# 2.45GHz RF microstrip Transmission Line design

Hello all,

I'm now working on the 2.45GHz frequency transmission line layout on the PCB.
I would like to know whether there is a design guideline for the

1) minimum length of the transmission line. how do I know or calculate? I can calculate the width of the TL to get the 50ohm impedance based on which layer for the ground plane, but length? how?

2) the PCB is 8 layer type, is there a good rule to have for the TL ground to be on which layer? can I calculate the TL width on layer 2 ground and layer 4 ground ? mixture of both, and the TL width will be different.

thanks.
spid3rx

Baluncore
You can run the line on the surface above a ground plane =microstrip, with some stray radiation and sensitivity to the environment. https://en.wikipedia.org/wiki/Microstrip

You can place the line between two ground planes with vias to tie the planes = stripline, which has low radiation and well controlled impedance. https://en.wikipedia.org/wiki/Stripline

If the transmission line is being used to transmit energy efficiently to a matched load, the waveform and line length will not be important, there is no minimum length.

If the line is being used with an open or short termination to emulate C, L or a resonant filter, then the line length is critical and the signal needs to be a sinewave. The physical length will need to be sdjusted for the velocity factor due to PCB dielectric constant.

tech99
Joshy
Gold Member
It's definitely a good idea to have the return path of the transmission line right below it. I've seen coplanar waveguides (CPW) without a back plate although I cannot think of any good reason to use it if you're going to have 8 metal layers. Striplines as stated above will have its return paths above and below it, and this'll provide good isolation from traces on the other layers.

I have heard of vias causing problems at 3 GHz, but I'm not so sure about 2.45 GHz. You might want to pay attention to its geometry because it could create a mismatch between your two "matched" lines, which will ultimately make your load look mismatched.

What kind of material are you using? Surface roughness? Dielectric loss? Determining a stack-up is very challenging work. What else is going to be on your board?

tech99