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Designing Microstrip on a PCB with unknown substrate - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.

Post subject: Designing Microstrip on a PCB with unknown substrate
Unread postPosted: Fri Mar 25, 2005 12:24 pm


I have bought some double sided PCBoard for Microwave work from Ebay, it was cheap, looks of good quality and is in bulk but unfortunately I don't know what the substrate material used in it is, or how thick the copper is. How can I find out, is there some test that I can perform? I will be etching 50ohm microstrip lines on it, how can I determine the right thickness for the lines without any knowledge of the board material or Er (relative substrate electric permitivity)? Any ideas?



Post subject:
Unread postPosted: Sat Mar 26, 2005 9:07 am

First check if there are any labels or markings attatched to any pieces. Post this info up, as it may contain the information you are looking for.

Check Rogers, Arlon and Taconic websites ( I know there are more manufacturers, but these are the most common)

The physical dimensions are easily measured with a micrometer or vernier caliper.

As a first guess, use a line width that is the same as the material thickness, although absolute value of impedance will be unknown, it will be reasonable to work with.

To measure Er, I would suggest that you etch some test resonators of known dimensions using this line width onto a spare piece, and measure the response on a VNA. If you have no VNA available, any means of measuring VSWR with frequency will do. I would recommend a ring resonator, coupled lightly as possible. Using ring resonator eliminates the capacitive end effects of a open line. The S11 (VSWR) response will dip at the point where the ring is electricaly one wavelength around (use centre line of strip). Be aware that this will repeat at integer wavelength multiples.

You can now apply the microstrip design equations (or goal seek in excel) to give a reasonably accurate value for Er.


Post subject:
Unread postPosted: Mon Mar 28, 2005 3:40 pm

There are no markings or stickers on the board itself but I'll try your procedure, thank you for the tips!



Post subject:
Unread postPosted: Tue May 17, 2005 9:29 am

The guest who suggested the resonator is correct, the method suggested is the standard procedure used to determine er ( dielectric constant).. The equation he was refering to is: Lambda=C/(sqrt(er)*F); where Lambda is the wavelength , c is the speed of light, f=the frequency.

Posted  11/12/2012
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