What Are TEM and Quasi-TEM Lines in Coaxial and Planar Interconnects?

TEM (Transverse Electromagnetic) and quasi-TEM lines

When we talk about TEM (Transverse Electromagnetic) and quasi-TEM lines, we’re diving into the world of radio frequency (RF) signal transmission. You might wonder, “What exactly does that mean for me?” Let me explain in simple terms.

In a coaxial cable, you’ll find that almost all the electric and magnetic fields lie in a plane that’s perpendicular to the direction the signal travels. We call these fields transverse because they are across, or ‘transverse,’ to the path of propagation. That’s why we refer to coaxial lines as TEM lines—Transverse Electromagnetic. Essentially, the electric field (E) and magnetic field (H) are both transverse, so they don’t have any significant components along the direction of the signal flow. This is why you can rely on coaxial cables to carry RF signals with minimal interference.

Now, when we look at other structures like stripline or microstrip lines, things change a bit. For example, the stripline is a type of planar interconnect that is sandwiched between two ground planes, often used in printed circuit boards (PCBs). It requires a solid dielectric material to support it, which keeps the signal confined and reduces loss. This structure is also great for controlled impedance environments, especially where you want minimal radiation loss.

How Do They Differ?

Then there is the microstrip line, which you might come across in semiconductor devices or multichip modules. Unlike the coaxial line, the microstrip line isn’t completely TEM. The fields are almost transverse, but there are some minor longitudinal components, which is why we call it a quasi-TEM line. As the frequency goes up, these longitudinal components become more significant. However, by keeping the dimensions of the line small, we can still make sure the fields stay mostly transverse, making it functionally close to a TEM line.

What makes these lines important to you and me is how they are used in real-world applications. For example, coaxial cables are great for shielding against external interference, while microstrip and stripline configurations are better suited for compact and integrated designs, like those on PCBs and microchips.

When deciding which type to use, it really comes down to the specific needs—whether you need low loss, minimal interference, or a particular kind of impedance control. So, next time you’re dealing with RF transmission or designing circuits, think about whether a TEM or quasi-TEM line is the right.