Click here to go to our main page on coupled line couplers
Click here to learn about Henrik Forstén's on-line 2D EM field solver tool
Click here to see PolyStrata coax analyzed with an Henrik Forstén's on-line EM tool
Click here to learn about the PolyStrata process
Click here to see a GaAs microstrip10dB coupler analyzed with Henrik Forstén's tool
New for March 2026. We used Henrik Forstén's on-line 2D EM tool to develop a 10dB coupler in PolyStrata coax. This is a TEM media, so 2D EM analysis will be very accurate in this case. You are not likely to be able to analyze this problem using linear models in ADS or Microwave Office. They don't have rectangular coax models, but maybe you could approximate the media as a "fat" stripline. Now that we have an on-line 2D EM tool, such approximations won't be necessary!
Below is a snapshot of the coupler geometry. In summary, it is an air-dielectric 10dB coupler, the outer jacket's inside dimensions are 400um tall by 850um wide. The strips are 200um tall by 120um wide. The gap is 90um. How did we come up with these dimensions? We started with a fixed height for the inner and outer conductors, then came up with the widths and gap by trial and error to get the targeted even and odd-more impedances. It took a few tries, and maybe to minutes of work.
Henrik's on-line tool allows you to save your work, click the link below to see the stored parameters for this coupler.
Link to geometry
For a 10dB coupler in 50 ohm system, Zeven should be 69.7 ohms and Zodd should be 35.2 ohms. The formulas for these parameters can be found on this page. We gave up on tweaking the dimensions when we got close enough for government work, here are the results.
Here are the S-parameters. You don't get to assign the port numbers, the tool picks them. It is almost exactly 10 dB coupling, and reflection coefficient (S11) and isolation (S21) are better than 50 dB. Further tweaking would be a fool's errand, you can never control dimensions well enough to get more than 50 dB isolation or return loss.
Now let's look at our PolyStrata coupler compared to a microstrip coupler, which was EM analyzed on this page, In short, the microstrip design is a sorry mess compared to the PolyStrata coupler, which has much better isolation and return loss. This is because PolyStrata is a TEM media and microstrip is only quasi-TEM.
Now you can design your own PolyStrata couplers. Put that on your resume!