What is Coaxial cable: Definition and 99 Discussions

Coaxial cable, or coax (pronounced ) is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric (insulating material); many coaxial cables also have a protective outer sheath or jacket. The term "coaxial" refers to the inner conductor and the outer shield sharing a geometric axis.
Coaxial cable is a type of transmission line, used to carry high-frequency electrical signals with low losses. It is used in such applications as telephone trunklines, broadband internet networking cables, high-speed computer data busses, cable television signals, and connecting radio transmitters and receivers to their antennas. It differs from other shielded cables because the dimensions of the cable and connectors are controlled to give a precise, constant conductor spacing, which is needed for it to function efficiently as a transmission line.
Coaxial cable was used in the first (1858) and following transatlantic cable installations, but its theory wasn't described until 1880 by English physicist, engineer, and mathematician Oliver Heaviside, who patented the design in that year (British patent No. 1,407).

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  1. G

    Power dissipated by a resistor on a coaxial cable

    I need help with part c. My solution: Is there an other way to do this other than dimensional analysis? P.S "dr an infinitesimal radius", it ofcourse should be dz.
  2. Kyuubi

    Speed of Current in a coaxial cable

    I've found the inductance and capacitance per unit length in a long coaxial cable. I even clearly see that if I multiply the two, I can get the speed of light. How do I begin to find the current wave and its speed though?
  3. A

    I Derivation of the magnetic flux in coaxial cable

    The magnetic flux ##\phi_m = \int{BdA}## The magnetic field of the coaxial cable B = ##\frac{I_{enc} \mu_0}{2\pi r}## since surface area of a cylinder = ##2\pi rdr L, dA = 2\pi L dr## where L is the length of the coaxial cable so ##\phi_m = \int{\frac{I_{enc} \mu_0}{2\pi r}2\pi L dr}##?
  4. H

    Magnetic energy inside a coaxial cable

    Hi, I have to find the magnetic energy inside a coaxial cable of inner radius ##a## and outer radium ##b##, ##I = I## By using Ampere's law if ##r<a## ##B = \frac{\mu_0Ir}{2\pi a^2}## if ##a<r<b## ##B = \frac{\mu_0I}{2\pi r}## if ##r>b## ##B = 0## Then, the energy in a magnetic field ##E_b...
  5. C

    Surface Charges on a Coaxial Cable

    To find ##\sigma_b## I can use a Gaussian surface of a cylinder of length ##L## and radius ##c>r>b##. Since that is inside of the outer conductor, I know the electric field is zero, so I have from Gauss' Law, $$0=2 \pi L\left(b\sigma_b+a\sigma\right)$$ and easily solve for ##\sigma_b##. For...
  6. E

    Characteristic Impedance of a coaxial cable

    Image 1: Image 2: I am attempting to learn about transmission lines and am having problems with this homework problem. For part a, I have derived an equation for the maximum electric field within the dielectric. I came up with: with r being the radius and the electric field decreasing with...
  7. adamaero

    Electric energy density in the dielectric of a coaxial cable

    V(ρ) = V_o*ln(ρ/0.0018)/ln(45/180) (Attached picture is where the unit vector of r is really ρ.) In cylindrical coordinates ∇V = ρ*dV/dρ + 0 + 0 ∇V =derivative[V_o*ln(ρ/0.0018)/1.386]dρ ∇V = V_o*0.0018/(1.386*ρ) E = V_o*0.0012987/ρ Work = 0.5∫∫∫εE•E dv Bounds: 0.0018 to 0.00045 m D = εE =...
  8. B

    Calculating Resistance of Coaxial Cable with Isolator

    Homework Statement Coaxial cable has radius a of copper core and radius b of copper shield. Between there is an isolator with specific resistance ζ. What is the resistance of this cable with length L between the core and the shield? Homework Equations First, I tried to solve this like this...
  9. Ignitia

    Ampere's Law with a coaxial cable

    Homework Statement A portion of a long, cylindrical coaxial cable is shown in the accompanying figure. A current I flows down the center conductor, and this current is returned in the outer conductor. Determine the magnetic field in the regions (a) R ≤ r1, (b) r2 ≥ R ≥ r1, (c) r3 ≥ R ≥ r2, and...
  10. B

    Refractive Index of Coaxial Cable

    Anyone know the refractive index of a standard coaxial cable and have a traceable source?
  11. A

    The magnetic field outside a coaxial cable

    Good day All, I have a confusion in my mind and i hope to clear it If we use the Amper law to calculate the magnetic field outside the cable, the value would be 0 because the electric current inside the amperian loop is equal to 0, but in case we apply the Biot savart law to calculate the...
  12. T

    What is the difference between PP(polypropylene) and CPP?

    To my knowledge, CPP means cast-polypropylene, which I've seen used in a coaxial cable. Some cables use CPP as dielectric material. My question is: 1. what is the difference between PP(polypropylene) and CPP? 2. How is CPP made? 3. Is the permittivity and loss tangent of CPP different from that...
  13. S

    Stray/Parasitic Capacitance - Impacts phase velocity?

    Hey everyone, I'm trying to measure the stray capacitance in a circuit comprised of an rf signal generator, an oscilloscope, a coaxial cable (short-circuited) and some capacitors. I measured the resonant frequencies of the coaxil cable for varying values of capacity (0pF to 850pF) between 4MHz...
  14. Marcus95

    Power Transmitted in a Coaxial Cable

    Homework Statement A coaxial transmission line consists of an inner cylindrical conductor of radius a = 1 mm and a cylindrical outer conductor chosen to make the characteristic impedance 75 ohm. The space between the conductors is lled with a gas which can stand a maximum eld of 105 V/m without...
  15. G

    Energy Flow in Coaxial Cable with Linear Free Charge Density

    Homework Statement An infinitely long cylindrical capacitor with inner radius a and outer radius b carries a free charge per unit length of ##\lambda_{free}##. The region between the plates is filled with a nonmagnetic dielectric of conductivity ##\sigma##. Show that at every point inside the...
  16. G

    Coaxial cable conductors with dielectric: polarization of charge

    Homework Statement Consider a coaxial cable which consists of an inner cylindrical conductor of radius R1, and a shell cylindrical conductor of radii R2 and R3. The 2 conductors are separated with a dielectric material of permittivity ε. Consider the length of the cable, ℓ, much larger than R3...
  17. T

    Displacement current in coaxial cables

    To calculate the displacement current in a coaxial cable (with equal and opposite currents on the inner and outer conductors), most standard texts use the magnetoquasistatic approximation, which ignores the time-varying electric field term in Ampere’s Law. Using this approximation, the...
  18. Abdul Wali

    Regarding Fiber Optics and Coaxial Cable

    Hi, "limitations/Constraints of Fiber Optics and Coaxial cable for average home user ?"
  19. J

    Gauss' Law for a coaxial cable

    Homework Statement I have attached the problem Homework Equations E*A = Qenc/ E0 The Attempt at a Solution At the moment I am looking at the problem more conceptually and seeing what is happening at each point and I wanted to know If I was on the right track. r<a As all charge would...
  20. Drakkith

    Magnetic Flux in a Coaxial Cable

    Let's say we have a coaxial cable with a 2d rectangular surface lying between the inner and outer conductors and running the length of the cable. I'm trying to understand why the magnetic flux through this surface only includes the magnetic field generated by the current flowing through the...
  21. O

    Electric field in coaxial cable

    Every physics books show that wave prograpation in coxial cable is TEM wave, like in the picture. But we know that J=σ E from ohms law, which says current in same direction with E field, which is not the case here. What do you think the reason is ?
  22. G

    Is current cancellation within coaxial cable always true?

    Hello. I'm using coaxial cable to transfer power to the load with original aim that radiation from the coax would be low or almost nothing as current on both conductor (central conductor and outer shield layer - ground) are equal in amplitude and opposite in phase so their sum is zero; zero...
  23. PrincessYams

    What is the electric field inside a coaxial cable with given charge densities?

    Homework Statement A coaxial cable consists of two concentric cylindrical conductors as shown here: http://puu.sh/luFZx/7f3e1ccb07.png . The inner cylinder has a linear charge density of +λ1, meaning that each meter of the cable will have λ C of charge on the inner cylinder. The outer cylinder...
  24. B

    Electric field of a coaxial cable

    We have a coaxial cable with inner radius a and outer radius b. The coaxial cable is modeled as two very long circular metal cylinders. I'm supposed to calculate the electric field E, the electric potential V and the charge enclosed Q when a voltage is applied between the metal cylinder...
  25. Cedric Eveleigh

    Calculating current distribution in a HF coaxial cable?

    Hello, I would like to calculate the current distribution in a coaxial cable where the skin effect is significant. I asked this question on stackexchange and I provided pictures and more details there...
  26. B

    Coaxial cable and Potential between

    Hello. I have DC going through a coaxial cable, and I have calculated the E fields of the two dielectrics in between to be E_1 and E_2 with help of their D-vectors. The dielectrics are cylindrically shaped like the conductors. As in, one is in contact with the inner conductor, and one is in...
  27. E

    Electric field in a resistive, shorted coaxial cable

    Homework Statement Consider a shorted coaxial line with a resistive inner conductor with radius a, and a perfect outer conductor located at radius b. A DC voltage is applied at the input end. Find the electric and magnetic fields in the dielectric region (a<r<b), assuming the battery sets up a...
  28. F

    What is the behavior of currents on a coaxial cable?

    Hello Forum, A coaxial cable is a two conductor transmission line. If we connect a DC voltage source via a coaxial cable to a resistor so that the positive terminal is connected to the inner conductor and the negative terminal to the shield , the current will flow through the whole...
  29. G

    Capacitance per length unit of coaxial cable

    Homework Statement Coaxial cable has double layered dielectric. a=2mm, b=4mm, c=8mm, εr1=3, εr2=4.5, Uab=1000V 1. Calculate capacitance per unit length of a cable. 2. Calculate inner and outer charge densities of a cable. 2. Homework Equations Gauss law C'=Q'/U σ = nD σp = nP 3. The Attempt...
  30. C

    Maximum voltage on coaxial cable?

    Homework Statement How to determine maximum voltage that coaxial cable, whose height is L and is filled with dielectric in such way that E is the same in the dielectric and in the part of the cable that isn't filled with dielectric (only vacuum),can handle if i have maximum electric field that...
  31. R

    Finding a 1.5 GHz Line Amplifier for RG59 Coaxial Cable Loss

    Hello. I 'm looking for a line amplifier schematics working around 1.5 GHz to compensate coaxial ( RG59/CU) loss behind a L.N.A
  32. 1

    Electric Field and Potential Difference in a Coaxial Cable

    Homework Statement : [/B] Co-axial cable, relative permittivity, capacitance, internal energy A long straight co-axial cable of length 1 consists of an inner conductor of radius r1 and a thin outer conductor or radius r2. The dielectric between the conductors has a relative permittivity εr...
  33. N

    Magnetic field for coaxial cable

    Homework Statement https://scontent-b.xx.fbcdn.net/hphotos-xpa1/v/t1.0-9/10846336_1517298108546407_4211362504714893270_n.jpg?oh=2f11c165933554a0e4003b398c877004&oe=5542789C Homework Equations ∫B⋅nda = Iencμo Ienc=∫J⋅da The Attempt at a Solution For r < a: Ienc = I...
  34. P

    What causes the magnetic field inside a coaxial cable?

    Hi. In this task we're asked to calculate the magnetic field inside the small cylinder (0>r>r1). The answer is zero, which I don't quite understand...Here is the picture: http://bildr.no/image/WTJ1VG5F.jpeg As the picture shows there's a current I going through the inner cylinder. Wouldn't this...
  35. B

    Resonant frequency of a coaxial cable

    What is the equation (and derivation of equation) to work out the resonant frequency of a coaxial cable? Given that the cable has a resistance, capacitance and inductance per meter which are known. Links and explanations are much appreciated Thanks :smile:
  36. T

    Power transported down a coaxial cable

    Homework Statement A long coaxial cable carries current I. The current flows down the surface of the inner cylinder (of radius a) and back along the outer cylinder (of radius b). Calculate the power transported down the wire. Homework Equations The Poynting vector is given by...
  37. R

    Potential Difference between conductors for a coaxial cable, conflict

    conflicting results. find the potential between the two conductors of a coaxial cable I have already found E... E = lambda / 2 pi r epsilon my question: What are the integration limits for finding the potential between the conductors? say we have a radius a (inner conductor)...
  38. Y

    Insulation Resistance in a coaxial cable of length L

    Homework Statement Determine the insulation resistance in a coaxial cable of length L, with conductivity, as shown in Figure assume that the current density J is radial direction. The Attempt at a Solution so i was wondering if this approach is correct, consider a sequence of...
  39. X

    Length of coaxial cable, based on signal reflection

    Homework Statement We measured the time between a signal source, and it's reflection coming back through our probe after going through an open-ended coaxial cable. My teacher told us this: the cable has a polyethylene insulator between central wire and the grounding web, which has a...
  40. L

    Flux density inside and outside a coaxial cable.

    For question 4b in the problem sheet attached below, is my working correct for B outside and inside the coaxial cable? Ans: Outside coaxial:∫B.dl = μI (the integral is between 0 and 2∏r) => B = (μI)/(2∏r) Inside the coaxial: \int_{2\pi a}^{2\pi r} B.dI = μI.\frac{a^2}{\pi...
  41. U

    What is the inductance of a length l of a coaxial cable?

    A coaxial cable is modeled as a long thin cylindrical shell with radius b concentric with a solid wire of radius a (the wire is inside the shell). Calculate the inductance of a length l of this cable. (Example 32.5 Here) ---> https://echsphysics.wikispaces.com/file/view/APPhysicsCH32.pdf...
  42. N

    Calculate Coaxial Cable Loss & Input Impedance at 1 MHz

    Hey Guys, How do I work out Transmission loses and input impedance for coaxial cable? I’ve been given this website to use http://fermi.la.asu.edu/w9cf/tran/ I do have the characteristic impedance of 75Ω and frequency and I am asked to work the attenuation in dB/100ft and “input...
  43. R

    Resistance of the insulation in coaxial cable

    Homework Statement Determine the resistance of the insulation in a length l of coaxial cable.inner radius = a and outer radius= b.Homework Equations R=\rho\frac{l}{A} Area= \pi (b^2 - a^2) The Attempt at a Solution I calculated the area between the two concentric 'circle' with radius 'a'...
  44. W

    Coaxial Cable Problem - Finding volume charge density.

    1. Homework Statement A long coaxial cable consists of an inner solid cylinder, radius a, and an outer thin coaxial cylindrical shell, radius b. The outer shell carries a uniform surface charge density σ. Find the uniform volume charge density ρ that the inner cylinder must have in order...
  45. PsychonautQQ

    Why do two opposing currents cancel each other out perfectly in Ampere's Law?

    Homework Statement http://grephysics.net/ans/9277/9 I'm confused why these two currents cancel each other out perfectly when the distance they are being measured from is closer to the outside current than the inside current... Like if the magnetic induction is proportional to the...
  46. R

    Testing Book for Coaxial cable problem in Physics GRE

    Book for Coaxial cable impedance problem in Physics GRE This is GR9277, Problem 18. Is this covered in any physics undergraduate course? What subject is this problem belong to? What book do you recommend for studying this? How can I make the picture appear bigger?
  47. A

    Inductance of Solenoid on Coaxial Cable

    Homework Statement The figure below shows a short coil which is coaxial with a very long solenoid (you may approximate it as infinite.) Coil: Has 120 turns of radius 1.8 cm and resistance 5.8 . It is not attached to anything. Solenoid: Has 22700 turns/m and a radius of 1.6 cm. It receives an...
  48. E

    Magnetic field of a Coaxial Cable

    Homework Statement A long coaxial cable consists of two concentric conductors with dimensions a = 1.20 cm, b = 5.40 cm and c = 7.30 cm. There are equal and opposite currents of 5.00 A in the conductors. Find the magnitude of the magnetic field at a distance of 0.36 cm from the center axis...