Index: impedance.c
===================================================================
--- impedance.c	(revision 31809)
+++ impedance.c	(revision 31810)
@@ -64,3 +64,67 @@
 	return 0;
 }
 
+
+/* As documented at https://chemandy.com/calculators/elliptical-integrals-of-the-first-kind-calculator.htm
+   refrerence: These recursive equations were originally given for use in calculating the inductance of helical coils and are taken from Miller, H Craig, "Inductance Equation for a Single-Layer Circular Coil" Proceedings of the IEEE, Vol. 75, No 2, February 1987, pp. 256-257.
+*/
+static double ellint(double k)
+{
+	double a = 1.0, b = sqrt(1.0-k*k), c = k, K, lastc = 100;
+
+	if ((k <= -1.0) || (k >= 1.0))
+		return 0;
+
+	for(K = M_PI/(2*a); (c != 0) && (c != lastc);) {
+		double an, bn, cn;
+
+		an = (a+b)/2;
+		bn = sqrt(a*b);
+		cn = (a-b)/2;
+		lastc = c;
+		a = an; b = bn; c = cn;
+		K = M_PI/(2*a);
+	}
+	return K;
+}
+
+/* Formula from https://chemandy.com/calculators/coplanar-waveguide-with-ground-calculator.htm
+   ref: Transmission Line Design Handbook by Brian C Wadell, Artech House 1991 page 79 */
+double pcb_impedance_coplanar_waveguide(rnd_coord_t trace_width, rnd_coord_t trace_clearance, rnd_coord_t subst_height, double dielect)
+{
+	double a = TOM(trace_width), clr = TOM(trace_clearance), h = TOM(subst_height), Er = dielect;
+	double b = clr+a+clr, z0, eeff, ktmp, k, k_, kl, kl_, ktmp2;
+
+	k = a/b;
+	k_ = sqrt(1.0 - k*k);
+	kl = tanh((M_PI * a)/(4.0*h)) / tanh((M_PI * b)/(4.0*h));
+	kl_ = sqrt(1.0 - kl*kl);
+
+	k = ellint(k);
+	k_ = ellint(k_);
+	kl = ellint(kl);
+	kl_ = ellint(kl_);
+
+	ktmp = (k_ * kl)  / (k * kl_);
+	eeff = (1.0 + Er * ktmp) / (1.0 + ktmp);
+
+	ktmp2 = k / k_ + kl / kl_;
+	return ((60.0*M_PI) / sqrt(eeff)) * (1.0 / ktmp2);
+}
+
+const char pcb_acts_impedance_coplanar_waveguide[] = "impedance_coplanar_waveguide(trace_width, trace_clearance, subst_height, dielectric)";
+const char pcb_acth_impedance_coplanar_waveguide[] = "Calculate the approximated impedance of a coplanar_waveguide transmission line, in ohms";
+fgw_error_t pcb_act_impedance_coplanar_waveguide(fgw_arg_t *res, int argc, fgw_arg_t *argv)
+{
+	rnd_coord_t trace_width, trace_clearance, subst_height;
+	double dielectric;
+
+	RND_ACT_CONVARG(1, FGW_COORD, impedance_coplanar_waveguide, trace_width = fgw_coord(&argv[1]));
+	RND_ACT_CONVARG(2, FGW_COORD, impedance_coplanar_waveguide, trace_clearance = fgw_coord(&argv[2]));
+	RND_ACT_CONVARG(3, FGW_COORD, impedance_coplanar_waveguide, subst_height = fgw_coord(&argv[3]));
+	RND_ACT_CONVARG(4, FGW_DOUBLE, impedance_coplanar_waveguide, dielectric = argv[4].val.nat_double);
+
+	res->type = FGW_DOUBLE;
+	res->val.nat_double = pcb_impedance_coplanar_waveguide(trace_width, trace_clearance, subst_height, dielectric);
+	return 0;
+}
Index: impedance.h
===================================================================
--- impedance.h	(revision 31809)
+++ impedance.h	(revision 31810)
@@ -5,3 +5,7 @@
 extern const char pcb_acts_impedance_microstrip[];
 extern const char pcb_acth_impedance_microstrip[];
 fgw_error_t pcb_act_impedance_microstrip(fgw_arg_t *res, int argc, fgw_arg_t *argv);
+
+extern const char pcb_acts_impedance_coplanar_waveguide[];
+extern const char pcb_acth_impedance_coplanar_waveguide[];
+fgw_error_t pcb_act_impedance_coplanar_waveguide(fgw_arg_t *res, int argc, fgw_arg_t *argv);
Index: lib_formula.c
===================================================================
--- lib_formula.c	(revision 31809)
+++ lib_formula.c	(revision 31810)
@@ -34,6 +34,7 @@
 
 static rnd_action_t lib_formula_action_list[] = {
 	{"impedance_microstrip", pcb_act_impedance_microstrip, pcb_acth_impedance_microstrip, pcb_acts_impedance_microstrip},
+	{"impedance_coplanar_waveguide", pcb_act_impedance_coplanar_waveguide, pcb_acth_impedance_coplanar_waveguide, pcb_acts_impedance_coplanar_waveguide},
 	{"formula_bisect", pcb_act_formula_bisect, pcb_acth_formula_bisect, pcb_acts_formula_bisect}
 };