designing microstrip antenna using HFSS

antenna + coaxial probe radius + hfss

hi
i want to simulate a H shaped microstrip antenna using HFSS with this dimensions(mm)
SL=15
hr=1.52
WA=40
LA=40
εr=3.38
SL=slot lengths
slot width= 1
ground plane=80×80
RESONANT FREQUENCY=1.397GHz
The patches are fed through a coaxial probe and are matched to 50Ω.
i dont know what dimension the coaxial probe must have.(inner & outer conductor radius)
please help me
thanks in advance

microstrip antenna on hfss

Lets assume your antenna is to be tested with a network vector analyser. Therefore the input characteristic impedance is 50 ohm. In order to get the maximum power transfer to the antenna through the SMA connector, the SMA connector must be designed to have a 50 ohm characteristic impedance. If you look through catalogues, they will specify the SMA connectors usually with 50 ohm Zo.

A SMA connector is basically a coaxial cable. So, let me ask you:

--- How would you calculate the characteristic impedance of a coaxial cable? ---

Answer: This is taught in transmission line theory. The Zo equation for a coaxial cable is defined as follow:

Zo = 60/sqrt(eps) * [ln(r_out/r_in)]

where eps => Permittivity of the dielectric cladding, usually Teflon = 2.1
r_out => Radius of the outer conductor = 1.67 (number I use)
r_in => Radius of the inner conductor = 0.5 (number I use)

Substitute into the equation, I get Zo = 49.9319 ohm. This is close enough.

From this point onwards, is just drawing the SMA / coaxial structure using the parameters calculated above. I have designed many low-loss microstrip antennas using the same feed parameters as above and always work well.

Bonus material:

Alternatively for a quick simulation, you might consider using the lumped gap source in HFSS or discrete port in CST Microwave Studio. This avoids the need to draw inner, outer and the dielectric cladding and just simply specify two points to form excitation or source, i.e., a point on the patch and a point on the ground.

There you are! I hope this resolves your uncertainty.

hi do you know how can connect sma connector to microstrip antenna in hfss12?