Microwave transmission lines are used to transmit electromagnetic energy in a controlled manner. In contrast to ordinary circuit theory where resistance (R), capacitance (C), conductance (G) and inductance (L) are represented as lumped constant elements, the R, C, G, and L of microwave transmission lines are considered distributed parameters. Hence, the microwave transmission line is a distributed element circuit. The electrical length of the microwave transmission line is a function of the physical length and the Velocity of Propagation. The principal mode of propagation in a coaxial microwave transmission line is the (TEM) Transverse Electro Magnetic mode. This means that the electromagnetic field has only radial components which include the vector electric field (E) and the vector magnetic field (H). TEM can exist in all transmission lines with two or more conductors or in free space. As the frequency increases, the wavelength will decrease. Therefore, the internal dimensions must be proportionally reduced for mode-free propagation in the TEM mode. If frequency increases and the internal radial dimensions remain constant, the next higher order mode may exist. This second mode in the coaxial line is transverse electric mode TE11. In coaxial microwave transmission lines, the TEM mode propagation is preferred because a second mode may cause resonance. A coaxial line may be used at frequencies that are slightly higher than the theoretical cutoff because the cutoff frequency does not mean that resonance will occur, it only means the possibility of resonance.

One of the first things to consider when selecting or designing a coaxial cable is determining the temperature requirements. The dielectric materials selected for the outer jacket and inner core are some of the limiting factors affecting the allowable temperature range.

Cable style (high flexibility, low flexibility or semi-rigid) should be the next determination. Some applications are able to use any of these styles. Since many flexible cables perform to the level of semi-rigid, and have a similar cost to semi-rigid, then the cost of installation should be considered.

High flexibility cables require a careful selection of materials and construction to ensure a long flex life. For low loss applications, a solid center conductor is usually preferred. However, a solid center conductor may limit flexibility and is not always the most cost effective for larger diameter cables.

Consider the cost limitations at all times when selecting a cable style or design. Overdesign of a cable may drive the cost unnecessarily high. A lower cost cable may appear to meet the requirements initially, but take care to consider the weaknesses of each individual style. For example, additional armor can be supplied over most cable assemblies to provide extra protection, however, it is costly.

In conclusion, specific requirements must be carefully considered with regard to the selection of cable and cable assemblies including but not limited to the frequency range, VSWR, insertion loss, mechanical and electrical requirements along with any environmental or application restrictions. A thoughtful and precise review of requirements will result in an optimal design.

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