A Disk Monopole Antenna

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PROCEEDINGS OF ISAP '92, SAPPORO, JAPAN

A DISK MONOPOLE ANTENNAWITH 1:8 IMPEDANCE BANDWIDTH

AND OMNIDIRECTIONAL RADIATION PATTERN

Satoshi H O N D A , Michiaki I TO , Hajime SEKI" and Yosbio JINBODepartment of Electrical Engineering , Nagaoka Unive rsity of Technology

Kamitomioka Nagaoka-shi 94()-21 Japan*E-mail address:[email protected]

1. INTRODUCTION

A fundamental thin wire monopole antenna. and it s varia.nt s(lJ have th e com mon featuresto be omnidirectional and structurally simple, bu t narrowband. By replacing th e wire elementwith a. conducting disk, the resultant monopole antenna. exhibits an extraor dinary ba.ndwidth of1:8(maybe more) , defined by return loss less than -10 dB, and maintain s omnidirectional coverage[2]-[4]. These experimental results are briefly reviewe d and the broadband omnidirectionality is

theoretically examined.

2. ANTENNA STRUCTURE

Th e lowest design frequency is 3GHz, and soth e diamet.er of disks is 2Smm (a quart.er wavelengt.h at. t.his frequency ). Two t.ypes of disks areused: one is simply made of copper plate O.Smmthick; the other is a printed disk of 3SIlm t h i c k ~

ness. This t.hin disk is backed up by the samesized PTFE laminate disk whos e thickness and

relative permittivity are 1.6mm and 2.S,r e s pe c ~

tively.An edge of either disk is soldered with a c e n ~

ter conductor of son SMA connector as shown inFig.1. Th e edge is just. in contact with the c o n ~

nector surface and so there is no gap betweenthem. The connector itself is flushmounted inth e center of th e sqnare ground plane sized300x300mm. Th e y ~ zplane containing th e diskis parallel to two edges of the square groundplan e and naturally orthogonal with th e r e m a i n ~

ing two.

3. EXPERIMENTAL RESULTS

z

e '

Disk

Ground

....

............... SM1I connector

Fig . l 11 disk monopole antenna .

Figure 2 shows l.he return loss for 2 to 20GHz measured using a Wil1.ron network analyzer.Th e lower limh of th e abovedefined bandwidth is recognized lower l.ban 2.5GHz , while th e upper

is ou t of th e frequency unge. Hence th e 1:8 bandwidth is at least confirmed.Impedance matching can be further improved by removing stray capacitance between the

disk and t.he ground plane[4]. This improvement was examined using a printed disk, whose lowerportion near l.he ground was cu t away by mean s of etching as indicated in Fig.l with cu t anglea . Th e results are shown in Fig.3. For 0'= 20 0 the return loss less than ~ l S d Bis achieved a.t

least in th e frequency band from 2.5 to 12GHz(1:5 ).Radiation patterns at four frequencies in th e band are shown in Fig.4. Th e patterns in the

vertical pla.nes were measured for azimuth angle if>fixed at every 10 degrees starting from 0 0 ( x~ z

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Frequency (GHz)D '~ ______ - 25~ ____________ ~ I ~O______________ ~ l f 5 ~ ____________ ~ ' , D

~~

~ - 1 0 .. ' ~ ~ ' . " " " . '_ . . --- -......... ...... .. -.- . . _. _. . . . . . . . . --_.- -_.- . .. .

•~..., -2 0

~,~

& 3 0

Fig.2 1 : 8 bandwidth o f a copper d i s k monopole antenna .

(a) 3 . 6 GHz

."

z

Frequency ( GHz)

0 ' 5 1 0 ,~~

"~ 60 '

• -10•

~

~ -2 0 20 ', a"'O·~

•" -3 0

Fig.3 Return l o s s fo r p r i n t e d d i s k s

with c u t angle a var i ed .

z_ _ - : ' - _ • LQtO!

( b ) 5 . 6 GH z __ 1 - _ _ 810(01

."

uel)

Z( e ) 8 . 4 GHz 8 I D ! ~ 1

~ - r - _ 'Z

( d ) 1 0 . 4 GHz 810UI

~ + - '." "

l U l l

"

Fig.4 omnid i rec t iona l rad ia t ion p a t t e r n s o f th e antenna as fo r Fig . 2.

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plane) 1.0 90 0 {y-z plane)[2]-[3]. All of them are plotted in one figure to observe omnidirectionalradiation. Th e three dimensional radiation pattern is almost unchanged in the band, altho ughfor higher frequencies crosspolar component appears (when ¢=30 ,40 ,SO ,60 degrees) and t hehorizontal radiation tends to cease.

For lower frequency applications the bulky disk element and the ground plane can as us ua l be

approximated by their approciate wire-grid models. As an example such a mode l was successfullydeveloped to cover aU over the Japanese television channels expanding from 90 to 770MHz(1:9)[3[.

4. THEORETICAL CONSIDERATION

According to the multipole expansion[S] , any current source whose maximum dimensionis much smaller than wavelength is equivalent to an electric dipole as far as the far field isconcerned. This is the basic reason for unexccptionaUy omnidirect ional radiat ion patterns ofvarious monopole-type antennas. In addition to this reasoning, there are symmetries of currentdistribution in the case of disk monopole antennas . In the following examined is the effect ofthese symmetries on the broadband omnidire cLionality.

Th e ground plane is assumed to extend infinitely and the image theory is applied. Let thereal current on the disk as well as the image current be denoted by J (r' , 8'), where the p rimedargumen ts refer to a source point as indicated in Fig.l. Th e symmetries , which are obvious fromthe structural symmetry . are expressed as follows.

J,(r', -8') = J,(r',O') J,(r', - 8') = - J,(r ', 8') (I )

J,(r'" - 8') = J,(r',8') J,(r' , , - 8') = -J,(r ' , 0') (2)

Th e conventional vector potential A is given by

(3)

(4)

cos'" = sin e in ¢ sin f)' + cos I) cos I)' (5)

By applying the addition theorem for a spherical Hankel function of the second kind, eq.(3)can be written as follows.

A = - i ( : ~ )[;.,(2n + I ) h ~ ')( k r )ff i.(kr')P.(cos,p)J(r',8')dS' (6)

Taking into account the above symmetries , an the terms for n odd are found to vanish. Especially th e seco nd term(n=l), which corresponds to a magnetic dipole as well as an electr icquadrupo le[S] and violates omnidirectional pattern with increasing frequency, does not exis tinherently. For this reason disk monopole antennas exhibit the broadband omnidirectionality.

Consider the remaining terms for n even. The Taylor series of a spherica.l Bessel function ofOlder n starts with the n powers of its argument , and the value of argument that gives the firstmaximum of the function increases with increasing order(6]. Hence in eq.(6) th e increase of nmeans the contribution {rom the current further from the feed point.

For n=O, Po(cos ¢ )= l and the integral over the real and image disks is a function of wavenumber (frequency) only , wltich is denoted by Co(.l; ). Then the vector potential ho m this te rm is

given by

(7)

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This is th e familiar form for a monopole or a dipole . I f th e frequency is no t so high that neglectof higher order terms is ju stifyable, th e radiation pattern is unaffe cted by the spe cific currentdistribution in accord with the a.bovementioned multi pole expansion. I t should be noted herethat eq.(7) is obtainable, unlike th e expans ion, without taking th e first term of th e Ta.ylor seriesof a sphe rical Bessel function because of th e symmetries.

For n=2, P2(COS '¢') = (3/2) cos 2 'I/; - 1/2 and a. straightforward analy sis following standardprocedure shows t.hat th e azimuthal and eleva.tio nal angle dependencies of copolar and cros spo larcomponents are given by

and

H ~ = C 2 1 ( k) si n 6 + Cn(k)sin6cos 2 6+C23(k)sin 3 Bsin 2 ¢+ D2(k)sinBcos 2 Bsin 2 ¢

H. = D,(k)sin(20) sin(2¢)

(8)

(9)

respectively, where C's are derived from J: and D2 £rom J IJ. Th e crosspoJar radiation observedin FigA can be understandable by the present result.

5. CONCLUSION

Th e disk monopole antennas a.re claimed to have 1:8(maybe more) impeda.nce bandwidth

and broadband omnidirectional radiation pattern. Th e la t ter is theoretically shown to be dueto the symmetries of current distribution on th e disks.

ACKNOWLEDGEMENT

Th e authors would like to thank Messrs. Makoto Megasawa and Yuji Takahashi of Anritsu

Corporat ion for measuring the re t urn loss in Fig.2 by their Wiltron network analyzer, andMr.Tsuyoshi Atan for re-editing OU I final manuscript using a Tex.

References

[1] B.A.Burberry: "VHF and UHF antennas " , in th e Handbook of Antenna Design,ed.A.W.Rudge et.al., pp.1428·1440 , lE E , London (1986).

[2] S.Honda, M.lto, H.Seki and Y .J inbo: ~ C h a I a c t e r i s t i c sof a disk monopole antenna", Pro-ceedings of th e ]991 IEICE Sh in etsu Chapter Conference, 4 (l991.1OY.

[3J S.Honda, M.lto , Y.Jinho and H.Seki: "On a broadband disk monopole antenna", TechnicalReports of Television Society Japan , ROFT91·S5(1991.1O)*.

[4J S.Honda, M.lto , H.Seki and Y .Jinbo: '1mproved input impedance of circular disc monopoleantenna ", PIoceedings of the 1992 IEICE Spr ing Conference , B·131 (1992.03)'" .

[5] J.A .Stratton: "Electromagnetic Theory ", ppA31-438, McGraw·Hill, New York (1941).

[6] H.A.Antosiewicz: "Besse] function s of fractional order ", in Ha.ndbook of Mathematica l Functions with Formulas , Graphs , and Mathematical Tables, ed.M.Ab ram owit z andI.A.Stegun, Fig.10 .1, pA3S, John Wiley & Sons, New York (1972 ).

'" Although these pape rs are oIiginally written in Japanese, their English versions translatedby the a.uthors are available.

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A Disk Monopole Antenna

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