What is patch antenna?
Microstrip patch antennas are used in mobile handsets due to simple look, lower cost, small size and lighter in weight. This antenna does not take any much extra size and can be etched on the same mobile PCB itself.
One such structure in our design is shown in the figure. As we know microstrip is constructed using dielectric substrate covered by ground plane on one side and metal conducting strip lines on the other side. In Microstrip one side is air and other side is dielectric hence pure TEM mode does not exist. Microstrip patch antenna can be constructed using photo-lithographic technique and later other RF and microwave active as well as passive components are assembled. The patch antenna is most widely used in cellular mobile terminals to operate with GSM,CDMA and LTE networks. We will see design of DCS band microstrip patch antenna used in mobile.
Mobile Microstrip patch antenna design example
Let us consider following specifications of patch antenna for our design.
• Centre frequency (Fo) : 1800MHz
• Dielectric Constant (εr) : 11.9
• Height of substrate (h) :1.5mm
• Calculate width of microstrip based patch antenna using following equation.
W = c / 2*Fo*((εr+1)/2)0.5 ;
C is about 3x108 for all the calculation on this page
This gives W of 0.03293 meter = 32.93mm
• Calculate Effective Dielectric Constant εreff using following equation.
εreff = ((εr+1)/2)+((εr-1)/2)*[1+12*h/W]-0.5
This gives εreff = 10.8323
• Calculate effective length of patch antenna using following equation
Leff = c/ 2*Fo*εreff0.5
This gives Leff of about 0.025m =25mm
• Calculate Length extension ΔL using following equation:
ΔL= 0.412*h* (εreff + 0.3) (W/h + 0.264) / (εreff - 0.258) (W/h + 0.8)
This gives ΔL of about 6.3528 e-4 mm
• Calculate actual length of patch antenna using L = Leff - 2*ΔL
L = 25 mm - 2 ( 6.3528e-4 )
L = 24.9987
• Calculate ground plane dimensions Lg and Wg:
Lg = 6h + L = 6 (1.5) + 24.9987 = 33.9987
Wg = 6h + W = 6 (1.5) + 32.93 = 41.93
• Calculate position of feed point (Xf,Yf):
Pls. note than center of the patch portion is taken as origin. Feed point is derived from this origin as reference. Feed point is located where input impedance is about 50 Ohm at resonant frequency.
Location of feed point is determinsed using trial and error method at various points. All the results are compared and feed point where return loss is most negative is taken as optimum point. In our patch antenna design, Yf is zero and Xf is obtained as per trial and error.
The antenna design can be simulated and results are optimized using rf and microwave antenna design softwares such as IE3D, AN-SOF, CST etc. This mobile microstrip patch antenna design can also be applied to all the other frequency bands in the similar way as outlined here.
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