Desert Sage replied the topic: Characteristic Impedance of Signal conductor

I found this that might be useful: http://www.fcarc.club/dokuwiki/lib/exe/fetch.php?media=estimating_characteristic_impedances.pdf

It shows for two wires, Z=276 x Log (D/r), where D is center to center spacing, r is the wire radius. For your configuration I calculate 214.7 ohms. When you add the second ground wire the impedance will be halved or 107.4 ohms.

avantec replied the topic: Characteristic Impedance of Signal conductor

Thanks, for response. I used that formula for my transmission line configuration but was uncertain of how to handle the second ground configuration in my other probe. I am in the process of trying to measure it using TDR. Are you sure it is halved? Brian

madengr replied the topic: Characteristic Impedance of Signal conductor

If the two wires are wide apart to begin with (i.e. high Z0 and the mutual capacitance <~ self capacitance), then it does not quite cut in half when adding the third wire. If they are already close together (i.e the mutual capacitance >> self capacitance) then it does. Try it with a microstrip calculator, then switch to stripline, keep decreasing the substrate thickness, and it approaches Z0/2.

Desert Sage replied the topic: Characteristic Impedance of Signal conductor

This boils down to the assumption used in designing combline filters based upon Cristal's curves. The charge distribution on one half of the conductor is assumed (to first order) to be unaffected by what is happening on the other side. It seems to be a relatively good assumption since I have designed many, many combline filters and never ran into a counter-example.

madengr replied the topic: Characteristic Impedance of Signal conductor

What are Cristal's curves? I'll have to look that up.

Desert Sage replied the topic: Characteristic Impedance of Signal conductor

"Coupled Circular Cylindrical Rods Between Parallel Ground Planes", Edward Cristal, IEEE-MTT July 1964 pp 428-439. These curves were used mostly to design combline filters but they can be used to design couplers (three rod dual directional for example). This paper is the logical follow up to Getsinger's paper (Coupled Rectangular Bars Between Parallel Plates, Jan 1962, IRE-MTT-10, pp 65-72).

madengr replied the topic: Characteristic Impedance of Signal conductor

Thanks:

ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1125843

avantec replied the topic: Characteristic Impedance of Signal conductor

Hi Desert Sage,
It's me again. I am trying to find out how far the fields extend laterally for the two configurations of probes I had those questions on 7 months ago. Someone from Berkeley Labs wanted to know exactly how far the fields would extend in different mediums, soil and water etc. I remembered in college that about three radius lengths outside of the probes would suffice to encompass about 99% of the fields, but now I am uncertain as I can't find the exact topic anywhere. Do you happen to know if there is closed form solution for this parameter and its dependence on the dielectric constant of the medium? Sincerely, Brian