CST simulation - Transmission Line Problem
CST simulation - Transmission Line Problem
(OP)
Hi,
I'm using CST2009 Microwave Studio to simulate transmission line characteristic impedance. The scenario is to have a pair of copper wires that are a distance above the earth ground plate. Additionally, I'd like to see the characteristic impedance of a shielded twist pair (STP) with the shield connected to earth ground. Currently I'm using the "Waveguide Ports" on simple 2 wires without problem. How do I add an earth-grounded shield? That is, I can draw a metallic shield, but how do I specify it to be earth grounded?
Thanks for answering my question.
B. Riverton
I'm using CST2009 Microwave Studio to simulate transmission line characteristic impedance. The scenario is to have a pair of copper wires that are a distance above the earth ground plate. Additionally, I'd like to see the characteristic impedance of a shielded twist pair (STP) with the shield connected to earth ground. Currently I'm using the "Waveguide Ports" on simple 2 wires without problem. How do I add an earth-grounded shield? That is, I can draw a metallic shield, but how do I specify it to be earth grounded?
Thanks for answering my question.
B. Riverton





RE: CST simulation - Transmission Line Problem
More seriously, because you're a brand new member asking a question that seems like it might (?) be a homework problem, we have to ask:
Is this for school?
RE: CST simulation - Transmission Line Problem
RE: CST simulation - Transmission Line Problem
RE: CST simulation - Transmission Line Problem
The characteristic impedance of a transmission line (RF) shouldn't change if nearby conductive surfaces are earth grounded or not. The equations used to estimate Z for balanced pair or coaxial cable do not include consideration of such external connections.
The clearest real-world example I can think of would be an aircraft equipped with 1Gbps Ethernet. The network works while the aircraft is at 30,000 feet, and it continues to work when the aircraft fuselage is firmly connected to earth ground during refueling. The characteristic impedance of the Cat5e wiring remains stable because it 'doesn't care' about any nearby planets.
There may be some pathological (extreme) cases where you could demonstrate a change, but that would perhaps indicate a fundamental defect in the system design.
RE: CST simulation - Transmission Line Problem
Inside a shield, by definition "it's a shield". You can add anything to it and expect no change.
CST and it's competitor HFSS are very tricky, so tricky that on complicated antennas, we use Jim Reed from Austin Texas for our CST setups. He's cheap. Then we run optimizations for months, then build our antennas.
RE: CST simulation - Transmission Line Problem
To be clear, the OP was asking about 'earth grounding' that nearby ground plane. That's the question I was addressing.
RE: CST simulation - Transmission Line Problem
RE: CST simulation - Transmission Line Problem
To be clear, removing the shield from the cable is different than your original question, which was about earth grounding (or not) the shield. "That is, I can draw a metallic shield, but how do I specify it to be earth grounded?"
The characteristic impedance of a transmission line is a function of inductance and capacitance per unit length. The presence of the shield will directly affect the capacitance from one conductor to the other because it provides a parallel capacitance (conductor to shield, two in series).
In practice, you should not remove the shield except at the very ends where you install a connector. And this should be just an inch or two.
RE: CST simulation - Transmission Line Problem
My though was, since the STP has 2 wires, say one is + and the other one is – (return). The field in between is confined within the shield. If there's a shield damage, more or less, the field leakage causes the capacitance change and that leads to characteristic impedance change.
I was trying to use CST to prove right or wrong on this, but was stuck on the usage. I found that I may be able to use the boundary condition to define the ground, but don't have a chance to try it out yet.
RE: CST simulation - Transmission Line Problem
(* not really...
RE: CST simulation - Transmission Line Problem
Operationally, you replace the cable or repair the damaged section because of the increased likelihood of further signal degradation.
The quick answer is that modeling the characteristic impedence doe not give you the answer being sought.
RE: CST simulation - Transmission Line Problem
RE: CST simulation - Transmission Line Problem
Darn varments.