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."
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