Input impdance of an inset fed microstrip antenna
For a 350 μm GaAs substrate with dielectric constant κ= 12.9 and with patch patch length and width almost equal at 5.7 mm , what can be said about the input imedance. CST-MWS simulations are giving me a value of about 1100 Ohms. Can this be expected ? Is there any independent verification ? Normally the impedance is expected to be between 100 and 350 Ohms. Is there any formula that relates the relation between input impedance of a patch antenna with the substrate permittivity ? Thanks for your time.
-svarun
From your given parameters, looks like the antenna will resonate at approximately 7.3 GHz (=Speed of light / p@tch_width_in_metres*2*sqrt[epsilon]). I didn't understand your saying input impedance normally be 100 and 350 ohms. Here I just go for 50 ohm. This is achieved by approx. 2.5 mm of inset. The width of the 50 ohm transmission line feed I calculated to be 0.255 mm. The feed is aligned center along one of the p@tch's edge. The first simulation did not include any inset. I resimulated the model by having the inset feed with 2.482 mm of inset distance from the edge toward the center of the p@tch; and with a gap width of 4 mm. No deembedding this time round! The resonant frequency is at 7.1427 GHz, with S11 = -26.29 dB. While the real and imaginary input impedance at 7.14 GHz is 44.04 - j3.605 ohm. For closer input impedance match of 50 ohm, simply calibrate the inset such that it is a little shorter.
Hope this help!
Hi Sassyboy,
Thanks for your nice answer. You have outlined rather nicely the standard design procedure. What I actually meant was the resonant impedance at the edge, that is without any inset. From your simulations it is okay to say that for the given return loss the p@tch is correctly matched to approximately a 45 ohm line ( neglecting the imaginary part and assuming that you tune inset to get rid of that ). Now if the resistance without inset is R0 then we have Rinset=R0*cos2(Π*x/L) (Balanis) where L is the length and x is the inset. Now substituting the values you have got by simulation, we get R0 to be around 1200 ohms. This was what I wanted to know , how this R0 depends on the permittivity of the substrate. Normal quoted values in the papers ( low permittivity substrates ) is about 100-350 . So I am a bit surprised at such a high value. Anyway thanks a ot for the simulation result thus reconfirming my design.
-svarun