CST simulation - Transmission Line Problem

[Bisonriver]

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

 

[VEBill]

Add a sphere of radius 6.37x10^6 m.

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?

 

[hacksaw]

hardly a class room problem, but you might be better served in a CST forum

 

[Bisonriver]

It's not for school. We are out of maintenance and don't have funds to renew the service contract this year. I'm new to this and the last guy retired. I've never used CST before, just don't know how to model a grounded shield. Googled, but no luck on it. Anyone can help? Or at least point me to the right direction. Thanks.

 

[VEBill]

I can't help you with CST2009, but perhaps I can shed some light on the more fundamental questions.

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.

 

[Higgler]

Twisted pair above a ground plane will be influenced by the ground plane if it's close. That's why us old people remember twinlead from our home TV antennas on the roof being located on standoff away from the house.

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.

 

[VEBill]

Higgler is perfectly correct that nearby conductors (such as ground planes) can have an influence on non-coaxial transmission lines.

To be clear, the OP was asking about 'earth grounding' that nearby ground plane. That's the question I was addressing.

 

[Bisonriver]

Thanks for you guys contribution. Here's the problem I would like to address (again). We have a tech at the other end of country saying that he measured an STP with a section of shield stripped off and the other section of the shield was grounded and intact. With a TDR, he sees an impedance mismatch, although not large, it can give us problems (high speed transmission). As you know, TDR has a high band width, it might fit the "extreme" cases you mentioned.

 

[VEBill]

"...an Shielded Twisted-Pair with a section of shield stripped off and the other section of the shield was grounded and intact. ...an impedance mismatch....not large..."

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.

 

[Bisonriver]

The shield was damaged unintentionally. In fact, there was a big hole on the shield although not completely broken apart. It might due to the abrasion or animal bite. Yes, the characteristic impedance is Z=sqrt((R+jwL)/(G+jwC)). My point is, does the damage of the shield (earth grounded) affect the characteristic of the STP (even though the shield is not part of the signal path)?
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.

 

[VEBill]

The change in Zo caused by the missing or damaged shield is very interesting (*), but the damaged shield is still a damaged shield. The primary function of the shield (shielding) has been compromised. Depending on the application ('Aerospace' vs. "animal bite"?) the cable should probably be replaced.

(* not really... ;-) )

 

[hacksaw]

The signifcance (meas. wise) of the shield breach depends in part on the signal wavelength and/or the resolution of your tdr meas.

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.

 

[Bisonriver]

Part of the cable is inside a conduit of a ground support facility. The previous design made it extremely difficult to reach. Techs had to disconnect/remove other equipment in order to get it. Often times this creates more problems than fixing it. Therefore, I was tasked to model the faulty shield in order to compare with the TDR result. We have several similar systems and would like to have a good confidence before starting digging the ground to replace the wire.

 

[Higgler]

If shield material was missing, and fell inside closer to the copper wires, you need to know that to model it accurately. That added capacitance will put a step in your pulses as the capacitor fills with charge. So a rising edge would have a stair step on the way up.

Darn varments.