiation pattern at 1.5 and 3 GHz is also depicted in Fig. 4. The patterns are

almost identical in both bands and agree with the conventional radiation pattern of

LPDA's. The patterns show a directive major lobe and small side and back lobes. The

maximum simulated and measured gain is 7.56dBi at 1.5 GHz and 8.9dBi at 3 GHz that

match well with the expected results from Fig. 2.

The measured results are promising and proved the concept, illustrating how significant

performance increases can be achieved with simple reconfigurable LPDA antennas.

Modem technological achievements such as the deposition of high-resistive lines onmicrowave laminates and the integration of MEMS on the antenna surface will allow

such designs to be realized in future communication applications. The integrating issues

and the mutual coupling effect results of the switched 'off' nearby dipole parts of this

reconfigurable LPDA will be presented.



Acknowledgement

This research was supported by NSF Award #0554609, "The 2010 Initiative:

Science-Based Leadership for South Dakota".

References

[I] A. A. Gheethan and D. E. Anagnostou, "Reduced Size Planar Log-Periodic DipoleArrays (LPDA) Using Rectangular Meander Line Elements," IEEE APS/URSI

International Symposium 2008, San Diego, CA, USA, July 5-12, 2008.

[2] R. L. Carrel, "Analysis and Design of the Log-Periodic Dipole Antenna," Ph.D.

Dissertation, Elec. Eng. Dep., University of Illinois, University Microfilms, Inc,. Ann

Arbor, MI, 1961.

[3] P. C. Buston, G. T. Thompson, "A Note on the Calculation of the Gain of Log

Periodic Dipole Antennas," IEEE Trans. on Ant. & Prop., v.24, pp.105-106, Jan.1976

[4] A. Galmon, G. Pacheco and Marco Terada, "A Novel Reconfigurable UWB Log

Periodic Antenna," Proceeding of the 2006 IEEE International Symposium on

Antenna and Propagation, pp 213-216, Albuquerque, NM, 2006.

[5] D. E. Anagnostou, G. Zheng, M. Chryssomallis, J. Papapolymerou, C. G.Christodoulou, J. Lyke and G. Ponchak, "Design, Fabrication and Measurements of a

Self-Similar Re-configurable Antenna with RF-MEMS Switches", IEEE

Transactions on Antennas & Propagation, Special Issue on Multifunction Antennasand Antenna Systems, Vol. 54, Issue 2, Part I, Pages: 422 - 432; 432, Feb 2006.

[6] IE3DTM is a trademark ofZeland Software Inc., 2006.

+

t.o. t.o.

:.§ :.§

~u UQ Q

a) b)

Fig. 1. The structure of an LPDA: a) The schematic of the reconfigurable (LPDA). The switches

are represented by a diode symbol. Only the largest antenna elements are shown, and b) A

photograph of the two fabricated LPDAs.



0.12

0.10

0.94 0.92 0.90

Scale factor (t)

0.88 0.86 0.84

Fig. 2. Computed contours of directivities versus ( f and r for log-periodic dipole arrays [2].

2 3

a) Freq. (GHz) b) Freq. (GHz)

Fig. 3. a) Simulated and b) measured return loss response of the reconfigurable LPDA. The widetotal frequency coverage [1-4GHz] can also be seen from the -IOdB dotted line.

90

210 Simulated (On mode)

••••••••••••• Simulated (Off mode)------ Measured (On mode)_ •• _ ••_. Measured (Off mode)

?70

330

Fig. 4. Simulated and measured H-plane radiation pattern of the LPDA at its

two different configurations.

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The Design of Reconfigurable Planar Log-periodic Dipole Array (LPDA) Using Switching Elements