Question about distance between microstrip and CPW

[DDavid]

Hi,

I have a little question?
I using in my design with microstrip,
To get Zo=50Ω: H=32mil, T=1.41mil=1oz, w=58mil, FR4 => ε=4.5 [F=800÷2200MHz]

What is intresting me how much I must to move from microstrip with ground plane so I wan't get CPW (Coplanar Wave Guide).
I thought that minimum distance is height of PCB (in my case 32mil),
But my colleague say that at least 2*width (2w) of the microstrip line.

Who is telling the truth???

David

 

[madengr]

Re: MICROSTRIP OR CPW

I use substrate height. What if you had a very wide line of Z0=5 ohms. Would you still use 2*W spacing? Use a 2.5D EM simulator to draw your own conclusions.

 

[DDavid]

Re: MICROSTRIP OR CPW

Hi,

Tnx for applying,
yesterday i spoke with other colleague he said that I must move a FEW wavelength from microstrip!!!!

David

 

[madengr]

MICROSTRIP OR CPW

If you don't currently have a 2.5D simulator you can use Sonnet Lite for free. It has enough capability to handle two coupled microstrips (4 ports). Just vary the spacing while observing return loss and coupling. I think you'll find that a spacing of 1*H to 2*H will get you between -20 dB to -30 dB of coupling depending on length. While this is too much for a high isolation switch it is probably for running traces next to a ground flood. You spacing shiould be determined by your needs and with a 2.5D simulator you can perform quick experiments in a few minutes.

**broken link removed**

Added after 10 minutes:

I tried your configuration in a 0.5"x0.5" enclosure. The response with no ground on either side is |S11|=-40 dB.

With 1*H spacing to ground (with continous via) is |S11|=-32 dB.

2*H spacing yields |S11|=-50 dB.