Switched Parasitic Antennas for Cellular Communications [1 ed.] 1580531547, 9781580531542

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Switched Parasitic Antennas for Cellular Communications

Disclaimer of Warranty

The technical decriptions, procedures, and algorithms in this book have been developed wirh the grean:St care; however, they are provided without a warranty of any kind. Anech House, Inc., and the authors of the book emided Switched Parasitic Antenru1.s for Cellular Communications make no warranties, expressed or implied, that the equations and procedures in rhis book are free from error, or will meet your requiremems for any panicular application. They should nor be relied on for solving a problem whose incorrect solution could result in injury to a person or loss of property. The authors and publisher disclaim all liability for direct, incidenral, or consequential damages resulring from your use of the procedures in this book.

For a listing of recent titles in rhc Anech House Antemuu and Propagario•i Libmry, tum co rhe baGk of ~his book.

Switched Parasitic Antennas for Cellular Communications David V. Thiel Srephanie Smirh

Anech House Boston • London www.artechhouse.com

Library of Congress Caraleging-in-J>ublication Dau Thiel, David V. Swirchcd parasiric-antennas for celluJa.r mmmunica.r:ians / Da.-id V Thiel, Stephanie Smith. p. cm.-(Anech House anrenn$ \Uld p~opa.ga:dn.n library) lndud(}S bibliogtaphi!!al teforenel!S and ind\, and ¢ 1 - B,r, is comp:m:d ro rhe signal scrength of che beam received ac ¢ 1• 1-f either one is stronger rhan that received when the antcmna. is in the r/> 1 directieo. then chc beam posicion is updared ro the new direcrion of maximum signal strengrh. This dithering operation mighr be initiated if the signal level inrn the receiver begins to reduce, or periodically after a specified, preprogrammed period of cirne. The riming of this update is dependent on the specific application. for example, in rhe c-.¾.se of an airport radar, the velocity and direction of the incoming or oucgoing aircraft must be taken inco accounc when pro.grarnming the dirhe:ring repea1 time ro ensure ceminuous tracking of rhe aircraft. It is also possible to retain the position history of the transmirrcr a.ad use f.hat ro predict rhe ne:>.."t position.

1.3.2 Two-Dimensional Tracking The algorithm for a 2D global search using sequential uniform sampling is given in Figure 1.5; rhe local search using the dither rechnique is given in F~gure I .6. In both cases, rhe SNR must be > Ir 'D, then the rwo veo:ors r and r- r' can be considered paF.ulel. Then [2].

(2.8)

l

y

II I

I

' ',

''

'•'

)(

Figure 2.1 Coordjnate system showing a 30 conductor ha-ving volume

v·

carrying volume current -"r,w) detected at a distant point from the origin at vect-0r , .

Switched Parasitic Antennas for Cellular Communications

28

- - - - - -- -- - -- - -- - -- - - - -

when'.~ is the angle benveen rhe two vectors rand r ' , as shown in Figur This general expre.~sion can be used wich (2. 7) co calcuJace the field ra< from a du:ee-dimcnsional current discriburion J.

2.3 Radiation from a Wire Element If the

current is confined co a single srrnight wire lying along the z ax:i

Figure 2.2), Il only, that is,

l=], and che elecuic field in the fur field has a 0 com pc

If the wire is very rhin, the currenr / ~c.in be written as: }

0

(z ')

= IJz')o(x')o(/) z

_,

_,

/lI

,

,, 0

,, ,,,,

,,,

,,,,

I I

_,

,/ y

''

',

''

', ',

X

Figure U

I I

_,,

Vertical cLJrrent element in the z direction.

I I 'J

29

U'lireAnrtnna Theory -

- - - - --

where ois the unit impulse funccion or the dirac delta function in the following wa,y:

{1 fr)r t5(x ) = I

1

X

-

Pl, defined

=1

(2. 11)

0 for x' :;t 1

Equation (2. 1 I ) indicarns that rhe currenr disuibution in me x' and y' clireccions is infinitely thin. Subsriruring (2.10) inro (2.8) results in: -1J.·r -

A
, Hmd.!er. €elorad. Jarn.JJU'}" I998, p. I 5.

I l 41

Richmond. J. H .. Mono-pole Antown Ml Girrular Sisk. Teehnieal Rcp,m 71 16.3'l- J. Ele

- - srn sm

(.3.6)

2

2

k is rhe free-space wave number, and .F,o is the field strength maximum. The E plane and H pfane polar patterns are given by [2]:

(3.7)

83

Patch Antnmas

. [kW sm. o]

sin

2

kW' -sin0

FH(fJ)=c0s0

(3.8)

2

The E plane of radiarion is the cf> = 0° cut when the radiating edges of the patch Lie parallel to the x axis in Figure 3.1 (a), and the H plane is r.he ¢ =90° cur for the same condition. A square parch amenna wir.h dimensions L = W= 55 mm, fabricated on duroid (relative permittiviry e, = 2.2 and loss tangent 0.001) with thickness t= 1.6 mm has a fundamental resonanr frequency of 1.795 GHz. The E and H plane radiation pam;:rn.s at this frequency are given in Figures 3.2 and

0 0

270

90

180

Figure 3.2 E plane radiation pattern (normalized dB) for a square patch antenna (¢, = 90° cut}; '-· Ensemble (7) result, 'o' (3.7) result

84

Switched Parasitic Antennas for Cellular Communications

0

300

I

/ 90

240

210

150 180

Figure 3.3 H pJane radiation pattern (normalized dB) for a square patch antenna (ip = 0° cut);,_. Ensemble 171 result, 'o' (3.8) result.

3.3, re.specrively. Ir has been assumed rhar rhe feed point is ar its oprimum for coupjjn.g to a 50-Q transmission line., and rhar the ground plane is of infinite exrenr in rhe :ry plane. These patterns have been calculated using the method of moments (MOM) code, Ensemble [7], with rhe feed position (x0 = 27.5 mm, Jo = 20.49 mm) . The normaliz.ed parrems from (3.7) and (3.8) have been induded for comparison. The maximum gain of die antenna lies in the 0 =0° direction (i.e .. perpendicular to rhe patch sur£ace). The E plane radiarion pattern is almost omnidirectional, with nulls along rhe surface of the didecmic only. The agreernem berween the results obtained usiDg rhe Ensemble [7] sofuvare and (3.7) and (3.8) is quire suong. In the E plane pattern, the Ensemble [7] resuh correctly shows a null at 0 = 90°, which is r10c included in (3.7).

8!i

Patch Antennas

Figure 3.4 illustraies the variation in rhe refleetion coefficiem of (he ~ame. square patch for a number of different feed positions measured relative to one end. The data shown was obtained using the caviry model equations given in [3, 8]. buc is almosr identical ro ~hat obr-ained using Ensemble [71. The impedance bandwidth (S11 < -10 dB) is a maximum when the feed p0inr is locacecl. ar a distance of 19 mm from the edge; howtlv~r, the best inpur march is obrained when the feed poim is located 2D mm from the edge. The input resistance RA at the edge of a resonant recmngular patch is given by theapproximare ~ression [91:

R.,,_

=

1

e E,.µ,, ( 90-'

pc1

-L )

(.3.9)

W

where e, is rhe radiation efficiency of a horizontal electric dipole on rop of the substrate, which is an appr0x.imation for rhe radiarion efficiency of a single

-15

S., (dBi Y,=18mm -20

Y,= l9 mm y,=20mm y, ,,21 mm y.=-22 mm

-Z6

-30

-35 ' - - - - - - - ' - - - - ' - - - -- - - - - --'-~ - ~-----'----'-1.86 1.7 1.74 1.18 1.8 1.82 1:84 1.88 1.72 1.76

--' 1.9

Frequency (GHz)

Figure 3.4 S11 (dBi of a coaxially fed square patch antenna plotted as a furiction of frequency for various feed positions.

86

Switched Parasitic Antennas for Cellular Communications

patch, and p is refer-red co as the p factor and is che racio of the power radiated into space by the parch co che power radiated into space by an equivalent dipole. The equation for pis given in [8]. 1 2 c, =1 - -- + t:,ft, 5(,,µ,)

P'' e, =

f

h

P,

h +PS\"V

(3.10)

(3.11)

where P," is che space-wave radiated power of a horizontal electric dipole on the substrate. The equation for P," can be found in [8). P}'w is the surfacewave _radiated f':,~er of ~ ho~izonral electric dipole on the substrate. The equanon for Ps,w 1s also gtven m [8). The equation for RA gives typical inpuc resistance values ranging from 100 co 400 Q wich the resistance reduced by increasing W. This can be useful, as there is no reactance ac resonance and so the value obtained for input resistance should be quite close co chat obtained using che much more complicated cavity model ac che resonant frequency. T his equation is dependent on frequency; however, rhe resistance value obtained does not change much with frequency, and the results are only accurate around che resonant frequency. As a result, ic is necessary co already know the resonanc frequency of the patch in order for the results to be useful. The currem clisuibution across the patcrh length L at resonance is a half-wavelength sinusoid, and the voltage distribucion is a half-wavelengrh cosinusoidal function. The impedance varies significantly across the parch. For a coaxial probe, the distance berween the probe and the edge of the patch (.xo) decreases the inpuc resistance by the factor [2]: cos

1(1rXo) - -

(3.12)

L

This faccor is similar to chat for the offset feed in a dipole wire ancenna give-n by (2.33). Figure 3 .5 shows a comparison between che input resistance obtained using the cavity model [8] and that obtained using (3.9), with the correction for rhe feed probe calculated using (3.12) as che feed position is moved &om rhe edge of the patch to the cemer of the patch. Figure 3.6 shows a comparison becween the same techniques as the width of the patch

87

Patc-h A.ntmnas 350

....

• • , approxlma1e :caylty molfel

300 ~

250

§: "'

C:

"'

.

..

200

t'i

-~

5C. 150 E

100

50

0

~ 0

5

10

15

20

25

30

Feed offset Y, Imm)

Figure 3.5 Input resistance (Q) of a resonant square patch antenna obtained using the approximation of (3.9) and (3.12) compared with t he cavity model as the feed position is moved from the edge. of the patch to the 0enter of the patch.

is varied from 30 mm co 80 mm with che feed offset (x0) sec conscanc at

20mm. The v.ridrh of the patch is importantfor several reasons. First, the widrh affects rhe radiation pattern through (3.8) and the input resistance of the antenna th~ough (3.9) (as shown in Figure 3.6). Figure 3.7 shows the variation in S11 as the width©[ the patch Wis changed from 45 mm to 65 1nm. The resonant frequency of r.he patch does not change significantly, which is expected because the resonant length of the patch remains constant. The impedance match rs affected by changing W Second, a coaxial probe will excite currents in boch rhe x and y directions. This may resulr in a large cross-polar component in the radiated field. This can be avoided by centering the feed probe along the width of the patch and offsetting the probe along the length of the patch.

88

Switched Parasitic Antennas tor CeJJuJar Communicatwns 200 •• , iipp10•1ma1, -c:aviw model

::f\ "'"".. C

t;

1-IO

,J

·~ C,

r

I

100 -

0.

C

80 t 60 -

:r 30

·····....······· 35

40

45

!if)

66 60 P~toh width Wlmml

65

70

75

Figure 3.6 Input resistance (Q) of a resonant square patGh antenna obtained using the approximation of (3.9) and (3.1 21 compared with the cavity model as the width of the patch is varied from 30 mm to 80 mm. ~

-5

~

7

J

- 10 l

-15 -

S,, !dBi

-20

W=4!imm -2li

~:-

W~ 5llrntn

W e 55 mm W: 60mm W=65mm

1

-JO

-;15

1.7

I 72

1.74

1.16

1.78

l ,8

I.Bl

I 114

1.116

1.811

19

f requency 1GHz)

Figure 3.7 S11 (dB) of a coaxial feed pa.tch as a function of frequency for various patoh widths.

Patch Antnmn..r

89

One disadvamage found in using the probe feed is chat the parch will nor be resonant when t > 0.1.A. 0 because of the inducranc~ introduced by rhe feed wire [21. The bandwidth Band radi'1.rion efficiency e, of a patch antenna both decrease as t,incrnases. The bandwidth increases as t increases, and the radiation efficiency decreases as t increases; hence, there is always a tradeoff berween bandwidth and radiation efficiency, as a high radiation efficien«y is required and a wide bandwidth is de-sirable. These rw0 effei::rs increase the sidelobe levels and cross-polar Gomponenc in the radiation field. An approrimation for the radiation efficiency was given in (3.11), and an approximation for the ll,andwidch is [91:

B=3.1~ 7 C1fJ( -1 e,

t,

X- X-W) L t

{3.13)

.A. 0

whore B is defined as the fractional bandwidth rdative to the c;emer frequency for a VSWR less than 2: 1. This is equivalent m an S11 value of -9.5 dB. The bandwidth and radiation efficiency of the square patch are plorred in Figures 3.8 and 3.9 versus rhe substrate height, for various values of substrate diele 90°. When the grnund plane has finite size, diffraction inm these angles results in a measurable fronr-to-baok ratio. This can be ca.lcu~ate.d using the geomel('ica.l theory of diffract.ion [I , l Ol or using a numerical solver such as Ensemble [7]. The ground plane should extend ar lease 3tpasr r.he edge ofthe patch, where tis rhe thickn~s of the dielectric. The square patch ( W = L = 55 mm, .x;-i = 20 mm, y0 = 27.5 mm) has been modeled on Ensemble [71 with square ground planes, ranging in length from 0.57'1.0 (equal to 6t+ 55 mm) to 1A0 • The E and H plane radiarion pamms are shqwt1 in Figure 3.10 and 3.11 and the S 11 is shown in Figure 3.12. As the ground plane size is increased, che radiarion below Ehe ground plane decreases, as expected, 1'!'_ __;:-- _

I

,,

1.84

1.82

Figure 3.22 Z11 (Q) tor arrays of one, two, and three patches; the patches are oriented for H plane coupling and the separation distanc:.e betwe.en patch edges is d = 0.0U 0 80 I

60

fl., l!l l

'

_,.,, - - ~

./',......

: l.. -< .-~-;::>

-.

, ~

"'

}

~-i, '--•·---_,.-•.,, -. ••--.;•.

r-----..:- -

0'----------10··

Ht

10 .

- rs·or.at,rd' pa1c~ · : • twa,patch affay - three-petch a.rrey - - fqur-pail:h arrav

Edge separatron distimce

dUJ

Figure J.23 Z, 1 (Q) at 1.795 6Hz tor arrays of one, two, three, and four patches versus the edge s€paration distance between the patches. The patches are oriented for E plane coupling.

103

Patch Amemu.s 100 ~ - - - -- ~ - - - - - -- - - -

R., lnl

....

40

-, I

2o r•:..,::·.: •_: ·.:·- -- : :; -::...,

-

__ _ . - ----

010·=

10'

10

60 ~ - - - -- - - - -- - .40 -

x. (nJ

10'

Fi11ure 3.24 Z 11 (.Q) at 1.7!!.5 GHz for arrays of on.e, two, three, afld four patches versus

I

the edge separation distance between the pa-tches. The patehes a.re oriented tor H plane coupling.

20 ___ ;.__L·::.:.~·.:-,,__ _ - , 0 ~------ ·:: .... ' -~----------·.. \ \ ,·i I - 20 ·. , i· , , 1

R. ffll --40 I - - - - - --

••• / \, ' j 1'

• • . two-parch·array

- 1h1ee-pa1ch array -60 ~ -=-lour-pstr:b

1

~tray

-80

I

- 100~~ - - - - -- - -1.7 1.72 \.74 1.76 I 78 1.8

40 ~ 20

x,, (.U}

0

1

-:

,

1.8'l

1.84

1.86

•\

1

1.88

I

1.9

•

,., '

.•f I • ...,,

----~----~- -o,:_,_____ _ - '

; Ir ·- ..~ .~-"""'•-

;

•

· --~. .......' ~

-20

• • •.,•'

,

J '

----·-

t I

~

I

'. I,

1.7

1.72

1.74

1.76

1.78 1.8 1 150

180

Figure 3.33 H plane radiation patterns (dBi) of the broadband stacked patch at the center frequency and the band edges.'...: 1.7147 GHz.·_ _ · 1.8258 GHz, and ._. _. 1.9369 GHz.

111

Patrh Antenna.

a.

-,

-5

\

- 10 - 15 $,, (dBi -20

'?

~ /

(

\I\

~

I

j

-25 -

-30

-35 ---40 - 1.6 1,65

1.7

us

1,8 1.85 1.9 Frequency 1GHz)

1.95

2.05

21

Figure 3.34 S11 (dB) for a dual-band stacked patch showing two separated resonant frequencies.

Figure 3.34 shows rhar rhere are two distinct resonanr frequencies, 1.704 GHz and 2.027 GHz, wirh VSWR > 2 bandwidths of 3. l % and 1.6%, compared with rhe bandwidrh of the single 55-mm square paTch of 1.02%. Figures 3.35 and 3 .36 sh0w rhe E plane radiation patterns of che stacked patch at che rwo resonant frequencies, along with the parrerns at the band edges. Figures 3.37 and 3.38 show the H plane patterns. Dual-frequency directional amenuas often have different radiation parterns at rhe two resonant free!juencies. If one frequency is used for the uplink aod the other is used simultaneously For the downlink, rhen there may be some uncertainty in determining the 0ptimal diren-i on of the amenna. It may be possible ro design a parasitic array of scaeked patches where the upper and lower patGhes are similar at the two resonam frequencies.

3.5 Switched Parasitic Patch Antennas As wirh rhe wire amenna elements out.lined in Section 2.7, ic is possible ro concml rhe effect 0n rhe impedance of nearby paras-itic parch elemems and thereby change the direction of che main beam of a patch antenna.

112

Switched Parasitic Antennas for Cellular Communications

,,,. oo ' \

90 I

I

I

I llO

,•

·•'

I

\

/

~_,-,.

210 •• ,_______ - - - -- - ---· 150 18()

Figure 3.35 E plane radiation patterns (dBi) of the dual-band stacked patch at the lower resonant frequency and the band ed.ges. ·.... ' 1.678 GHz, '__ ' 1.704 GHz, and ·_ . _' 1.730 GHz. ·

300

\

/

60

(\

- ---,-- -t-=~ ~ ::....~+ - --'---- ---:1 90

270 ......

I /

i 12Q

/ .,ii..

,/ /

IIIO

Figure 3.36 E plane radiation patterns (dBi) of th e dual-band stacked pateh at the upper resonant frequency and the band edges. ' .... · 2.011 GHz, ·__ · 2.027 GHz, and '_ . _' 2.043 GHz.

113

Patch A ntennas

60

llO

I

\

/

\

I

170

I

I\

911

) /y

\ \

120

240 /

180

figure 3.37 H plane radiation patterns (dBi) of the dual-band stacked patch at the lower resonant frequency and the and· . · 1.730 GHz.

band edg.es. •.... ' 1.678 GHz,·_ _• 1.704 GHz.

300

/

\

I

I

210

I

go

I

\

180

Figure 3.38 H plane radiation patterns (dBi) of the dual-band stacked pat&h at the upper resonant frequency and the band edges.·.... • 2.011 GHz.·__' 2.027 GHz. and '_ . _' 2.043 GHz.

114

3.5.1

Switched Parasitic Antennas for Cellular Communications

Switched Active Patch Arrays

The simplest switched parasitic p.ar.c h array is shown in Figure 3.3.9. lt consists of rwo patch elements oriented for maximum E plane coupling. Om element is active and the other dement has the feed position open-circuited Tbe position of the feed can be switched berween either element, and so th( beam can he direcced in two positions separated by 180° in azimuth. This i1 similar co r:he wire SASPAs discussed in Chapter 2, and is referred ro as , parch SASPA. The array consists of two 55-mm square patches, with feec offset (y1 =y,J of 14 mm constructed on a subsffate wirh f , = 2.2 mm and t = 1.6 mm. The separation distance between the edge of the two demenrs is d= l.5 mm. This separation distance equates to 0.009}, 0 and gives rhe best directional radiation pattern. Figure 3.40 shows the reflection coeffici.ent obtained frnm Ensembl< [7], with a VSWR < 2 bandwidth of.0.6% in switch position J and 1.1 % ir switch position 2. Figures 3.41 and 3.42 show rhe E and H plane radiatio.r patcerns also from Ensemble [71 in both switch posirions. Position 1 is defined when elemenr I in Figure 3.39 is open circuit and elemem 2 is acrive and. pos:icion 2 is when element I is active and demenr 2 is open circuit. Th( maximum gain of the beam in position 1 is 8.114 dBi and in position 2 ii 8..306 dBi. 1n the E plane radiation pJor, the change in signal screngrh between the two switch positions at 0 cc: 30° and 60° is 8 dB and 15 dB rnspectively. As expected, there is almost no change in rhe H plane radiacior pattern. For chis panicular design, the direction of the maximum radiacior has been shifred significantly ( + 18° and -1 6°). The addition of more para· si.tic elements either side 0f che currenc ones will puU the beam down further. This is demonsrrated in Chapter 4.

-

d

+ -- +

(1)

RF input - - - C Y

Figure 3.39 Two-element switched active patch array. The parasitic element foed posi· tion is always set at open circuit.

115

Patch llmmnas

°L -2

-4

"'

~

"\ \

r

,

'\

/;,

I

,--

-6

/

I

I

S,, (dB)

~

-8

- 10

Position 1 P'osition 2

- 12

-14

1.7

1.72

1.74

1.16

1.8 1.82 1.78 Frequency 1GHz)

1.84

1.81i

1.88

1.9

Figure 3.40 S11 (dB) of the two-element patch array shown in Figure 3.39 in both switch positions.

3.5.2

Switched Parasitic Patch Arrays

There are some advantages to fixing the position of rhe active element in an arr~, as described. in relation to rhe wire arrays. These arrays are referred ro as fixed active switched parasitic arrays (FASPA). A rhree-dement switched parasiric parch array is shown in Figure 3.43. This switched parasitic parch arr~y has a fixed active el€ment located cenu:ally, ancl a direct·ional beam is formed when one of the oursidc elernenrs is open-circuited and rhe other is shon-circuiced. The beam has cwo possible directions separated by 180° in a2i.muth, which are achie,•ed by switching the position of the snort- and open-circuited elements simultaneously. Figure 3.44 shows the reflection coefficient in bod. switch positioa_s for an a:rray of three 55~mm square parches wirh feed offier of 14 mm on a rnbstrace with E, = 2.2 am:! t = 1.6 mm, and edge-to-edge separation of 1.5 mm. Position 1 is defined when element l is active, elemenc 2 is open circuit, and elemen c 3 is short-circuited. Position 2 is wben element 1 is ac-mve, element 2 is shorc-circuiced, and element 3 is open circuit. The VSWR < 2 bandwidrb for position J is 1.2% and for p0sitio11 2 is 1.4%. Figurns 3.45 and 3.46 show rhe E and H plane radiation

116

Switched Parasitic Antennas for Cellular Communications

O

:lSG

10

--~----- - - - -- •

30

180

Figure 3.41 E plane radiation patterns (dBi) of the two-element patch array for both switch positions·at 1.776 GHL ·__ • Position 1; ·___· position 2.

90

180

Figure 3.42 H plane radiation patterns (dBr) of the two-element patch array for both switch positions at 1.776 GHL ·_ _ · Position 1; ·___· position 2.

117

Parch Amennas

-----> d

/,----r/j - - -,-, • ~- -----~

d

lex switched parasitic antennas.

References [ I J B,1.la.ni$, C A, Amtrinll J'tuory A,cafy,is ,md DeJig,,, 2nd ed., New York: John Wiley and Son~. 1997. [21 Snm.man, W. I.., and G. A. Thiele. t lnmma Themy ,md D~sig,1, 2nd ed .. New York: John Wile\· and Som, 199$, [3] Sai1mi, R. A.. CAD of /1,ficro;rrip Amm,rm for Wirekss llpplimtions. Nonvood, ,"-lA: Am:ch House;, 1996. [4] Zurl' her, ,I. f .. and F. E. Gardiol. Br11tulba11d Plf(Th Ant,•n·n,u, Norwood. MA: Arr~:ch House, 1995.

[S·] Himsawa. K., and 1\rt. Haneish1. Ana~J•-•is.. D,,,1gu. and ]V/ea.,urnmmt of Small muL L()u1l'roft!e Antm,u,s, Norw0od. MJ\.: Arrnch House. 1991. [6] (;u pta, K. C, and P. S. Hall (tds,), AnaljsiJ ,rnd De,·ig11 of hu(gmrul Ciro,ir ,foren11r; Mod.1,/n, New Y0rk: John Wll\:'y an.d Sons, 200.0.

[7) Ansofr Cnporntion, En~cmblc, 200(1. ht.tp: //www,ansok,mm, [8] J.1ckson, D. R., anJ I\'. e,;, Alc,xopoulos, "Simple, Apprnxiniace Formulas for lnpllt Reshiancc, Bandwidrh and Efficiency of a Rcctarlgular Patch," IE.EE Tr.-ms. .r!J1tcl/11tt.< aud Pmp,igaw>11, \lc,I. .3, M:irch 1991. pp. 407--4 10. [9] Huang, J.. "The Finitc CrounJ l'la.nc Eff"n on tlit: Microsuip Aruenna Radiation PJttern, - IEEE Tmm.. Anteml/lS ,md P;·,1pwi{,11tio1J, Vol.JI, July 1983, pp. 649-;b53. [10] Lee, K. F., and W. C...htu1111t1J, New York: John Wiley and Son~. 1997 . (11 J Jame~, J. R .. anJ I'. S. H2II. H,mdbonk t1/Micr1JJ·trip A11remws. Londc,n: rerer Peregrinus, 1989.

[ 12] Co-JI ins. R. E.. Field Theory• "(Gttidd W,we.~ 2nd ed., N] Dcrrlsed

Closed Open

Closed

Closed

CICJ_sed

Open

Closed

Closed

Closed

Open

t----

Open

Closed

180°

OP..en

Open

Closed

Open

Close.cl

270°

Open

Open

Open

CIOSEJd

Closed

\

I - 20

l

- 25

j

S,, ldB)

"

I

- 30

- 35 ' - - --'-- - ---L--- -- - .L-----'-- - L--- - -- - 1,1

l.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

i 2

Frequency l,GHil

Figure 4.17 Measured frequency variation in the S11 (dB) of the four-element monopole SASPA shown in Figure 4.16 in one switch position.

Design E...-ample1 efSwitched Parasitic Antennas

m

147

~

\ \ ' "'\,✓·

I I

/1 ••

210' - - - - - - - - ~ 150

180

!al

lbl

Figure 4.18 fa) Measured E plane radiation patterns (dBi) of the four-element monopole SASPA shown 'in Figure 4.16 ·, none switch position.·_ _ · 1.35 GHz; ·__ _· 1.40 GHz; ·_ . _· 1.45 GHz. (bl H plane radiation pattern (dBi) of the fourelement SASPA shown in Figure 4.16 rneasured atBmax = 52°, for one switch position. ·_ _· 1.35 GHz;·__ _· 1.40 GHz; ·_. _· 1.45 GHz.

148

Switched Parasitic Antennas for Cellular Communications

\

I I

\,

270

90 I

\

\

'-

271)

(bl

Figure 4.19 (a) Measured E plane cross-polar radiation pattern (dBi) of the four-element SASPA shown in Figure 4.16 in one switch position. ·_ _ · 1.35 GHz;·_ __· 1.40 GHz: ·_ . _· 1.45 GHz. (bl H plane cross-polar radiation pattern (dBi} of the four-element SASPA shown in Figure 4.16 measured at Bm~~ "' 52°, for ane switch position. •_ _ · J.35 GHz; •___• 1.40 GHz; ·_. _· 1.45 GHz.

Dl!sign E-.;amp!:es r,_(Switched Pa:rtl5itit· Antmnas 149 - - -- - - -- - 90 10

0

180

270 Figure 4.20 H plane radiation pattern (dBi) of the four-element SASPA shown in Figure 4.16 at 1.4 G.Hz measured at Om.,= 52°, for all switch pasitions.

4.3.2

Five-Element FASPA

A five-element monopole FASPA on an infinite ground plane deaigned for 2-GHz ope-ration is shown in Figure 4.2 1. The center demem has length L, = 0.224J..0 , and rbe parasitic elements all have the same length L2 = 0.267210 • The radius of the ring is r = 0.25A0 • A dircccional beam is formed by an appropriate ch.oice of the parasitic demem termination, and the beam is steered by switching rhe rnrmination on the parasitic c::lemenrs around the array. The swir-ch position of the four parasitic el~mcms in Figure 4.21 is marked wirh an idenrifier, Dn (n = 1,2, 3, and 4), t0 illusuate that rhese elements are under diode control. Table 4.5 gives die switch configurations required ro generate four identical beams separated by 90° in azimuth.

15-0

Switched Parasitic Antennas for Cellular Communications

vf

◄ --

T

Figure 4.21 Five-element monopole FASPA on an infinite ground plane.

The array was 1:nodeled with an intinire groU:nd plane and a wire radius of 0.0059.-l.0 . The 5 11 is given as a function of frequency in Figure 4.22, and rhe H plane radiation pattern for one switch position is given in Figure 4.23. The impedance bandwidth is greater c.han 43%, B,:, = 120°, and c.he fromr0-back ratio is 11.7 dB. The maximum gain of rhe array is 9.98 dBi. The

Table4.5

Switch famfig-ur.nions of a Five-Element FASPA Required

t-0 Give

Four Syrnmetrrcal Beams

Beam Direction

1Prnax1°I

0 90

Dioile Switch State

D1

m

D3

D4

Open

Close:d

Closed

Closed

Closed

Open

Closed

C!.osed

180

Closed

Close:d

Open

Closed

270

Closed

Closed

Closed

Open

Design Examples ufSwitched Parasitic A11temu1..f

151 ,---

0 -2 -4 ~

-8

t

S,, ldBl - 10

~

- 12 - 14 - 16 - 18

- 20 1.2

1

14

1.6

1.8

2

2.2

2.4

2.6

2.8

Frequency (GHz)

Figure 4.22 Frequency variation in S11 (dB) for the five-element monopole FAS PA shown in Figure 4.21. 90 10

150 I

/

I

'DO

Figure 4.23 H plane radiation pattern (dBi) of the five-element FASPA shown in Figure 4.21 in one switch position at 1.9 GHz.

152

Switched Parasitic Antennas for Cellular Communications

five-element FASPA can obtain full 360° azimuthal coverage within l dB of the maximum gain. There are a number of rules dun can be followed when designing an FASPA wirh a central active demem and a single ring of switchable parasitic elemrum. The center frequency and the impedance bandw:idch ca:n be ser by using parasitic elements slightly longer rhan 0.25l0, and then reducing the length of the feed monopole from 0.25). 0 . The from-ro-back rario and antenn-a gain are determined principally by the number of parasitic elemems and the radius r of rhe army. Significant directionafoy is obtained when r < 0. 5A0 • The nurn ber of parasitic elements thar are set at open circuit for a fixed beam direcrio11 should be chosen to ensure rhat rhe angle from the feed monopole is greater than 60°. Higher gain and beam control in rhe 0 pla:ne can be obtai.necl if additional rings of para.~itic elements are added to the array [10, l I].

4.4 Multibeam Wire Array An array using a combinatioB of SASP As and FASP A.~ is capable ohupponing multiple active clements. The array is referred to as a mukibeam switched active switched parasiric array (MSASPA) [42, 44J. The arr.1y consists of a central fLxed reflecror clemenr surrounded by at least two concentric rings of elements. The acrive dernem(s) lie on the inner ring, a.nd all elements rhar are nor active may hie eirher open- or short-circuited parasitic elemenrs. An example of a 13-elemenr monopole MSASPA is given in Figure 4.24. lo order to generate a direccional beam, the parasitic element located on the ourer ring io line with rhe required direcrion of maximum propagation must be open circuit, and the remaining elemems "behind" the active dc;ment are shorr-cireuired. This forms a comer reflector around rhe active elcmtent, giving sufficienc isolation for multiple simultaneous active elements. ln Figure 4.24, elements 1-6 h;¾Ve both a diode at the feed posirion to swirch the elements between open and shore circuit and an RF switch ro cont-ml the position of the accive element. Elemenrs 7- 12 only have a diode ar the foed posirien ro swirch between open and sh.ore circuit. Table 4,6 lists the switching configurations requirnd to gen.erate six single beams separated by 60" in azimuth. Table 4.7 gives the i.witching configurari0ns required co generate the nine possible combinations of rw.0 simulraneous beams, and Table 4.8 gives the configuracions required to generate the two combinarions of three simulraneous beams.

Design Examples ofSwitched Parasitic Animnas

153

! --

'4

010

SID~♦------- r, ___ ___ :::

l

t

X

►

!

Figure 4.24 Thirteen-element monopole MSASPA.

For all MSASPAs, the number of beam-switching combinations)' can be calculare,.,.x("I I

o•

I

600

1200

180°

240°

300"

RF

S1

Closed

Open

.Open

Open

Open

Open

SWitch

S?. 83·

Open

Closed

'Open

Open

Open

Open

Open

Open

Closeo

Open

Open

Open

S4

!)pen

I State

I

Open

Open

Open

Clos~d

Open

So

Op~n

'Open

Open

Open

Closed

Operr

$

Open

13pen

Open

Open

Open

Closed

' Diode

01

Qpen

Closed

Closad

CIO'.ied

Closed

Closed

[swiMh

fJl.

Closed

Open

Closed

Closefl

Closed

Closed

£Jd

Closed

GJosed

Open

Closed

Closed

Closed

I

I

I

IState I

I

D4

Closed

Closed

Closed

Open

Closed

Closed

[Jj

Closed

Closed

Closed

Closed

Open

Closed

Yo

Closed

Closed

Closed

Closed

Closed

Open

[J1

Open

Closed

Closed

Closed

Closed

Closed

lJd

Closed

Open

Closed

Closed

Closed

Closed

D9

Closed

Closed

O_pen

Closed

Closed

Closed

Lno

CJosed

Closel'I

Closed

Open

Closed

Closed

01 1

Closed

Closed

Closed

Closed

Open

Closed

012'

Clos£d

Closed

Closed

Closed

Closed

Open

I

I

gle accive element and also for the case when chere are three aetive elemencs. Figure 4.26 shows r.he E plane and elevared H pl.me radiarion parrnrns, togerher wich che cross-pt1lar radiation pam:rru for clle case when there is a single active elemenr at r.hree frequm cies. T he main-bean1 direction 0 ma. = 58° ar all r.hree frequencies, wich a gain of 5.9 dBi ,ll 1.36 GHz and 7.4 dBi at 1.46 GHz. The H plane radiation patterns were measured at an elevation angle oF 0md,, = 58° and the beam width is B,p ::= 67° at 1.36 GHz and 92° at 1.46

155

DS!!d

Closl!d

Clos'eil

tin.et!

Ojlen

011

Closed

Close1J

Opei,

Closed

Open

Closec!

Open

Closed

Closed

012

Cl9sed

CJQSed

Clc.>sed

dosed

Clo~ed

Open

Closed

Open

Open

I

I

I

I

I

GHz. The front-to-back ratio is JG dB at 1.36 GHz and 12 dB at 1. 46 GHz. The cross-polarization levels are all less rhan -20 dBi. Figure 4.27 shows E and H plane radiatio n patterns fur rhe case when there are three active elemems in che arrav. The radiatio n characrnristics are similar to tho se for 1he case of one active beam st-town in Figure 4.25 . Tl-te main-beam di.met.ion is 0m= =:c G0°, with a slightly higher gain of 8 dBi. The H plane r-adiari0n pattern was mea.rnred ar an elevarion angle of 0 "'·" and shows a be.am width B'I> = 58° and a front-ro-b.ick rario of greater rhan l 2 dB for aJJ frequencies. The cross-polari1..arion levels are all less than -20 diji_

156

Switched Parasitic Antennas for Cellular Communications

- -

- -

- ---

--

- -

-

-

-

Table 4.8 Switch Configurations of a 13-Element MSASPA Required 10 Give the Two Possible Combinations of Three Simultaneous Active Elements Beam1 1p_.(0 ) Beam 2 l/>mu( Beam3 1/>,,,..(0 )

Ir

so•

121r 241r

181r 300°

RF Switch

Closed

Open

Open

Closed

0

)

State

Diode Switch State

S1 S2 53 S4 S5 S6

Closed

Open

Open

Closed

Closed

Open

Open

Closed

[JI

Opeh

Closed

m

Closed

Open

03

Open

Closed

04

Closed

Open Closed

{J5

O'pen

[Ji

Closed

Open

UJ /Jd

Open

Closed

Clos~d

Open

[JJ

010 011 [)12

IOpen

Closed

Closed

Open

Open Closed

Closed

I

Open

The mutual coupling berween rwo or more active elements in che array is of importance in duplex communications applicarions. When one active clement is transmining.a high-power signal and rhe ochers are receiving lowpower signals, there is a possibility of incerference in the receiving channels. Figure 4.28 shows char che mutual coupling berween che three simulraneously fed accive elements is as high as - 13 dB at some frequencies in rhe range 1.2 GHz- 1.6 GHz. Across the frequency r.mge 1.385-1.52 GHz, rhe coupling is always below - 29 dB.

157 De.sign Ev:amp/e, ofSu;itched Pamsitic Antmnas - -- - - -

-

- -- - - - - -- - -·

- - - --r--:--- -2

S,, (dBi

➔I -8 -

- 10

L

- 12 ~ _______. ~ 1.4 1.5 1.6

- 14 1

\.1

1.2

1.3

- - l.7

19

1,8

Frequency IGHzl

!al

o~ -

-

-

-

,-

-

-[

~,r\0

~. ~/\

(V/~\

6

(

I

S,, (dBi -

-a l

\~

/

\

.J

rnf\ I

\

j

\

~

-I

V

- 10 -

I

--

- 12 - 1

11

1.2

I3

1.6 1.5 Frequency !GHz!

1.4

1.7

---'-

1.8

__ J 1.9

2

lbl Figure 4.25 (a)S11 (dB) for the 13-element MSASPA shown in Figure 4.24 with a single active element lb) S11 (dB) for the 13-element MSASPA shown in Figure 4.24 with three simultaneous active elements.

168

Switched Parasitic Antennas for Cellular Communications

O 10

330

JOO

\\ i40

180

la) 0

0

\ 90

270

\

I

,/ 180 !bl

Figure 4.26 (a) Measured E plane radiation pattern (dBi) for one switch posrtion of the 13-element MSASPA (Figure 4.24) with a single active eternent. ·_ . _· 1.36 GHz. ·__ _ · 1.41 GHz, '_ _· 1.46 GHz. (b) Measured E plane cross-polar radiation pattern (dBi) for one switch position of the 13-element MSASPA

(Figure 4.24) with a single active element.·_._· 1.36 GHz,•___· 1.41 GHz. •__'1.46 GHz.

Design Example; o_fSwitched Parasitic Antennas

159

270

le! 90 0

1~0

I"

/

240 270

Id!

Figure 4.26 (c) H plane radiation pattern (dBi) for one switch position of the l3-element MSASPA (Figure 4.24) with a single active element measured at 0m,~• ·_. _· 1.36 GHz,' ___· 1.41 GHz,·_ _· 1.46 GHz. (d) H plane cro.ss-polar radiation patt.ern (dBi) tor one switch position of the 13-element MSASPA (Figure 4.24\ with a single active element measured at 8 ms.- ·_ . _· 1.36 GHz, · ___· 1.41 GHz,'_ _. 1.46 GHz.

160

Switched Parasitic Antennas for Cellular Communications

10

JOO

\\

\ 90

270 (

I

\

\

240

180

lal 90 10

60

210 \

\...•,,, 240

' ' --------1.- ---~ 270

lbl

Figure 4.27 (a) Measured E plane radiation pattern (dBi) of one beam for the 13-element MSASPA (Figure 4.24) wrth three simultaneous a-ctive elements. ·_ . _· 1.36 GHz,·__ _· 1.41 GHz,·_ _• 1.46 GHz.. (b) H plane radiation pattern (dBi) for one switch position of the 13-element MSASPA (Figure 4.24) with a single active element measured at 0m•x· ·_. _· 1.36 GHz, · ___' 1.41 GHz, ·_ _•

1.46 GHz.

161

Design Examples ofSwitdmi Parasitic Antennas -10

- -~ - - - -- -- ~ - - - - - -~ ~ -""""T""- - ~- -,

- 20

-30 ~

m

11 I

:!:'.

cJ

-40

i'I

ii I

(If

I I

c,f' -50

u] I

--60

- 70

L___

....L...-

1.1

- - - - - ' - - -----'---

1.2

1.3

___J.__

_

__,___

___.__ _ _ ___,__

1.6 1.4 1.5 Fr-equency (GHz,)

1.7

1.8

_

_ ___J

1.9

2

Figure 4.28 Measured mutual c·oupling (dBi between teed points of the 1'3-element MSASPA (Figure 4.241 with three simultaneous active elements.

4.5 Switched Parasitic Patch Antennas Switched parasiric and switched acrive patch arrays were introduced in Chapter 3. In chis searion, rwo examples of optimized switched parasitic patch arrays are discussed. The switched parasitic patch array bas been published [45). A five-element linear array of lineacly polari1.ed patches is shown in Figure 4.29. 1n th.is array, the central active element is 41.6,8 ram square, th e parasitic elemems on either side of che acrive element are 41.32 mm square, and the outer parasitic dements are 40.86 mm square. The edge separation distance be~n rhe active element and the par-asiric elements is 2.04 mm. The aetive demenr feed probe is ccmre.red in rhe x dimension and offset by 11.35 mm in they dimension. The parasitic element short probe is also centered in the xdimension and is offset by 2.04 mm in chc _ytlimension. The substrate is l. 56 mm chick with a dielectric constant oh:, = 4.35 and a loss rnngenr of ta.no = 0.01.

162

Switched Parasitic Antennas for Cellular Communications y

X

□□□□□

Figure 4.29 Five-element linear switched parasitic patch antenna.

In Figure 4.29, the -= 90° beam I. ·__· Modeled, ·___ · Measured. (b) E plane radiation patterns (dBi) of the patch array shown in Figure 4.29 with 01 open and 02 closed ( = 270° beam). ·_ _ · Modeled, ·__ _ · Measured.

Design Examples ofSwitahed Parasiti, Antl'mtds -

165

--------

0 0 330

------,_

30

60

300

I I

\

\\ \

I

i

90

270

I

/

·' 120

180 (c)

Figure 4.31 (c) Me.asured E plane cross-polar radiation patterns (dBi) of the patch array shown ifl Figure 4.29 with D1 closed and 02 open.

the parasitic elemenrs on either side of the active element are 57.75 mm square, and the 0uter parasitic.:: elements are 57 .12 mm square. The edge separarion distance between all adjaccm clements is 2.23 mm. Feed probe RFI is cemered along th~ x dimemion of rhe patch and offset by 17.61 mm from the edge of the patch in they dimension. Feed probe RF2 is centered

LJ• m □ □ D1 •

01 •

D1 •

X

• 02

1

• D2

Figure 4.32 Five-element circularly polarized switched parasitic patch array.

y

166

Switched Parasitic Antennas for Cellular Communlcatfons

along rhe y dimension and offset by 16.68 mm in che x dimension of rhe patch. The parasitic probes are evenly spaced along che x dimension and all are offS'et by 2.86 mm from the edge of rhe parch in rhe y dimension. The swirnh configurarions given in Table 4.9 apply ro rhis array as well. Figure 4.33 shows ch.e measured S11 , S'i2 , and 5 12 for rhis array in borh switch positions widl a ground plane size of 355.5 mm by 118.5 mm. The same sc

\: Ji \ : ;

S,. ldB) ~35

(, { ~•

\ :; 1;; !•

--40

I

I I

11

'

:j

;

I I 1 J I I

I

;:

-'

I

I

IJ

··1 I

1

~' ' '-- -Ensemble -- - -- ,p= .90" beam ~

• • • Measured: = SO' beam - - Ensemble,?= 270' bl!aiTI

'~

:'

-50 - 55

c __

1.5

_

_.___

1.55

_

.J..__

1.6

_

_,___

1.65

-

1

_

. L . __

Me,!l_;;t1red ¢ = 270' ~e"q m _

1.7

-".-.__

1.75

_

_ , __ __ . __

1.8

1.85

____,

1.9

Frequency lGHl)

le)

Figure 4.33

(cl Measured and modeled S12 (dB') of the five-element circularly polarized FASPA shown in Figure 4.32.

where c is rhe velociry of light in a vacuum. /is the freq uency of the radiation, £, is the relative permittivity, andµ, is the relative permeabiliry of the medium. In free spacet:,= landµ, = 1, and so.A.,,, reduces coA. 0 . The half-wave resonance condirion occurs when the lengch of rhe wire I is given by:

(4.5)

In most communications siruarions, the region berween the transmitting and receiving antennas is free space (i.e., aid and so rhe embedding material surrounding one antenna is nor of infinite exrenc. le is sciJI possible co reduce che length of the wire demenc by coating ir wicli a layer of dielectric or magne tic mate rial chat has a finite width. In this case, (4.4) is no longer accurate and the reso nant length of the antenna depends on the thickness of the coating as well as e, and/lr

Desig:n Examples ofSwitched ?ar11.si1ir Antennas

169

O 10

!I 90

\\

I

I I

no

2'.40 \ \,

"'I

~ - -- _____1______ 180

!al

300

60

/

'\

I

\

/ \

24ll \

~-1 ' 210

.

·--------180 (b)

Figure 4.34 (a) Me-asured and modeled RHCP radiation patterns (dBi1 of t he circularly polarized FASPA shown in Figure 4.32 with 01 closed and 02 open (0 cut when tp = 90°). •_ _• Modeled, ·___· Measured. {b) Maasurad and modeled RHCP radiation patterns (dBi) of the Circularly polarized FAS PA shown in Figure 4.32 w·1th D1 o·pen and D2 closed \9 cut when = 210"). ' _ _• Modeled, ' ___· Measured.

170

Switched Parasitic Antennas for Cellular Communications

When the coating is thin compared to 0. 52,n, only one surface wave mode can propagate, and rhe effective lengrh of r:he antenna is independem of the type of foed. 1n this case, the antenna can be treated in a manner similar co wire anceuna structures in free space, bur with the effenive length and separacion berween rhe wires larger Ghan the true dimensions. When the coaring is greater rhan or equal co 0.5.Am, more rhan one surface-wave mode and a number of caviry modes are possible. The location of the feed point is significant in determining which modes are launched. T he ra.diacion charac:tcristic_s are dominated by resonances in the die.l.eccric material alone r-arher rhan in the wire elements. This dass of ancennas is referred ro as dielectric resonator antennas (DRA). These rwo antenna technologies are disoussed separately.

4.6.1

Dielectric Coated Wire Antennas

Consider the case of a monopole antenna with radius a, on a ground plane of i.11£nire extent (Figure 4.35). The monopole is located ar the center of a dideccric cylinder having outer radius band elecrromagneric propenie.s t,and

i

~. I I

b

j I

:,.._., a : I

I

t,

,,, Ground plane

Coai :~?~" ~:.:;; L. __

I

-- -

.....

Figure 4.39 SASPA embedded in nylon for 2.4-GHz operation from [54).

- 10 -

-id I

S,, (dBi - 20 -

- 25

If

- 30

I-

- 35

1

--40 ' 2.1

2.2

2.3

2.4 Frequency !GHzI

2.5

2.6

Figure 4.40 Measured S11 (dB) tor the SASPA embedded in nylon (Figure 4.39).

2.7

176

Switched Parasitic Antennas for Cellular Communications

90 O

0

/

----~o ~--'----270

Figure 4.41 Modeled H plane radiation pattern (normalized dB) for the SAS PA embed· ded in nylon (Figure 4.39) at 2.4 GHz using a finite-difference time-domain model.

4.6.2

Dielectric Resonator Antennas

At high frequt-ncics, the siLe of anrenna structures becomes very small and che appropriate scaling o( the conducting elements makes construcrion djffi. cult. The length, diameter. and spacing of wire demenrs arc critical and subject to fine mechanical tolerance. The depth of penetration of the current along the wire (the skin depth) is related ro the frequency [48}. This is discussed in Chapter 6. The nee result is chat rhe wire res:isrance increases a.-; rhc frequency increases. The fin ite conducrivicy of wire elements of small diamccer exacerbates this effect. To overcome chese problems, ic was suggested chac a resonant dielectric cavicy could serve as a radiating element [55]. The antenna consists of a dieleetric volwne lying on a ground plane fed b)' a shorr conducting probe located inside che dielectric or an open-circuit microstip line located immediately below the rudectric. The shape of the ORA can be cylindrieal [55).

177

Design Exampl~s ofSwitched Parasitic Antennas

hemispherical [56J, or rectangular [57) . This DRA is similar in principle to a meral cavity resonamr where there is field leakage through rhe dielectric-air interface. The radiation pauern and d1e input impedance of the DRAs can be cialculared analytically using the boundary ea:ondirions appropriate to the resonant cavicy mode ex.cited [55-57] . A cylindrical ORA fed wirh a feed probe offset from the cenrral p~>sition has a figure-of-eight H plane polar pattern [55] when excited at the fundamental HEM 11,1 mode. It is possible to switch the main-beam dfret:tion in the H plane by changing rhe feed position ro one of a number of probes already locared inside rhe dielectric [58] (see Figure 4.42). The unused probe feeds are nor oonnecred ro rhe ground plane and so play lirde role in the overall characreriscics of rhe antenna similar to the open-circuit elements in FASPAs and SASPAs. Kingsley and O'Keefe [591 e~rendecl chis concept by using a power splirrer/power combiner to feed more than one probe simultaneously. This antenna was designed for monopulse radar applications at VHF. ln chis way, the number of beam positions can be increased significantly, and rhe array has characteristics similar lO rhar of a phased array. When che power spliner provi.des unequal power to the rwo prob~. addirional positions of rhe main beam can be achieved [59]. As the antenna beamwidtb is quite kirge, the shift in rhe posirion of the nulls in the radiation pattern is more imporram for maximizing d:1e signal-~o-noise ratio rather than a significant change in gain at angles close to the main-beam direction.

D ♦- ---------- --- ------- -- ---- --------- -►

.. f., .

--

I I I

I I

:H I I

I I

• I

Ground plane

Figure 4.42 Electronically steerable ORA.

178

Switched Parasitic Antennas for Cellular Communications

A switchable ORA structure was designed for operarion at approximately 55 MHz using water as the dielectric material. The water was contained in a PVC cylinder (diameter D = 550 mm and height H = 200 mm) mounted on a small, conductive ground plane. Tbe length of the probes was approximately L= 100 mm. The I 0-dB impedance bandwidth was measured ro be approximarely 1 MHz [59]. As the relative permitriviry of water is Er= 84 ar a rernperarure of J 5°C, a significant decrease in anrenna sizt! was achieved.

4.7 Tin-Can Antenna Ir is often desirable co incorporate antennas into existing infrasrrucrure or co design antennas thac are unobtrusive in the urban environmenc. The adv.anrages in chis approach indude improved aesthetics, a reduction in the number of attacks by vandals, redm:ed wind resistance, and reduced cost$ associated with c:be inscallation of additional support sr-ru.crures. The tin-can antenna is a five-elemenc monopole FASPA antenna designed to fit inside a 10-cm-diamerer PVC pole (see Figure 4.43). Problems associated with the finice ground plane (sec Secrion 2.6) are reduced by the addition 0f a conducting cylinder with a lengch of approximately }, 0 /4. The design frequency was 1.36 GHz and so the monopoles are 55 mm long wirh a radius of 2 mm and are mounred on a flat circular ground plane of radius 32 mm, char forms rhe cop of a hollow cylinder with lengrh t = 70 mm. The control electronics can be housed immediately benearh the ground plane. The ground plane therefore resembles a tin can wirh rhc feed cable end opened. The four parasitic elemencs lie very close to rhe edge of the grLtnd plane, and the lengtli of the cylinder is optimized co conrrol the elevation angle of the radiarion pac-cem. The resonant frequency was measured robe Jo= 1.39 GHz. The tin-oan antenna is an exce.nsion o f a coaxially fe.d monopole with a conducting sleeve shown in Figure 2. I 5 (a). The cylinder provides a quarter-wave resonant ground reference, giving a performance similar co an infi.nite ground plane, but wich rhe additional bcmefir of being able co control che elevacion angle of rhe radiation. In many siruacions it is desirable char the ancenna is lo cated above the level of rhe surrounding objects so that li.ne-of-sight illumination of the area is possible. For many applications, the beam direction shouJd lie close co or even beneath the horizontal pl.ane.

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In Figure 2.17 it is evident thar the effect of a finite circular ground plane on a single monopole antenna is ro elevare the radiarion above the horizontal plane. The radiation can be directed toward the h0rizontal plane by increasing rhe length of the tin-can t. This has only a minor effect on rhe resonant frequcncy of rhe antenna. In Figure 4.43, if element I is open circuit and elements 2, 3, and 4 a.re short circuit. rhe radiarion is directed in the

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Figure 4.46 Parabolic-dish reflector antenna used in line-of-sight microwave links. The feed is located at the toe us.

182

Switched Parasitic Antennas for Cellular Communications

also makes precise pos.irioning of che antenna difficult in conditions of high wind. Ruze [62) demonscrared char the principal beam direcrion is changed slightly by mechanically moving the feed position in the venical plane. Such a movement, however, subscancially degrades the performance of the amenna i.n terms of beamwidch and gain, as rhe feed is no longer at the focus. An alternate solution proposed by Durnan [63] uses a switched parasitic anrenna located at rhe focus as the feed. Figure 4.47 illusrrares a parabolic dish antenna with cwo parasitic elemenrs displaced vertically and symmetrically about rhe driven dipole feed element. The driven antenna remains at the focal point of rhe dish. When rhe top parasitic element is shore-circuited, significant currenc is induce-cl in chis element. The total radiation pattern is the summation of the cwo components. The beam can be adjusted by switchi.ng the pamsiric elements ro increase the radiated power in the position of the first null. Figure 4.48 shows the measured H plane radiation patterns of the array shown in Figure 4.47, with a 32-cm dish for the case whe.n both parasitic dipoles arc open-circuiced and also when the lower parasitic dipole is shorr-cin;uitcd. This shows char rhe beam is shifted by apprnx_imardy 10° in the¢, plane wirh linle or no degradar.ion in the gain and beamwid.th. These patterns were measured at l GH1.. The optimal position of a verrically displaced parasiric elcmeor was found ro be ar approximately 0. 5A 0 • With chis separation distance, the current magnfrude in the pai:asicic element is approximately 40% of rhac i11 the feed antenna, with a phase delay of slightly les.., than 0753. April

[18] Pvker, E.G.. Antenna Pancm Symhesis and Shaping, U.S. Parenr No. 4260994, April 7. 1981. [19] Henderson. A. S .. Antenna Having Electrically Positionable Phase Ce-nrer, U.S. Patent No. 4387378, _lw1e 7 . 1983. [20J Audren, J., and P. Brault, High Fr~quency Antenna with a Variable Direcring Radiation Parrem . U.S. l'a1enr No. 5235343, Aug,ist 10, 1993. [21] Sanford. G . G., a.nd P. M. Wcsrfeldt, E.loccron.ieally Remnfigurnhle Anrmna, U .S. P~tenr No. 5294939, March 15. 1994. [2-2) Pri1chcrt, D . M .. Communication Sym:m and Method~ Utilizing a Reacciv~l_v Concrolled Din'Ct:ive Arrn.y, U.S. Parent N o. 5767807.June 16. 1998. (23] Taen1.er, J. C., .Adjusrablc Array Antenna, U.S. Parent No. 59054 7 3. May 18, 1999. [24] Koscica, T. E .. and B. J. Liban, Azimmh Sceerabk Antenna. U.S. Parent No. 6037905, March 14, 2000.

186

Switched Parasitic Antennas for Cellular Communications

[25J Dinger, R. J., ~Adaptive Microstrip Antenna Array Using Reamivdy T erm-lnined Parasit-ic Elements,'' IEEE AJ>S lntemational Sympo;ium Digm, Albuquerque, NM, 1982,

pp. 300-301.

J., "A Micmmip Powc:r Inversion Array Using Parasiric Elements," !£El:: APS lntmuuional Symposi1i?11 Digut, Houston, TX, J 983, pp. L9 l- l 94.

[261 Dinger. R.

[2 7] Dinger, R. J., "A Computer Study of Interference Nulling by Reactively Steered Adaprive Arrays," .IEEE APS lmematio11al Syrn.pmium Digm, Busron, MA, 1984. pp. 807- 810. [28] Dinger, R. J., "Reaccivd}' Steered Adaptive Ar_ray Using Mierosrrip Patch Elements ar 4

GHz: !EEE TranL A,,.m,muand'Prqpagatio•~ Vo\. 3~. No. 8. 1':>84. pp. 848-856.

J.. "A Planar 4.0 GHz Reacrively Steered Adaprive Army," IEEE MTJ:s lmema.tio1u,/ Sympo.,ium Digat, San Francisco, C..A, 1984, pp. 303-305.

[29) Dinger, R.

[30] Dingu, R. J., "A Planar Vcrsian "i a 4.0 GHz Reacrivd y Steered Adaptive Array," IEEE Tram. Anrenmu and Propagation, Vol. 34, No. 3, l 986, pp. 427-431 .

[31 ] Dinger, R. J., and W. D. MeyI.

he gradienr method offers the most direct method ofobtaining 1he global minimum, although the technique suffers from local minima napping. The simulared annealing technique required the smallest number of calls m rhe solver, bur also suffered from local minimum trapping. As the sc.i.cisti

Elur1r11mflg11cn'; Fields, 3rd

ed., Boscon: McGraw-Hill, 1998.

[6] Ansofr Corporncion, Ensemble. 2000. htrp:l/www.-.1 nsofu:om. [71 BaJanjs, C. A., Anten:1111 Theory: A11a~ysis ,md De.-c, ~

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information typically nor foun d in other books, the authors cxumcrow; advan ragcs ofLhese antennas- includin g hig h-speed sigLtion , fixed input impedance, low loss, and small footprint.. us find practical design examples, strat.egies, an d optimv.atio11 tr design ing economical swilcbcd parasitic antennas for a pplicaas direnion finding and muhih(-am nmmmnications !;)Ste ms. ~c: technologies and applications such as ME.Ms RF switches are ed.

a chanerc:d professional enginee1~ is tlw clirect-01 ol the Radio >0rarory al the School of Microelectronic: Engineering at GriOith n Brisbane, Ausu-alia Dr. Thiel received his Ph.D. in «-kctromag• 1is M.Sc in antenna design and modeling atjamc.s Cook Urriveri.;it, le. Queensland, Au.c;u-aHa. l le e?.m\:d his R.Sc. in physics and J1cmatics at the Uni,er-sil) of Adelaide in South Australia.

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