Stripline
Updated January 13,
2014
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Stripline, according to David
Pozar's textbook "Microwave Engineering"
was invented at by R. Barrett in the 1950s. Airborne Instruments
Labs (Long Island New York, gone but spawned present day companies
such as MITEQ) coined the term "stripline", while others
such as Sanders (Nashua, NH, now part of BAE) applied the trade
name "Triplate".
Stripline is a conductor sandwiched
by dielectric between a pair of groundplanes, much like a coax cable
would look after you ran it over with your smallmanhood indicating
SUV (let's not go there...) In practice, "classic" stripline
is usually made by etching circuitry on a substrate that has a groundplane
on the opposite face, then adhesively attaching a second substrate
(which is metalized on only one surface) on top to achieve the second
groundplane. Stripline is most often a "softboard" technology,
but using lowtemperature cofired ceramics (LTCC), ceramic stripline
circuits are also possible.
All kinds of interesting circuits
can be fabricated if a third layer of dielectric is added along
with a second interior metal layer, for example, a stackup of 31
mil Duroid, then 5 mil Duroid, then 31 mil Duroid (Duroid is a trademark
of the Rogers
Corporation). Transmission lines on either of the interior metal
layers behave very nearly like "classic" stripline, the
slight asymmetry is not a problem. Excellent "broadside"
couplers can be made by running transmission lines parallel to each
other on the two surfaces. We'll add more about this later!
Other variants of the stripline
are offset strip line and suspended air stripline (SAS).
For stripline and offset stripline,
because all of the fields are constrained to the same dielectric,
the effective dielectric constant is equal to the relative dielectric
constant of the chosen dielectric material. For suspended stripline,
you will have to calculate the effective dielectric constant, but
if it is "mostly air", the effective dielectric constant
will be close to 1.
Advantages and disadvantages
of stripline
Stripline is a TEM (transverse
electromagnetic) transmission line media, like coax. The filling
factor for coax is unity, and "Keff"
is equal to ER. This means that it is nondispersive. Whatever circuits
you can make on microstrip (which is quasiTEM), you can make better
using stripline, unless you run into fabrication or size constraints.
Stripline filters and couplers always offer better bandwidth than
their counterparts in microstrip, and the rolloff of stripline BPFs
can be quite symmetric (unlike microstrip). Stripline has no lower
cutoff frequency (like waveguide does).
But is stripline really nondispersive
at all frequencies? Read about the low
frequency dispersion of TEM media, something to think about
when you are designing between 10 MHz and 1 GHz...
Another advantage of stripline
is that fantastic isolation between adjacent traces can be achieved
(as opposed to microstrip). The best isolation results when a picketfence
of vias surrounds each transmission line, spaced at less than 1/4
wavelength. Stripline can be used to route RF signals across each
other quite easily when offset stripline is used.
Disadvantages of stripline are
two: first, it is much harder (and more expensive) to fabricate
than microstrip, some old guys would even say it's a lost art. Lumpedelement
and active components either have to be buried between the groundplanes
(generally a tricky proposition), or transitions to microstrip must
be employed as needed to get the components onto the top of the
board.
The second disadvantage of stripline
is that because of the second groundplane, the strip widths are
much narrower for a given impedance (such as 50 ohms) and board
thickness than for microstrip. A common reaction to problems with
microstrip circuits is to attempt to convert them to stripline.
Chances are you'll end up with a board thickness that is four times
that of your microstrip board to get equivalent transmission line
loss. That means you'll need forty mils thick stripline to replace
ten mil thick microstrip! This is one of the reasons that softboard
manufacturers offer so many thicknesses.
Stripline Rule
of Thumb
Time for another Microwaves101 Rule of
Thumb! This one was contributed by an Yaroslav, from beautiful
Butler, New Jersey. The minimum width for a stripline that is encased
by metal on the edges is 5 times the line width, in order for the
impedance to calculate with the "normal" closed form equations.
The drawing below
is a 3D electromagnetic model of stripline with perfect electrical
conductors encasing all four sides along the z and y axes (created
using Ansoft's HFSS). The width of the stripline is 0.284 inches,
its thickness is 0.050 inches, the height of the enclosure is 0.750
inches, and the relative dielectric constant of the material is
1 (it's air).
The width of the
enclosure was varied to examine its effect on characteristic impedance
(see figure below). As the width increases, the impedance increases
(less fringing capacitance to the edge walls), but the rate of increase
eventually reduces to zero when the enclosure becomes infinitely
wide (the enclosure becomes two parallel plates). The goal of this
design was to determine the dimensions for a 100 ohm transmission
line, which happens to occur near a width of 1.4 inches. Coincidentally,
the 5X width rule is 1.42 inches. Past the 5X width point, the impedance
only changes about 2% all the way out to infinity. An error of two
percent in impedance is so small it is usually negligible. After
all, it represents a voltage standing wave ratio (VSWR)
of only 1.02:1!
For grins we tried
the Microwaves101 stripline impedance calculator
on Yaroslav's dimensions of 0.750 inch height, and 0.284 inch
strip width. We calculated 114 ohms. Looks like our calculator wasn't
all that accurate in this case, but we will look into this further.
Stripline equations
A simplified (approximate) equation for characteristic
impedance of stripline is given as:
Thanks to David for helping us correct this in January 2014!)
We seem to have misplaced the
reference for this equation, any stripline solution comes with a
lot of caveats about what range of geometries it performs accurately.
We'll try to dig that up one of these days.
If anyone wants to tackle the
job of presenting a more accurate formula for stripline impedance,
please contact us. We also need:
Synthesis equations for stripline (input Z0 and partial geometry, solve for the rest of the geometry)
Attenuation (metal and dielectric loss) for stripline
Try out our stripline
calculator!
