Rectax transmission lines

Updated October 3, 2006

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What do you call a coaxial transmission line that is square or rectangular in cross-section, instead of the usual round variety? The general case of rectangular transmission lines we'll call rectax, although we've heard the term "recticoax" used. Reminds us of an old joke about an unfortunate cow, but we'd better not go into that here...

The word "coaxial" implies that the conductors nominally share the same centerline. However, we don't want to make such restrictions on the technology, that's another reason why we'll use "rectax" in favor of "recticoax".

When would you use a rectax a transmission line? Whenever it is economical! In the near future, you might consider using the technology that DARPA is developing under the 3D-MERFS program described below, which provides a very low-loss transmission line structure that is capable of carrying millimeter-wave frequencies at unheard of low loss and is manufactured in a batch process.

Advantages of rectax

What are the advantages of rectax, versus waveguide, stripline, or microstrip? The tradeoffs of any transmission line start with its attenuation characteristics. Read our page on transmission line loss, to become familiar with the four loss mechanisms _C, _D , _G and _R .

Compared to stripline, you can get about half the RF loss due to metal (_C) in the same height. This is because all four sides of the center conductor are are exposed to the RF current. Recall that the loss of a transmission line is related to skin depth, which limits how far into the conductor the EM wave penetrates. The metal loss comparison to microstrip is even more favorable.

Like coax, rectax provides a pure TEM mode, which means no dispersion, which you would encounter in waveguide and to a lesser extent in microstrip.

The shielded structure rectax offers excellent isolation for adjacent signals, potentially as good as coax or waveguide. In microstrip or stripline, bringing traces anywhere near each other spells isolation problems that are not easily solvable.

Rectax has similar properties to coax, in that its characteristic impedance doe not change when the cross-section of the squarax is scaled up or down. There is a limitation on upper frequency that the squarax line can pass before funny modes that you want to avoid start to occur. Just like coax, rectax has a cutoff frequency that you don't want to exceed.

Squarax versus rectax

The square case (height = width) we will call squarax (square-coax, get it?)

Now it's time for a Microwaves101 rule of thumb!

For air-filled squarax, a characteristic impedance is achieved when the center conductor's height and width are 40% of the width and height of the cavity.

Rectax equations

Coming soon?

Characteristic impedance

Cutoff frequency

Attenuation

Power handling

DARPA's 3D-MERFS program

This is one of the few new transmission lines to come along in a lifetime, ranking up there with stripline, microstrip and coplanar waveguide. This cool technology is being worked on DARPA's 3-D Micro Electromagnetic Radio Frequency Systems (3D-MERFS) program. We found this image on www.darpa.mil. 3D-MERFS creates miniature air-dielectric rectangular coax by sequentially plating thick layers of copper, much like a printed wiring board process.

Dr. Zoya Popovic (UC Boulder) has promised to help us out with this topic soon. We'll keep you posted in our monthly newsletter. Some of the data on 3D-MERFS may be subject to ITAR, but you might learn more about it at the darpa.mil web site. We want to error on the side of caution here so we'll bite our tongues until we hear from Zoya. But we did copy this info from DARPA's web site so you don't have to go looking for it...

T"he 3-D Micro Electromagnetic Radio Frequency Systems (3-D MERFS) program was launched in mid-2004 to revolutionize the performance, cost, and form factor for advanced RF and MMW radar and communications systems, thereby fulfilling the unmet military & national security need for widely deployable high-performance multi-function MMW systems. Such systems include secure high-data rate communications, vehicle protection radar, on-the-move satellite communication systems, and compact solid-state missile seekers. Although prototype systems with some level of performance can be realized for most of these applications, these prototypes are typically heavy, bulky, fragile, un-mass producible, lack desired capabilities, and often cost 1000 times more than established cost targets.

Much of the difficulty associated with MMW systems arises because there exists no MMW analog to the Printed Circuit Board (PCB). Thus, instead of integrating MMW components compactly on a single system-scale substrate as is done in digital computers, MMW components are typically manually assembled, first at the module level, then the rack level, and then at the system level, thereby degrading performance and reliability, and making the systems un-mass producible.

Through developing a printed circuit board analog for MMW systems, the 3-D MERFS program seeks to extend the same system-level integration paradigm used in digital systems to MMW systems. The program is developing a novel high performance printed circuit board technology for MWW/RF, that instead of using planar micro-strip waveguide structures, uses 3-dimensional recta-coax structures - recta-coax much like the wire that carries cable-TV to your home, but 1000s and 1000s of them, miniaturized and integrated into one monolithic substrate.

Though made only of low cost metal and plastic, these structures outperform transmission lines printed on the most expensive semiconductor materials - with 10x less loss, carrying 10x-100x more power, with 100x component density, and having >1,000x better isolation. They also have much better dispersion, enabling broad-band operations.

The primary contractor on this program is BAE Systems, with subcontractors Rohm & Haas, the University of Colorado Boulder and Innovative Micro Technologies (IMT). This program is scheduled to complete in 2007."