How to set Rectangular wave guide port for nano size antenna in CST?

my antenna is in nano meter size... and got a frequency in 800 to 1000 thz. below -10 db of return loss.used Gold and silicon dioxide as matarial. got 4.81 dbi directivity.... but very very low minus scale efficiency... then i understood it was a port mistake... we can not simply generate the rectangular wave guide port for achiving thz frequency... so Help me about the port or exitation in the nano antenna (nantenna) for CST? or please say me what type of port is better for nano antenna? i am not designing the ARRAY... designing a very simple unit cell antenna...so how to excite it.thanks in advance

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