# Gap Between Microstrip and Ground Plane - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

ljoseph
Post subject: Gap between Microstrip and Ground Plane Posted: Tue Sep 04, 2007 8:03 am

Captain

Joined: Tue Sep 04, 2007 7:54 am
Posts: 13
Location: Dallas
Hello

I have microsrtip line on the PCB with following specifications:
Er = 4.25
H = 0.3mm
T = 32.5um
With this, width of 50 ohms line is 0.56mm.

On the PCB it is better to provide ground fill on both sides of Microstriop line so that fields radiating from the line will be terminated on the ground fill. But, depnding on the distance between the Mcirostrip line and Gnd plane, CPW effects will take over. What is the ideal distance between the Microstrip line and the ground fill so that CPW effects won't dominate and affect characteristics impedance of the line?

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IR
Post subject: Posted: Tue Sep 04, 2007 2:19 pm

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello ljoseph,

From my experience, the best distance from both sides of the Microstrip line is 1.5 to double the line width.

Don't forget to stitch plated vias in a distance which is around 1/10 wavelngth along the GND fills.

Hope this helps.

Good luck!

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ljoseph
Post subject: Posted: Tue Sep 04, 2007 6:00 pm

Captain

Joined: Tue Sep 04, 2007 7:54 am
Posts: 13
Location: Dallas
Hello

Could you please explain the function of ground fill. Still, I am not very clear about it.

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Noise
Post subject: Posted: Wed Sep 05, 2007 2:06 am

Captain

Joined: Tue May 02, 2006 4:59 am
Posts: 15
hello,

In my opinion the distance from GND should be 4 time the copper width.
Else it will act like Coplanar and not as microstrip.
For Coplanar as far as ratio a\b is bigger , the characteristic impedance is bigger.
It can be seen as two parallel impednace (Coplanar and microstrip)

Noise

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IR
Post subject: Posted: Wed Sep 05, 2007 2:28 pm

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
The function of the GND planes is exactly as the shielding of a coaxial cable.

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Post subject: Posted: Tue Sep 11, 2007 4:55 pm

Captain

Joined: Fri Feb 02, 2007 5:22 pm
Posts: 5
Location: Overland Park, KS
I use a spacing of one or two substrate heights, depending on how long the trace is. Often people use N*trace_witdth, however lets say you have a low impedance line of Z0=10 ohms, would you still space at N*trace_width? EM simulators are inexpensive or free these days. Maybe try your configuration and see how much you can get away with.

http://www.sonnetusa.com/products/lite/

Posted  11/12/2012

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