Calculate Impedance of SMA

Hi all,

I want to calculate the impedance of an SMA. Is teher any formula available. On what factors is its resistance depending on? outer dia, inner dia, pin in between, dielectric in between?

Thanks in advance.

This subject has just been spoken about over the last couple of days
see this thread .... https://www.edaboard.com/thread191484.html

the impedance of an SMA connect is nominally 50 Ohms unless specially made for say 75 Ohms

I put a link in that thread to Microwaves101 site showing how to calculate impedance

Dave

Thanks for the link. I am trying to calculate the impedance of SMA using the following data

https://emersonconnectivity.com/OA_M...1-0701-602.pdf

It has the innder dia = 0.035 or 0.037 inch
Outer dia = 0.181 inch

insulator is Teflon permittivity = 2.1

It gives me a impedance of 65 Ohms instead of 50 Ohms as per data sheet. Why is that?

Thanks for your reply in advance.

ok I assume the .035 was the diameter of the centre conductor ?
and the .181 was the diameter of the teflon dielectric ?

ok reading the pdf, I see where you got the .035-.037 from so I took the median of .036

I see where you got the .181 from, NOTE.... that is NOT the diameter of the teflon
which is actually smaller maybe ~ .178

I dont know where you got an Er for the teflon from, couldnt see that noted anywhere in the document. if you got that from elsewhere you dont know if the teflon they have used is the same Er ?

anyway
using .036, .178 and 2.1 .... I got 66.1 Ohms
using .036, .178 and 3.0 .... I got 55.3 Ohms

even little variations in those numbers, particularly the teflon diameter and the Er
have a big effect on the impedance measurement

In reality, using something of 55, 60 or even 66 Ohms impedance in a 50 Ohm
transmission line is hardly going to have a noticeable effect

heck have even had to use 75 Ohm coax in a 50 Ohm system and the resultant %'age
of reflected power is well under 20%

Dave

Thanks for your help. YA you may be right about Er of Teflon. But the resistance matter when you are working at higher frequencies.

Im using SMA's up to 10GHz, there are precision ones for 20 GHz and up but I prefer to use waveguide up that high :)

50 Ohms + - 10 Ohms is neither here nor there the additional losses incurred due to a small impedance bump aren't worth worrying about

Dave

See also here to calculate the SMA impedance and frequency limitations:

Tools & Converters

Just calculate the SMA impedance is something meaningless, if you require very accurate, you should build model in HFSS for SMA and say microstrip and the connection between them , then you can see the final result of your sma link with the circuits. One of my colleagues had done this job for SMA in HFSS which soldered on a 4-layers substrate and get almost very close result to the live.

I agree for the basic connector geometry, unless you put it into a more complex design that involves tuning for minimum reflections. Of course, it can be meaningful as a homework or lab exercise.
Yes, but no SMA 75 ohm version is available from major manufaturers, as far as I'm aware of. It's common for BNC, N and some other standard connectors.