Effects of Tuning Stub on Microstrip Patch Antenna
Indian Journal of Radi o & Space Physics Vo l. 34, Ap ril2005, pp. 139- 141
Effects of tuning stub on microstrip patch antenna
Surendra Kumar Roy & Lalan Jha Un ive rsity Department of Phys ics, L N Mithil a Uni ve rsity, Darbhanga 846 004, Bi har, India Received I September 2003: revised 10 February 2004; accepted 27 December 2004
Use of tuning stubs to trim th e phase of a mi crostrip feed line or to tune the reso nant frequency of a mi crostrip patch ant enna is discussed in thi s paper. The variation of resonan t frequency with stub-length of microstrip patch antenna i also di sc ussed. The ex periment al results regarding phase shift and resonant frequency wi th stub-length are in good agreement wit h the th eoretical result s.
Key words: Tunning stub, Mi crostrip patch antenn a. Resonant frequency J>ACS No.: 84.40 Ba IPC Code: HOIQ-9/00: HOIQ-21/00; HOIQ-23/00
1 Introduction The coefficients Xro and X11 can be approx im ated as Microstrip elements have proven to be very versatile and have found uses in a vari ety of Xrn "" 1/ 2 (X if+)+ X(f. _)) + R/4Q applicati ons from satellite arrays to automobile anti colli sion warn in g systems. This versatility stems and mainl y from their st ructural simplicity, ease of Xr, "" Qlfo [ X if+) - Xif _) + 2R] constru cti on, ruggedness, confonnability, low profile, and low cost. In additi on to th ese features microstri p where R = 1/G. What remain s IS to determine the elements are capable of producing a va ri ety of input reactance of the ideal cavity at the upper and 1 5 different pattern s and polari sa ti ons • • lower freq uenci es!+ an df_. The fo und at ion upon whi ch th e stub lin e theory is To do thi s, one si mply views the inpu t reactance at built is the so-call ed cavity model6 as mod ifi ed by the feed port as th e input reactance of a loaded ,wo 7 Rich ards et a/ • This model is best suited for port network. The z parameters of th e two-port understanding loaded elements, because it can network are the;:: parameters of the empty cavity with reconstruct the localized fie ld in the vici nity of the port 2 located at the feed and port I located at the loads, yet it is simple enough that basic physical load. The mutual reactance between the ports with principles useful in design can be developed and is respective position vectors r 1 and r2 is just also surpri singly accurate. Ray and Kumar8 has studi ed tuneable and dual band circ ul ar microstrip antenna with stubs usin g improved transmi ssion line model. In thi s paper mainl y th e effect of tuning stu b The input reactance is th en on the characteristics of mi crostrip patch antenna, is analysed. X !f ) 2 Theoretical analysis Circuit model of the loaclecl mi crostrip patch c + antenna is shown in Fig. I . The feed reactance can be Vt mode II eel as ~ G Xr = Xrn + if-fo) Xn
Let
F+=( l + 1/2Q)fo.J- =(1 - I/2Q)fo Fig. !-Circuit moldel of th e loaded microstrip an tenn a 140 IND IAN J RA DJO & SPACE PHYS. APRIL 2005
180 0 .. ~ ./ ' \ ' ·1 't \ I ·"' \ .I . -] I I wh ere X22 is the input reactance of the empty cavity at I I I · -] r2 • and is computed in the sa me way as X11 I l I I ------ATTENUATION I - ~ ' I 3 Numerical analysis PHASE SHIFT I Use of small tuning stubs to trim the phase of a -s I ' a> mi crostrip feed line, or to tun e the resonant frequency I .6 " I z of a mi cros trip patch antenna is di sc ussed here. I ~ I ·7 ... <1 Because of fabri cati on tolerances and oth er effec ts, I ::> I z the electrical characteri sti cs of a microstri p or I -8 w I ~ ... stripline circuit may de vi ate from th e design I <( r -9 spec ifi ca ti ons. In a corporate feed netwo rk for an I array, fo r example, power di viders, bends, juncti ons, I -10 I "'~ dimensio nal tolerances, and wbstrate inhomogeneiti es 0 . can lead to phase (a nd amp litude) errors at th e array ,_ I u I I elements, es pec iall y fo r long length s of feed line. In I -so r I "' I I I mi crostrip antenn a fab ri ca ti on, dimensional tolerances w -~0 I I I and subs trate inh omogeneiti es can result in signifi cant "'<( I I I c.. - 100 I I variati on in the resonant freq uencies of the el ements II of a patch array. This can lead to mi smatch loss and - 120 phase errors clu e to va ri abl e dri ving point impedances - 1W lr =2 -55 across th e array, and is a problem exacerbated by th e d = 0 ·1 6 em typica ll y narrow band width of mi cros trip ante nnas. - 16 0 f =, .5 By proper design, the tec hni ques described here -18 0 ~-+------r------~------~ can be used to compensate fo r phase errors over a o o·2 o·, o·6 o·a ro n n r6 ra 2·o
resonabl e range, with out substantiall y affec ting th e STUB LE NGTH _1, e m loss of a feed lin e, or th e patt ern s of an antenn a cl ement. Fig. 2-Ph ase shift and att enuati on for a si ngle tun in g stu b on a mi crostri p lin e stu b width = 0.125 em A short open-circuited stub in parall el with a mi crostrip feed line can in crease th e in serti on phase radiati on pattern , although the stub should be ori ented of th e lin e. Fi gure 2 shows an example of a 50 Q sy mmetri call y with the resonant directi on of th e patch mi cros trip lin e on a su bstrate with E, = 2.55 and d = to avoid cross-polarized spuri ous radiati on. 0. 16 em. The stub is 0.125 em wide, and th e in serti on In Fig. 3 the measured resonant frequency is plott ed ph ase and attenu ati on of the Ii ne are pl otted versus versus stub-length for stu b widths of I mm an d 3 mm. stu b length /, norm ali zed to zero stub length. Observe Greater than 10% tuning range is easi ly ac hi eved for th at ph ase delay increases as th e stub length increases, 0 :S I :S I em, with the wider stub giving a greater with neg li gibl e attenu ati on, for /up to about 0.7 em, va ri ati on. This should be more th an enough to offse t at whi ch point the phase delay is about 20 deg. As I dimensional tol erances, whi ch usually res ult in errors app roaches 1.15 em, th e stub goes through a of no more than a few per cent. resonan ce, with substantial att enu ati on on th e feed The dimensions of th e single stub circul ar lin e. Thi s loss is du e to radiated power. So, to avoid mi crostrip patch antenn a for operati on in th e L-bancl attenu ati on on th e lin e and undes irabl e spu rious were chosen as radius r = 3 em; feed poi nt x = 0.95 radi atio n, the stu b-l ength is prac ti call y limited to be em; and stub width = 0.4 em. The width of the stu b less th an about 0. 7 em . Thus, it is better to use narrow has been kept small , so th at the radi ati on from it is (hi gh impedance) tuning stubs. neg li gibl e. The substrate parameters are thi ckness h = Short open-circuited stu bs can also be used to load 0. 16 em, E, = 2.33, and tan 8 = 0.00 I. As th e stu b micros trip antennas, in order to adjust th e operating length increases fro m 0.0 to 2.2 em, the measu red frequency . If the stub is small (say less th an half th e resonant frequ ency decreased from 1. 868 to 1.695 patch length ), it will no t noti ceabl y affect the GH z (th eoretical : 1. 866 to 1.700 GH z), and th e ROY & JHA: EFFECTS OF TUNING STUB ON MICROSTRJP PATCH ANTENNA 14 1
5"1r------, turned to less than -45 dB, at any frequency over the range 2.95-3.05 GHz, i.e. a tuning range of 3.33%, without degradation of the match. s· 4 Conclusions These types of stub tuners can be applied to many 4"9 ' ' . other configurations, including stripline feeds, slotline feeds and circular patches. A varactor may be added ' W .1mm ' ' at the open end of the stub, to facilitate electronic ''\ tuning, instead of the manual tuning described. Bi as \ \ for this mass be connected to a suitable point on the /~ \ patch, through a filter. Finally, we might take th is / ..,. W ' Jmm ' \ opportunity to dispel a myth about the mi crostrip antenna, which says that its width should be small er >- \ ~ \ w than its (resonant) length, to avoid the excitation of an ::> \o 0" w \ undesired cross-polarized mode. This is not always cr \ ... \ necessary, as the patch geometry of Fig. 3 shows that .... \ the patch width can be greater than the length (to z \