this page you will find some informations about some coaxial cables which may be usefull for you.GeneralitiesProperties of dielectricals used in coaxial cablesProperties of some coaxial cablesLinks to some coaxial manufacturers
Velocity of PropagationWhen an electromagnetic wave travels in a medium other than air or vacuum, the velocity for the wave is reduced by a factor of the square root of the dielectric constant of the media. The velocity (v) of the propagation of a signal is given by the formula :
Where c is the speed of light, 3 x 108 m/sec or 1.18 x 1010 in/sec, and is the dielectric constant of the medium.The wavelength of a signal is given by the formula:
A wave loses energy (attenuates) in several ways: 1.The resistance of the inner and outer conductors is small but can be significant over long lengths and will produce some heat.2.The dielectric may cause loss; it's resistance is high but not infinite, and some energy is lost. 3.Electromagnetic energy radiates at high frequencies; significant energy losses are caused by radiation of electromagnetic energy (the cable acts like an antenna).4.Energy is reflected due to impedance mismatches or impedance discontinuities. The combination of these four types of losses are referred to as the insertion loss of a transmission line system.
A parameter which defines the behavior of a cable, connector, or any propagating system.The characteristic impedance Zo of a lossless cable is related to the inductance per unit length, L, and the capacitance per unit length, C, as follows:
The equivalent circuit of a transmission line is shown in Figure 4. R represents the conductor resistance for a unit length.
For a coaxial cable the characteristic impedance is given by:
where "D" is the inner diameter of the outer conductor and "d" is the outer diameter of the inner conductor, respectively. Similar equations apply for other geometries such as two parallel wires. As can be observed from this equation, the impedance is a function of the diameters. Generally the conductor diameter can be very accurately controlled, but the dielectric diameter can vary based on the accuracy of the process. If the impedance changes are a consistent spacing of one 1/4 wavelength, this can cause signifigant signal loss.
When the characteristic impedance changes in a transmission line system, part of an incident wave is reflected. The reflection coefficient can be calculated as:Where Vi and Zo are the incident voltage and impedance of the first media. VR and ZR represent the reflected voltage and impedance of the media that caused the reflection. The decibel loss due to reflection is given by:
The traditional way to determine the reflection coefficient is to measure the standing wave caused by the superposition of the incident wave and the reflected wave. Traditionally the voltage is measured at a series of points using a slotted line. The ratio of the maximum divided by the minimum is the Voltage Standing Wave Ratio (VSWR). The VSWR is infinite for total reflections because the minimum voltage is zero. If no reflection occurs the VSWR is 1.0. VSWR and reflection coefficient are related as follows: Present instrumentation measures the return loss. Figure 5 represents the relationship between VSWR and its equivalent in return loss (expressed in dB).
If there is a series of impedance changes, each one will cause a reflection. The total reflection is the vector addition of each of the individual coefficients accounting for the distance between discontinuities. Even though the calculations are difficult, a total VSWR can still be measured.