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defining 'air' substrate

Question asked by tobebadger on Apr 23, 2009
Latest reply on Apr 28, 2009 by RANDY_RHEA

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Can someone tell me if I have correctly defined a substrate for an air dielectric design incorporating a .212" Diameter coaxial resonator within a .625" 'slabline' ground plane?
Er=1
Ur=1
Tand=0
Rho=1
Thick=1.42
Sr=0.094
Height=625

RANDY_RHEA Apr 28, 2009 3:37 AM

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I couldn't tell if you were using an EM or circuit theory simulator. The GENESYS circuit theory simulator has a model for a round rod between ground planes: it is called slabline. The input parameters for it override the thickness and height partameters in the substrate dialog.

By the way, the relative dielectric constant for dry, dust free air at 20C and 1 atm is not 1 but 1.00059. The dielectric constant increases linearly with atmospheric pressure.

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SERGEYTRU

Apr 27, 2009 3:10 AM

EM simulators (Momentum, Empower) are planar ones, and do not allow to simulate designs with metal conductors changing width with substrate height, as round robs.

You need using 3D EM simulator, as ADS EMDS to simulate the slab line with circular conductor.

In planar simulators it may be simulated the only after replacing round rod by rectangular only. In this case the substrate stack must consist of 2 air dielectric layers and thick metal layer for the central conductor having constant width. The top and bottom air layers must take into account the central metal height, which depends on what expansion for the thick metal do you use.

So for "UP" thick metal expansion:

Top air: Height=625/2-212/2=312.5-106=206.5 mil
Bottom air: Height=625/2+212/2=312.5+106=418.5 mil

For alternative "Down" thick metal expansion the Top and Â Bottom air layer heights will be swapped.

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NWHITE

Apr 27, 2009 5:39 AM

In case you're having troubles reading that...

"rob" => "rod"
"thich" => "thick"

Basically, Empower & Momementum cannot simulate a round conductor, parallel to the substrate. You need to simulate a "simplified" rectangular-cross-section conductor instead.

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RANDY_RHEA Apr 28, 2009 3:37 AM

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SERGEYTRU Apr 27, 2009 3:10 AM
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Correct Answer
EM simulators (Momentum, Empower) are planar ones, and do not allow to simulate designs with metal conductors changing width with substrate height, as round robs.

You need using 3D EM simulator, as ADS EMDS to simulate the slab line with circular conductor.

In planar simulators it may be simulated the only after replacing round rod by rectangular only. In this case the substrate stack must consist of 2 air dielectric layers and thick metal layer for the central conductor having constant width. The top and bottom air layers must take into account the central metal height, which depends on what expansion for the thick metal do you use.

So for "UP" thick metal expansion:

Top air: Height=625/2-212/2=312.5-106=206.5 mil
Bottom air: Height=625/2+212/2=312.5+106=418.5 mil

For alternative "Down" thick metal expansion the Top and Â Bottom air layer heights will be swapped.
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NWHITE Apr 27, 2009 5:39 AM
Mark Correct
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In case you're having troubles reading that...

"rob" => "rod"
"thich" => "thick"

Basically, Empower & Momementum cannot simulate a round conductor, parallel to the substrate. You need to simulate a "simplified" rectangular-cross-section conductor instead.
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RANDY_RHEA Apr 28, 2009 3:37 AM
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I couldn't tell if you were using an EM or circuit theory simulator. The GENESYS circuit theory simulator has a model for a round rod between ground planes: it is called slabline. The input parameters for it override the thickness and height partameters in the substrate dialog.

By the way, the relative dielectric constant for dry, dust free air at 20C and 1 atm is not 1 but 1.00059. The dielectric constant increases linearly with atmospheric pressure.
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