Drawing Curved Coplanar Waveguide in KiCad: A Practical Walkthrough
Most people hit a wall when they try to make a curved CPW in KiCad. You start by drawing a nice arc for the center conductor, set your gap to maintain 50-ohm impedance, and then realize the ground planes on either side don't follow the same path cleanly. The outer ground plane ends up with a different radius than the inner one. If you've tried the arc tool and gotten frustrated, you're not alone. Here is what actually works. The fundamental geometry problem is that a coplanar waveguide has a center trace and ground pour on both sides, all separated by a consistent gap. When you bend it, the center trace follows one radius, the inner ground edge follows a smaller radius, and the outer ground edge follows a larger radius. All three need to maintain the same gap width. KiCad does not have a built-in "curved CPW" tool, so you build it manually. Start by determining your trace width and gap based on your stackup. For a typical 4-layer FR4 board with a 0.2mm prepreg dielectric height, a 50-ohm CPW might need a center trace of about 0.35mm and a gap of roughly 0.2mm. These numbers change with your actual dielectric thickness, so run a field solver like Saturn PCB Toolkit or the online version before committing to anything.
Draw the center conductor first. Use the arc tool from the main toolbar, or the parametric curve tool in KiCad 6 and 7 if you need more control. Set your track width to the value you calculated. Route the arc to your desired angle and radius. Once that is placed, you need to add the ground planes on either side. For the ground planes, you cannot simply duplicate the arc and offset it. Each ground plane edge needs its own arc with a slightly different radius. The inner ground plane edge radius equals the center trace radius minus the gap. The outer ground plane edge radius equals the center trace radius plus the gap. Draw these as separate arc segments, matching the same start and end angles as the center trace. Fill the area between the inner and outer arcs with a copper pour, or draw them as solid polygons if you prefer explicit geometry over pours. Ground pour is the more common approach. Create a dedicated copper layer region or use the filled polygon tool, making sure to set the net to GND. Use the same outline you drew for the ground plane edges as the polygon boundary. You'll need to adjust the clearance settings so the pour does not encroach on the center trace gap.
There is a trick most tutorials skip: the gap between the center trace and the ground pour must remain constant along the entire curve. When you create your polygon clearance, do not rely on the default global clearance. Set a specific clearance value for the center trace net that matches your calculated gap. In KiCad 6 and later, you can set this per-net in the design rules editor under Clearance. This prevents the simulator or DRC from happily leaving you with a tapered gap that destroys your impedance. I ran into a specific problem last year that cost me two days. I was routing a curved CPW section on a microwave filter board, and everything looked fine in the layout. When I checked the impedance with a field solver, the effective gap at the bend was shrinking by about 8 percent compared to the straight sections. The center trace radius was too small relative to the gap. Tight bends on CPW distort the electromagnetic field distribution, and the simple geometric offset method I described above does not fully account for that. The workaround was to increase the bend radius to at least ten times the trace width, which brought the impedance variation down to under 2 percent. For tighter bends, I ended up taping the curve into short straight segments instead, each with properly spaced ground planes on either side. It is more work but gives predictable results.
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Parametric Curve Method
If you are using KiCad 7 or later, the parametric curve feature gives you a bit more precision. You can define a curve using polar equations rather than freehand arcs. For a CPW bend, the center conductor path can be defined parametrically, and then you manually construct the ground plane boundaries around it. The advantage is that you have exact control over the radius at every point along the curve. The disadvantage is that it requires some familiarity with the parameter editor, and you still have to draw the ground planes separately. Here is a practical workflow that saves time: draw the center trace arc first. Then use the dimension tool to read its exact inner and outer edge coordinates. Create two additional arcs with those adjusted radii. Use those three arcs as the boundaries of a filled polygon netted to GND. Set the polygon clearance equal to your gap value. This takes about five minutes once you know the numbers, versus the twenty to thirty minutes it takes to fumble through manual polygon placement.
Common Pitfalls
One thing beginners consistently mess up is the connection between the curved section and the straight CPW runs on either side. The ground planes must connect properly, and the gap must not change abruptly at the transition point. If you just butt two sections together, you will get a discontinuity that reflects signal. The fix is to blend the transition over at least five times the trace width. Taper the center trace radius gradually from straight to curved, and let the ground planes follow along with matching radii at every point. Another issue is the via stitching. Curved CPW relies heavily on the ground pour being well-connected to the reference plane. If your ground pour is floating or has insufficient vias, the return current path gets disrupted. Place stitching vias along the outer edge of the ground planes at intervals no greater than a quarter of the wavelength at your highest operating frequency. For a 10GHz design, that means vias every few millimeters. Simulation validation is non-negotiable. KiCad's layout tools do not simulate transmission lines. If you are building this for anything above a hobby project, export the geometry to a 3D field solver like Ansys HFSS or Sonnet. At minimum, use a 2.5D solver like Q3D Extractor. The hand calculations and geometric construction methods are only approximations. The actual performance depends on your specific stackup, surface finish, and manufacturer tolerances.
When Manual Drawing Is the Wrong Call
If your design requires many curved CPW sections, or if you are generating a filter bank with numerous coupled lines, doing this by hand is inefficient. A Python script using the KiCad PCB API can automate the process. Define the center trace path as a sequence of points, then programmatically generate the ground plane edges with the correct offsets. This is how I handled a twelve-resonator bandpass filter layout. The manual approach would have taken a full week. The script took about three hours to write and then produced all the curves in under a minute. There are community-developed scripts floating around on GitHub, though they tend to be written for specific KiCad versions and break with updates. Writing your own is usually faster than troubleshooting someone else's code.
Bottom Line
Drawing a curved coplanar waveguide in KiCad comes down to understanding the geometry, setting the right design rules, and verifying with a solver. The tool itself is straightforward. The impedance behavior at bends is what will trip you up if you skip the simulation step. Keep your bend radius generous, stitch your grounds properly, and do not trust the layout to tell you the impedance is correct just because the DRC passes.
