The concept of a geometric form factor is very useful when thinking about transmission lines. It bundles up all the messy 2D geometry of cross-sections and field line patterns into one scalar quantity that is fixed for a particular configuration. Then you can think about the general concepts without getting bogged down in field integrals and field solvers.

To explore this idea, let’s start with a uniform relative dielectric constant ε_{r} like you’d have with stripline. (Later we’ll generalize to the case where the field lines “sees” a mixture of materials, for example dielectric and air for a microstrip.)

First define a unitless geometric form factor, F, such that capacitance per unit length, C, is:

…where ε_{0} is the vacuum permittivity, 8.9pF/m (or 0.22pF/inch) and ε_{r} is the relative permittivity (relative dielectric constant), ~4.2 to 4.6 in FR4 glass/resin.

In general, we need a 2D electrostatic solver to determine *F* but let’s first look at a geometry for which there’s an exact, textbook, analytical solution: a rod of radius *a*, with its center a distance *b* above a ground plane like this:

The form factor is:

Couple of thing to note about F:

**Unitless:** F is a function of a geometric *ratio*, *b/a*.

**Logarithm of a smallish ratio:** In practical cases, the numerator and divisor are not of wildly different orders of magnitude. You wouldn’t create a transmission line with *a = 1 mm* and *b = 1 km*! No, for the realistic range of values of *b/a*, the natural logarithm of the function here is of order 1. For example if *b=2a*, then *F* is 0.21.

Let’s plot it and two other configurations (illustrated below) and you’ll see that (apart from the “short circuit” corner case at *a = b*) the blue curve varies slowly with *b/a*:

The neat thing about the form factor is that when you have it for capacitance, you get inductance and impedance “for free.” To see the first of these, consider the propagation velocity, v, which is both:

…where c = vacuum speed of light = 0.3 m / ns or 12 inches / ns

(Strictly speaking, we can only use “pure” insulator ε_{r} if the metal is a perfect conductor, the skin depth is zero (surface currents). If the conductivity and skin depth are finite, we would need a “blend” of the dielectric and metal ε_{r}s.)

With a little algebra you can see that the inductance per unit length, *L*, is given by:

*L = Fμ _{0}*

…where

*μ*is the vacuum permeability, 1.3 μH/m or 32 nH/inch.

_{0}Thus, you can think of

*F*as “how inductive the geometry is versus the free space value of 1.” If

*F < 1*the geometry is “more capacitive” than free space, if

*F > 1*it’s “more inductive.” The second benefit is that once you have L and C, you also have impedance. Again a little algebra shows that:

…where

*Z*is the vacuum impedance, 377 Ω. The form factor and the relative dielectric constant determine how far the line impedance is from the “natural” value of 377 Ω. Lowering the form factor (making a “more capacitive” geometry) lowers the impedance, as does a higher relative dielectric constant. If ε

_{0}_{r}= 4.2, and you want Z = 50 Ω, then pick your geometry such that

*F*is 0.27.

If you take the ground plane and wrap it around the rod, you get co-ax:

The form factor (magenta line in the plot above) is a bit lower (more capacitive) as you’d expect from bring the ground plane closer.

If instead you squash the rod into a ribbon and put a second ground plane on top, you get stripline:

The result is a bit less capacitive (red line in the plot above), because the squashing increases the separation from ground.

A power and ground plane pair forms another, simple, special case that’s useful. In a power distribution network (PDN) the source (VRM) and load (the chip) impedances are low, so the design goal is ultralow impedance. We use wide, closely spaced planes. The form factor of such a geometry is easily derived from the parallel plate capacitor equation recast into “per unit length” format: C = ε_{0}ε_{r}a/b. If a >> b, then F is simply b/a, where a is the copper width and b is the height of the dielectric between the planes. I didn’t plot it above, but it would be a straight line through the origin of slope 1. The inductance per square is simply μ_{0}b or 1.3 pH per square per μm of height or 32 pH per square per mil of height. For a typical 4 mil (~100μm) height that’s ~130pH per square.

You can generalize this idea to non-uniform dielectrics by using a blended or “effective” relative dielectric constant. An example is the IPC recommended approximation for microstrip. In a future posting, I’ll also show how it’s useful for lumped elements like inductors in the form of flat coils, and short and long solenoids.

Mort Harwood// Sep 2, 2009 at 10:30 amColin, this article is useful; thank you for providing the link on si-list.

Colin Warwick// Sep 2, 2009 at 10:35 amThanks, Mort! I appreciate the feedback!

Best regards,

— Colin

Mickenzy// Feb 1, 2010 at 5:40 pmThis didn’t really help.

Colin Warwick// Feb 4, 2010 at 4:31 pmHi Mickenzy, Sorry to hear that. What kind of help were you looking for? I’d reply via email directly, but the address you gave N/A@yahoo.com is clearly not a real one

— Colin

David// Dec 1, 2010 at 6:45 amVery useful indeed. One query though, the arguments above are clearly helpful rather than precise (in a mathematical sense). Could you point me towards a rigorous discussion? I’ve also seen conductance (g) being treated as g = sigma/F where sigma is the conductivity.

Colin Warwick// Dec 1, 2010 at 2:04 pmThanks, David.

With the exception of the stripline formula (which is approximate) the formula here are exact for the case of ideal conductors (perfect electrical conductor or PEC) and dielectrics (zero loss tangent and perfect insulator). The text book I use is old (like me 🙂 ) “Electricity and Magnetism” by the husband and wife team of Brebis Bleaney & Betty I. Bleaney. I don’t have a version that accounts for conductance loss, but I’ll look for one.

Thanks again.

— Colin

Alessandro// Dec 13, 2016 at 12:05 pmthank you Colin,

very useful, do you think that with ADS i can evaluate power plane ac profile during the pre-layout using your formulas?

Colin Warwick// Dec 19, 2016 at 4:18 pmHi Alessandro, You can get a zero-order approximation but for a more accurate answer it is better to do an EM extraction. We have a tool PIPro that is an application-specific EM field solver that can do it. More info at http://www.keysight.com/en/pd-2625864-pn-W2359EP/pipro-power-integrity-em-analysis-element . Best regards, Colin