# Questions tagged [transmission-line]

A transmission line may refer to a cable or assembly that is designed to carry radio frequencies and must consider the wave nature of the signal. Coaxial cable is an example of a transmission line. It may also refer to high-voltage power transmission lines, used to transfer power from generating stations to consumers.

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### CAN Protocol traced routed with 90 ohm characteristic impedance

I am using ISO1042BDWV in my board. The schematic is given below. I know that the characteristic impedance of the CAN bus is 120 ohm. The layout team routed CANH and CANL as 90 Ω differential. The CAN ...
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### AC Circuit Analysis with Square Wave and Transmission Line

I'd like to solve the AC analysis for a circuit with a square-wave function (with a DC offset, so it goes from 0V to some positive voltage with each step) in series with a resistor and a shorted ...
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### Impedance matrix of a GSG probe

I want to design a CPW that allows for high transmission. Therefore, my first guess was to find the impedance of a CPW for the even mode. I have decided to solve it from the ground up. I have used a ...
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### Why do these secondary transmission wires coil around the primaries?

All three primary transmission wires in the picture below have smaller wires coiled around them. These smaller wires (secondaries?) connect to a transformer and onto a nearby house. The coils follow ...
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### Using flex PCB for RF signal instead of coax cable

I have been looking into reducing the number of different cables needed for my project: I have a couple of photodiodes on separate boards, with a total of 7 analog outputs. They have signal bandwidths ...
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### How does this HF 'coupler' work?

I came across this coupler design in this link in another question. It's designed to be used to couple a small monitoring amount of signal off a 1 kW HF (3 MHz to 30 MHz) signal on a through line. ...
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### What is the most electricity that can be transmitted over a typical power line of the US electricity grid?

I am trying to understand how much electricity can be sent over the long-distance high voltage interconnects in the US (not sure if this is what is more relevant to me -- the context of the question ...
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### Coax cables and input impedances

I'm doing a voltage measurement with the help of an Active Differential Probe (TESTEC TT-SI 9010) with an output voltage of +-7V. This voltages have rise times of the order 500ns. The set up looks ...
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### Contradiction between wave impedance mismatch and transmission line theory

Suppose we have two transmission lines connected with each other, each transmission line has the same characteristic impedance: $$Z_0 = \sqrt{\frac{L}{C}}$$ however they don't have the same ...
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### Solving for current flowing through R and reflection coefficient

This is my attempt at solving it. I'm not sure if it's correct, so I thought of asking for some advice. Image of circuit: Values: ABCD matrix: So I'm supposed to solve for the reflection ...
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### How to calculate reflection coefficient of a transmission line with discontinuity [duplicate]

I have a question, let's say that we want to calculate the Reflection Coefficient that we see by watching from left to right of the red-line this circuit: Let's say that: ...
1 vote
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### How to design for an impedance matching device for non-TEM waveguides?

Let us start from the assumption that we are working with RF waveguides which do not support TEM modes. In such scenario, there is no physical (only formal) meaning to an equivalent voltage and ...
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### Why are there two wires connected to each terminal of the RS485 port in energy meters

I want to connect the energy meters of our clients to a gateway so that the gateway can communicate with the meters using Modbus. I have visited several electrical rooms that have gateways installed ...
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### Validity of RLGC model for QTEM transmission lines with numerically calculated characteristic impedance and propagation constant

My problem In my understanding, any system that supports a QTEM mode can be represented by an RLGC transmission line model with negligible error. Indeed, when I perform a calculation in CST microwave ...
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### Why is electricity transmitted at high voltage? [duplicate]

I've learnt that step-up transformers are used to increase voltage and hence decrease current so we can reduce power losses in transmission. If the product V * I is constant with or without a ...
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### DDR3 U-DIMM Signal Reference Plain

I am doing the PCB layout of a DDR3 U-DIMM and I have run into a perceived gap in the JEDEC DDR3 DIMM standards and am hoping to get some input from a DDR3 SME who could clarify the Address/Control ...
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### Should long PCB traces be treated as transmission lines?

Suppose I have a BNC in connector, terminated in parallel with a 50 ohm resistor. The PCB trace extends for about 5-7cm before the signal is 'read' by a power amplifier. Should I treat this PCB trace ...
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### Is a gap needed with LVDS with metal layer below it?

I have a 1.8 V, 350 MHz, 100 Ω LVDS signal on an FFC on the bottom plane. Below the FFC is an aluminum metal layer; the aluminum is pretty much unconnected from the FFC. The air gap/solder mask gap is ...
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### Intuitive explanation of shorted stub behaviour

Can someone please give me an intuitive explanation of shorted stub behaviour? NOTE: This is not a duplicate of Intuitive explanation of open stub behaviour which has been narrowed to focus on open ...
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### Intuitive explanation of open stub behaviour

Can someone please give me an intuitive explanation of open stubs? I have the length-based mathematical derivation and explanation from my textbook, but I am looking for an intuitive one. For example,...
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### Does coiling a transmission line affect its characteristic impedance?

If I take an advertised 50 ohm impedance cable and turn it into a coil will this change its characteristic impedance from 50 ohm to something else? My intuition tells me it should as if we look at one ...
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### How to simulate a 30 inch FR-4 lossy channel (20dB loss) using ltline symbol in LtSpice?

For designing a high speed serdes system, I need to model a lossy channel of FR-4 substrate of length as 30 inch and 20dB attenuation. How can I implement it in LtSpice tool?
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### Why aren't baluns needed for a monopole to unbalanced coax connection?

I've asked before regarding dipoles connected to unbalanced lines. From what I gathered, the current in the inner shield when reaching the dipole (in transmitting case) can either go into the pole ...
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### What actually causes the need for a balun on a dipole fed to coax

My current understanding: The line in the center of the coax and the inner side of the shield are the current paths, as in the signal/field propagates between these two, (i can't therefore imagine it ...
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### Differential pairs / Single ended and the need for baluns

Excuse any blatant mistakes, I'm very clearly learning this new. Anyway from a purely transmission line perspective, it seems you can have a single ended line and a differential pair. I see the single ...
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### If RF signals require matched impedance to remove reflections, why do approaches to shunt noise need only a “good path to ground?”

From my understanding, for signals to propagate without reflection the impedance of the path must be constant and can be shown via the telegraph equation. When it comes to noise on the line, for ...
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### M-LVDS differential signals through multiplexers

I'm experimenting with M-LVDS via USB 3.1 cables, passing the differential signals through the SuperSpeed lanes. I'm using the DS91M040 for driving M-LVDS. So far no issues, however, I need now to ...
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### Why do the metal sticks (for the ground) on the ground plane antenna reflect and not absorb?

From my current knowledge, any metal whether grounded (and or earthed) or not will reflect radio waves as seen on dishes, Faraday cages, etc. On the other side the ground in transmission lines or the ...
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### Safety of grounded and isolated AC in medical systems

Introduction I try to make more sense of safety and reliability of grounded and isolated electricity. I divided the problem into 3 stages: Power-transmission line Household electricity Electrical ...
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### RF Reactive power splitter S21 and S12 is equal, when return loss is not

When measuring a 4-way reactive power splitter the S-parameters at the frequency of interest(400 MHz) looks like this: S11: -34 dB S22: -3dB S21 = S12 = -6dB With IN being port 1 and OUT 1 being port ...
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### Integrated Circuit z-impedance

I have an OBU (On-board Unit) circuit and it has a DSRC (Dedicated Short-Range Comm) IC, but I don't have any information about the IC from S11 and Z etc. I want to ask how can I find the Z-impedance ...
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### How do I correctly calculate the S-parameters of cascaded lines?

Suppose I have two coupled transmission lines cascaded. I know the S parameters of both lines. From these S parameters, I can extra SA and SB S = [SA SB; SB SA] Now, I can find S_odd = SA-SB and ...
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Below is the statement from AD application note MT-097. I have see this explanation many places. But no one explains how transmission line effects comes when rise time is smaller than 2*times ...
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### Ultrasound System Design: Navigating Transmission Line Challenges and Impedance Matching

Hello community! I'm currently working on the design of an ultrasound scanning system, and I'm seeking guidance, particularly in the realm of high frequency PCB design and transmission lines. My ...
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### Why does disconnecting the receiving end of a transmission from the sending end increase the voltage at the receiving end? [closed]

Some background information: I have a transmission line circuit (included below). The rated voltage is 63.639kV and the rated current is 325A. The characteristic impedance is 30.3 Ohms. I also have ...
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Assuming that I use a high temperature low sag line that can withstand 210°C CIGRE. These lines have a thermal limit around 20x surge impedance loading (SIL). If I use such line: Do I still need to ...
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### Matching network for a radio

I’m trying to understand how the matching network functions on the radio side of a transmission line. As an example, here’s an excerpt from the Nordic nRF52840 DK schematics. As I understand it, when ...
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### Downside/tradeoffs to clamping signals with zener diodes

I have a signal source that sends fast pulsing square waves in the MHz range (encoder pulses from a motor) down a long transmission line to a receiver. I have tried many solutions and different types ...
1 vote
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### Derivation of input admittance of a lossless T-Junction Power Divider that is matched at the input port

I'm trying to understand how the equation for the input admittance for a lossless T-Junction power divider was derived, which is only matched at the input. Let $Z_0$, $Z_1$, $Z_2$ be the ...
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### How to determine the length of a quarter wave transmission line which would be added to an existing circuit for impedance matching?

Are the values I have calculated accurate? With this setup, there seems to be a 90 degree phase difference across Tx2. I'm not sure if the 90 degree phase shift is supposed to across just Tx1 or ...
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### Classification of transmission lines

The general classification of transmission lines based on their length into short, medium and long is - Short if length < 80 km Medium if 80 km < length < 200 km Long if length > 200 km ...
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### What is the key circuitry in a form of high frequency signal generator that means power would be sent and reflected when there's an open load

Ive asked enough questions to explain why when there is an impedance mismatch there is power reflected which from my understanding is because there is a reflection of the waves, and also the cause of ...
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### How to calculate the via fence spacing between two differential CPWG?

I designed a very compact 4 layer PCB that has many PCIe 2.0 differential pairs spaced in close proximity. In addition to the internal ground planes, I want to fill the signal layers with copper, and ...
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### How to detect leak or shortcircuit in electrical wires undeground?

Recently, the circuit breaker was triggered when I activated the electricity in a cable buried in my garden. Probably water entered into a junction or I nicked the line by accident... The line is not ...
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### Reflection coefficient of a transmission line with discontinuity

I have a question, let's say that we want to calculate the Reflection Coefficient that we see by watching from left to right of the red-line this circuit: Let's say that: Za= Equivalent Impedence of ...
147 views

### Propagation speed and capacitors charging speed of transmission line

I am a bit confused on the speed of voltage propagation in the transmission line and I hope someone can enlighten me. I understand the speed of step up voltage propagation is in the same order as the ...
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### Scattering matrix of quarter-wave transformer

I am trying to solve for the scattering parameters of a quarter wave transformer. The example that I am given is a transformer with characteristic impedance 100 Ohms, and the reference impedance (...
1 vote
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### Problem understanding lossless T-Junction divider

I am studying how the lossless T-Junction divider works. I am going to assume that the susceptance in the junction is 0 for simplicity, and it is not relevant to my question. The structure of a T-...
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### Is propagation delay in coax cable a function of signal frequency?

For this coax cable, the propagation delay is given as a single parameter in 1.54 ns/ft as follows: But isn't the propagation delay function of signal frequency?
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### How can I intuitively understand input impedance?

Let's say we have a lossless transmission line with Zo impedance, terminated by a ZL = R+jX load. The question I was asked is for what input impedance the voltage would be maximum or minimum. What I ...
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