# how to find inset feed point ? microstrip line ?

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#### waqas jaan

##### Member level 1
please please help me out to find the microstrip line in patch antenna. i have found the width and length for it. but i don't know how to calculate:
1. feed point.
2. thickness of a substrate
3. width and length of a ground plane

please tell me, is there any formula to calculate all these parameters ?

Yes, see Balanis or Pozar.

Yes, see Balanis or Pozar.

i studied balanis but there is no such kind of formula to find the substrate thickness at resonant frequency.
please tell me what will the thickness of substrate at 10GHz. if i am using two different types of substrate FR-4 and duriod 5880 substrate.

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also how to find the ground width and length ?????
is it 6h+W (w=width of the patch)
and 6h+L (L=length of patch, h = height of substrate) ????

The problem is that there is no explicit definition for the values you are looking for.

In my Balanis (3rd Edition), page 821, a rule of thumb is given that you h/lambda0 < 0.1. The largest impact of the dielectric height is the effective dielectric constant, for which he also gives equation 14-1.

To my knowledge, the ground plane size really just determines the amount of 'ripple' seen in the radiation pattern's main lobe. The bigger, the better, but of course practicality has to factor in somewhere. From my (limited) experience, 6x(L,W) should be fairly good. People with more expertise on patches, of which there is at least one on these forums, may be able to give you a few more pointers.

please tell me about the feed point. i can't find how to calculate the feed points ?

thank you so much

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and one last question please if you dont mind. how much will be the gap between the patch and microstrip ?????

and one last question please if you dont mind. how much will be the gap between the patch and microstrip ?????

I assume you mean for a gap-couple patch antenna. You want the gap capacitance to be a value such that the total input impedance is real (and ideally equal to that of the microstrip). See Pozar (3rd ed. ) Chapter 6.6

thank you so much. i am talking about the small gap between the patch and microstrip. i hope you understand. when i insert the microstrip in patch. there is a small gap on both side of the microstrip with patch.

thank you so much. i am talking about the small gap between the patch and microstrip. i hope you understand. when i insert the microstrip in patch. there is a small gap on both side of the microstrip with patch.

Yes, the inset is discussed in Balanis. Equation (14-20a) states

Rin(y=y_o)=Rin(y=0)*cos((pi/L)*y_o)^2

Where y_o is the length of the inset and y=0 is the edge of the rectangular patch.

Thank you so much. i really appreciated your help.
i m using this equation but there is a problem and i m doing it for the last one month but i m always failed. on this point stops me for further calculation.
the problem is that in equation
Rin (y=0) = 1 / 2 (G1 + G12)
i couldnt find this G1 and G12. there are equations for this in balanis but i can't find it
as G1 = I / 120 (pi)^2 => I = integ (from 0 to pi) [sin (koW cos Q )/cosQ ]^2 (sinQ)^3 dQ.
and G12 from equation (14-18a) balanis 3rd ed on page 823. in this equation there is a Bessel function so how can i calculate it.

secondly i took y_0 = L/3 =2.44 so i took 2.50 approx and i put this value in above equation (mentioned by you) and i got Rin(y=o) = 50.178 ohms. so is it correct ?

please help me out in this situation because this is the only problem (feeding method) which stops me for further designing.

Hi waqas jaan,

I'm not sure about the equations you mentioned, but you can find the input impedance Rin(y=0) roughly by feeding the patch without an inset, and extracting the input impedance with basic TL theory.

I have also seen a few methods for calculating patch input impedance (without an inset) on google.

Good Luck

I believe you mean for a gap-couple spot aerial. You want the gap capacitance to be a value such that the complete feedback impedance is actual (and preferably similar to that of the microstrip). See Pozar (3rd ed. ) Section 6.6

thank you so much....

im using CST microstudio but now im calculating all the parameters manually. so this is the only parameter left to calculate. please help me how to calculate the width of microstrip line

im using CST microstudio but now im calculating all the parameters manually. so this is the only parameter left to calculate. please help me how to calculate the width of microstrip line

*******************
transmission line calculator:
https://www.emtalk.com/mscalc.php?e...ts_list=Wmm&La=8.01576372242&L_units_list=Lmm

Patch calculator:
https://www.emtalk.com/mpacalc.php?...Lmm&Wa=31.2066874359&W_units_list=Wmm&Rin=144

i m using these links right now. HOpe they help.

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