I have calculated surface wave cut-off frequency from the given substrate heights , now I just have to decide which height to go for.
Dear Mrs. Iqal,
yes, look please to the spinwave theory.
All the best
F. Gräbner
Selection of substrate height is depends on your application.
Overall, for microwave application you need to keep power inside the substrate, so you need a thin one with higher er; while for antenna applications these is opposite. But to be more accurate you must have a look at substrates datasheet and see how dielectric parameters change vs frequency.
You also need to check the copper height (usually there are 17, 35, ... um available), which is important for surface waves, to find the proper one for your application.
You can also check for example Rogers webpage for its substrate selection guides.
the frequency limitation depends on the coupling of energy to desired quasi TEM mode from surface wave mode or next higher order mode. Surface wave mode has no cut off. It is present for any dielectric discontinuities but its coupling to Quasi TEM mode of microstrip line becomes significant after a frequency value. This limiting frequency can be determined by using available numerical approximated formula.
The frequency of operation in microstrip line is also limited by the generation of next higher order mode. This frequency limit can also be determined using available formula.
Both of these two limiting frequencies depends on substrate thickness and dielectric constant of the substrate. In practice the lowest frequency value of these two is taken as upper limit.
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