Microstripline inductance and capasitance at the same time? - Page 1
Forum section: electronics / rf-microwave

author: LM21
author_rank: Regular Contributor
timestamp: October 07, 2024, 12:21:55 pm
content: I wonder what is happening. I studied web calculators for air dielectric micro strips, that is a copper strip over ground plane. I found web calculators for both capacitance and inductance. But I know it is pretty hard to a line be both at same time. I didn't study the calculators too well, so there may be some length limitations, but transmission lines are either capacitive or inductive at regular interwalls.And is it true that Er(relative permittivity) of the insulator has very little effect for the microstrip inductance. Because I may use air and not FR4

author: tszaboo
author_rank: Super Contributor
timestamp: October 07, 2024, 12:47:03 pm
content: Any transmission line will have 4 parameters "at regular intervals" in the lumped model.Series inductance (L), because it has a length, and it has self inductance.Series resistance, because it's made out of metal, that has limited conduction.Parallel capacitance (C), because you have two metal object next to each other.Parallel resistance because even isolators conduct a little bit.L and C, the ratio of it will give you the characteristic impedance which is 50Ohm or 75Ohm or something like that depending on what you are connecting.C depends on Er. L depends on relative permeability, which is very close to 1 in most cases unless you have ferrite or something as dielectric (which you don't)

author: LM21
author_rank: Regular Contributor
timestamp: October 07, 2024, 02:07:40 pm
content: It is easier to ask here than think, but I think open short stubs have capacitance and lines in series have Edit: inductive (capacitive) reactance.Are you saying that insulation and its Er, have no effect on inductance

author: nctnico
author_rank: Super Contributor
timestamp: October 07, 2024, 02:40:05 pm
content: Quote from: LM21 on October 07, 2024, 12:21:55 pmWeb calculators are not the best source as these use limited formulas.Any piece of microstrip or stripline has capacitance and inductance at the same time. Using a lumped (R, L, C) model only takes you so far. A field solver like Sonnet is a better tool to study & simulate microstrip structures. IMHO a very good book on microstrip design is 'Microstrip Filters for RF/Microwave Applications' by Hong and Lancaster. Not cheap but well worth the money as it covers both theory and practical applications / implementations without much fuss.

author: tggzzz
author_rank: Super Contributor
timestamp: October 07, 2024, 02:54:22 pm
content: "All models are wrong, but some are useful"Inductance and capacitance are simplified models of how electromagnetic waves behave. Those models are valuable in many circumstances, but they have their limits. One such limit is in transmission lines.As for stubs and wider/narrower lines having inductance or capacitance, that isn't quite correct. A more useful statement is that - over a limited range of frequencies - such linesbehave in similar waysto inductors and capacitors.That is the basis for using filter design techniques and programs to design filters, where the output is expressed in terms of LCR components. Even so, not all filters are easily expressible in that kind of way, and EM field solvers are often used to "tweak" the design parameters.

author: TimFox
author_rank: Super Contributor
timestamp: October 07, 2024, 02:54:28 pm
content: Short transmission lines, with electrical length shorter than (wavelength/4) at that frequency, look like a capacitor when the other end is open circuit (or high impedance), and like an inductor for short circuit (or low impedance).As the frequency approaches the quarter-wavelength condition, the impedance goes to series resonance (open circuit termination) or parallel resonance (short circuit termination)—double check me for correct description.At (wavelength/8), the line looks like a capacitance or inductance with reactance equal to the characteristic impedance of the line (open or short terminated, respectively).There are multiple resonances at higher frequencies, as mentioned above.

author: LM21
author_rank: Regular Contributor
timestamp: October 07, 2024, 08:41:03 pm
content: I think I have got good info, thank you. I wrote rubbish in my earlier post. I should have written: "I think open short stubs have capacitance and lines in series have inductive reactance."What bothers me still. is that what is between ground and the micro strip have no effect on inductance.

author: TimFox
author_rank: Super Contributor
timestamp: October 07, 2024, 09:03:38 pm
content: Between the line and ground plane is a dielectric, which affects the capacitance per length.Some very rare transmission line cables included magnetic material that could affect the inductance for a given geometry.