Calculation of wave resistance is very important in the radio technician and the electronic engineer. Finding of the correct value of this size helps to determine the range of the maximum distance of signal transmission and prompts as far as it should be strengthened for obtaining the best quality of reception.
What is the wave resistance?
Any mass media transmit a signal to long distances by means of electromagnetic waves. One of properties of such wave also is wave resistance. Though characteristic units of measure of resistance - Ohms, it not the "real" resistance which can be measured by means of the special equipment, such as ohmmeter or multimeter.
The best way to understand that such wave resistance is to imagine an infinitely long wire which does not create the reflected or return waves at loading. Creation of alternating voltage (V) in such chain will lead to emergence of current (I). Wave resistance (Z) in this case will be in number equal to a ratio: Z = V/I
This formula is fair for a vacuum. But if it is about "real space" where there is no infinitely long wire, the equation takes a form of Ohm's law for subcircuit: R = V/I
Equivalent scheme of calculation of the line of transfers
For the microwave oven of engineers the general expression determining wave resistance is: Z = R+j*w*L/G+j*w*CZdes R, G, L and With – nominal lengths of waves of model of the line of transfers. It should be noted that in a general view wave resistance can be complex number. Important specification is that such case is possible only if R or G are not equal to zero. In practice always try to reach the minimum losses on a signal transmission line. Therefore usually ignore a contribution of R and G to the equation and, finally, quantitative value of wave resistance accepts very small value.
Internal resistance
Wave resistance is present even if there is no transmission line. It is connected with distribution of waves in any homogeneous environment. Internal resistance is a measure of the relation of electric field to magnetic. It pays off as well as in transmission lines. Assuming that there is no "real" conductivity or resistance in the environment, the equation comes down to a simple square form: Z = SQRT(L/C) In this case inductance per unit length comes down to permeability of the environment, and capacity per unit length – to dielectric permeability.
Vacuum resistance
In space the relative permeability of the environment and dielectric permeability are always constant. Thus, the equation of internal resistance becomes simpler to the equation for the wave resistance of a vacuum: n = (SQRTm/e) Here of m is permeability of a vacuum, and e – dielectric permeability of the environment. The value of wave resistance of a vacuum is a constant and approximately equally the 120th peak.