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suspended microstrip line for 15GHz

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shahid78

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Hi all

i am trying to design an air suspended transmissio line which should work from 0.5GHz to 20GHz.

i have calculated its width dimension using CST built in micro .
but i can not get the good results for the my desired frequency range.

the S21 becomes worse as frequency increases which i think is due to miss matching. i want a smooth horizontal line close to 0dB throughout the range assumng low loss dielectric .

so i am just wondering how to create such a transmission line which is matched for desired frequency band, the cST built in macro gives the width of the line for the desired 50 ohm characteristic impedance but i think this width may not work for my desired frequency range.

anyone having some experience in this area , i would appreciate their comments

thanks
 

biff44

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What are your using for a channel width? It needs to be pretty small. At 18 Ghz, you have to be sure that waveguide modes are not propagating in parallel with your suspended stripline.
 

shahid78

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hi biff44

well how can i make sure that waveguide modes are not propagating along the microstrip line.

thats why i asked in my post . how would you proceed to design a microstrip line for such a wide band ? specially if there is air involved as well between the ground and the dielectric substtrate on which microstrip line is etched.

thanks
 

biff44

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You want the substrate to be suspended halfway between the top and bottom cover, and you want it bounded to the left and right by a channel wall. If you want only to propagate energy in the trace itself, the dimensions of the channel have to be below a certain width.

An approximation, assuming the width is much wider than the cover to cover height, is that the cuttoff frequency for the lowest waveguide mode will be:

Fc = C/2A, where A is the width in cm, and C is the speed of light 30 cm/ns.

For a cuttoff frequency of 22 GHz, you need A = 30 [cm/ns] / (2 * 22 GHz) = 0.68 cm.

https://www.microwaves101.com/encyclopedia/waveguidemath.cfm#cutoff

Rich
 
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