Coax to Microstrip Transition with HFSS

[goswami]

I'm trying to get some field patterns from a microstrip (fed by a 50 ohm coax liine) on a Silicon substrate. I'm new to HFSS and am running into a few problems. Hope someone can help me out :

1) Assigning the port : I've used a waveport set smack against the inner conductor of the coax, a radius slightly greater than the radius of the inner conductor. I notice that changing this radius dramatically changes my field patterns. Also, my port and the radiation boudary condition cuboid have their surfaces overlapping. Is that alright ? If I don't satisfy this, I can't seem to get the simulation to run.

2) In a realistic situation, I would like the outer conductor of the coax and the back plane of the strip to share a common ground. How do I do this ? In my model the outer conductor and back plane are simulated by just assigning a perfect E condition. Should this be good enough ?

I'm attaching a jpeg of my model for reference.

Thanks
Goswami

 

[ddrmax]

1) the radius ratio between the inner conductor and coax determine your characteristic impedance. If you have 2 items are overlapping. you need to subtract one from the other (leave the clone form of the one you want to keep)
2) make sure the outer conductor is connect to the ground. assigning perfect E is a good approach and that wouldn't effect result significantly.

 

[jallem]

Are you trying to design the transition or an antenna?
For:
a) transition: you should model the transmission line with two connectors.
dont need to worry about radiation unles you want to. I think you should do it
even if you want to design an antenna, so you can have an idea about the
parameters of importance that affect the transition alone, for example: the
radius ratio.

b) antenna: you should have already a model or a way for the feeding of the
antenna. In others words, the transition should be chosen and you should be
confident what you get. After that, the variation of the pattern and/or return loss
will be mostly due to model size, this imply aspect as closer distance to the radiation
boundary, number of iterations, meashing, etc. there is a lot of information in
this board about it.

You model is very simple, if you post the file someone can help you better.