Gain improvement of an array of sequentially rotated ...

Gain Improvement of an Array of Sequentially Rotated Circularly Polarized Microstrip Antennas Using Stacked Parasitic Patches' Yanun Liu'*,Mahmoud Shahabadi*,Safieddin Safavi-Naeini', Shady Suleiman' 'Depament of Elcceical and Computer Engineering, University of Waterloo

'Vhegard Co., Bwlington, Iowa, USA

1.lN~RoDucT,o~ Sequentially rotated microstrip array antemas have been extensively used for ~ireularly palarized radiation due to their wide VSWR bandwidth and polwization purity [I, 21; however, in many cases these arrays do not mccl gain requirements. One por8ible solution i s to YSZ a larger number of elements in special constellations [3] in cost of adopting more complicated feed networh and allowing lower a p e m e efficiencies, If is well b o w n that stacked parasitic patches (also called e1ed"gnetirally coupled patchesj can improve the VSWR bandwidth and AR bandwidth of single microstrip anlemas and thur their arrays [2. 4111. Previous researches have found that adding one parasitic patch causes the original resonant frequency of the fed patch to split, which contributes to 1he improved bandwidth [SI. However, gain improvement by stacking parasitic patches has not been fully sludied, especially its dependence on the height ofthe p m i t i c patches.

In this research we focus on gain improvement of a sequentially rotated circularly polanzed microstrip antenna a m y using stacked parasitic patches. It is shown that there exist multiple local maximums in the E W ~of the gain vcrsu the height of the parasitic patch, which was not reponed in the literature. The maximums o c c u at heights near to multiples of a half of wavelength in the spacer medium. By adding another parasitic patch at the second maximum the gain C M be funher Unproved. We also rNdy haw the size ofthe parasitic patches affects the overall gain. 11. IMPROVISC GAIN BY ADDING ONE PARASITIC PATCH PERELEMENT

Fig.1 shows the geomehy of a 2x2 microseip array antenna with one parasitic p t c h per element. The m a y is fmt designed as a scquemially rotated circularly polzrired array antenna Without parasitic patches, i.e. without I, 2 and 5 in Fig.1."he intended working Frequency band is from m-2%m to m 2 % m with m somewhere in the range of 11 GHZ to 14G H Z~ h simulaled c axial ratio is below 2.ldB for the band. The gain is 1 t.ldB@(fO-2%R)), I1.4@tlXHz and 1I .3@(m+%2fD).To improve the gain, each element of the m y is stacked with a parasitic patch which is fabricated on the bonom side of a 0.06& ulick low dielectric constant substrate ( &, = 2.2) and separated by a layer of low peminivity foam from the excitation patch. To simplify the design, the pmasitic patch has the same shape as the excitalion palch but scaled by a factor. It is found both the rcallng factor and the foam thickness affect the gain and axial mi0 (AR), therefore their effects are sNdied through ADS simulation.

' '&is

work was suppolfed by Winegard Ca., Iowa, USA.

0-7803-7846-6103191 7.0002003 IEEE

614

A. Dependence ofG& and AR on the Thdzzness ofrhe Faam

Fip.2 shows the dependence of gain and AR of the 2x2 array on foam ulicloless. The top patch is the scaled version of the bonom patch with a scale factor of between 0.8 to 0.9. From Fig.2 we can see that there are two maxima, one at 0.45 , and anmher at 0.9 to 0.95 They are quite close IO half a wavelength 1 t U and one wavelenglh at in foam. The AR at there two thic!exsrer is below ZdB.

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(b) side view

Fig.1 Layout of the 2 x 1 m y wlth ow pm%iticparch per dcmmt. l'hc cop patch is 0.8 CO 0.9 ofthc batom patch 5 i3 the Lop parch. 6 is banom patch 7 i s giound plans berwesn 3 and 4 8 is freding micmshp line cm 1.9isvia. IOisaholeon7for9toparrihrovgh.Thrhole IOllldmicmihpfcedingnc~ork8arenot s h o w in rhc figuro.

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foam thickness for thc 2x2 a m y with o w parasitic patch per element.

E. Dependence ofGain and AR on the Szre of rhe Top Porch

Now we fix the foam thbchess as OA6& and change the lop p m h sire.From Table2 we can see the lop 10 bonom ratio doer no1 area the gain much, and the ratio 0.9 is bcner than 0.8 and 1.0.

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Tsbli.1, Dependence ofgain and AR on the size ofparasitic patch. R i r i h ~ r a t i o o f t h e ~ i i r o f f h r t o p p a r c hbonompaich. io Gain (dB1 AR (dB1 m-%2m I m 1 m % 2 m m.%2m 1 m

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m%2m 1.08

1.55 2.65

615

111. IMPROYINC GAINBY A D I ~ TCW O PARASITICPATCHES PEP ELEMENT

To further improve the gain we add a second parasitic patch lo each element ofthe array. From Fig.2 we h o w that the second patch may be added at the second muimwn but it may need some optimization. The geomehy o f t h e design is s h o m m Fig.3. The dependence of gain and AR on the upper foam thickness and Size of lop patch are studied.

(a) top view @) side view Fig.3 Layout ofrhe 2x2 amay with two parasilic patcher per element. The top patch i s 0.8 m 0.9 ofthe horlom path. I I is fop patch. 5 is the middlc patch. 6 is bottom patch. 7 i s ground plane belwecn 3 and 4 8 IS fcsding microsmp linc on 4. 9 is via. 10 is a hole on 7 for 9 LO pms &rough. The hole 10 and micro~mp feding nework 8 am not shown in the Rgurc.

A. Dependence of Gam ond.4R an rhe Thicberr of Upper Foam L a w

Fig.4 shows the dependence of the gain and AK on the upper f o m thickness. while the lower foam thickness is fixed at 0 . 4 b 4 . From FigA we can see that the highest gain occurs at 0 . 4 2 4 .

which is close to the prediction by F i g 2

I UPPET foam thicknois for 2x2 array with W Oparasitic patches per clement. n e Fig.4 Cain and AR V C T ~ U Uthe middlc patch is 0.9 ofrhe bosom patch. The fop patch is 0 8 afths homm patch. The 1ow