I am trying to make a patch antenna for 28GHz and THz waves, but I m having confusion with the dimension of coaxial feed. wherever I find coaxial cables for millimeter dimensions, only the outer diameter is given for the cable, which dimension I should use for the inner diameter???
is it okay to use only two pec structures for the coaxial cable or is it mandatory to make all material layers of coaxial cable for antenna design?
The outer diameter in your table is outside the plastic outer cover, or whatever else it has, so has no relevance to the microwave or THz performance.
The column on the far right gives a reference cable, which for the top row is .085"
semi-rigid. Look up the dimensions for 085 semi-rigid, or RG405. You should be able to find a manufacturer's data sheet Another column says the impedance is 50 ohms. To model it correctly you need the correct outer diameter of the centre conductor and the correct outer diameter of the dielectric, which is the same as the inner diameter of the outer conductor. You also need the correct dielectric constant between them, or the impedance will be wrong.
085 cable will be very poor for THz.
You can use PEC for the metal, but if you want realistic losses you will need to use the right metal, such as silver, and the right loss parameters for the dielectric.
From your question it seems as if you don't know much about microwaves, THz or coaxial cable, so I suggest you read up about it, perhaps starting on simple sites like Microwaves 101. A THz patch would be 0.1 mm across.
The coax cable is a poor choice for frequencies above some limit. That limit corresponds to waveguide modes propagation or when when cable circumference to double lambda in coax dielectric: https://www.microwaves101.com/encyclopedias/coax-cutoff-frequency
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