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Where does it go? (mismatched power)
On Jun 11, 12:09*pm, Roy Lewallen wrote:
Can you write an equation giving the supposed dissipation caused by "reflected power" as a function of transmission line length? I'll just quote the following from Chapter 24-11 of REFERENCE DATA FOR RADIO ENGINEERS, 1975 Edition: QUOTE A load of 0.4-j2000 ohms is fed through a length of RG-218/U cable at a frequency of 2.0 MHz. What are the input impedance and efficiency for a 24-foot length and a 124-foot length? (Omitting the number crunching shown...) Tabulating the results, Input Length Impedance Efficiency Loss (feet) (ohms) ( % ) ( dB ) 24 0.106-j95 1.1 19.6 124 1.8+j55 0.03 35 The considerably greater loss for 124 feet compared with 24 feet is because the transmission passes through a current maximum, where the loss per unit length is much higher than at the current minimum. END QUOTE As the input Z for each of these lengths is not the same as their 0.4- j2000 ohm termination, this shows the dependence of the performance of such systems on the electrical lengths of the transmission line in use. I realize we have strayed into the real world here, because RG-218/U transmission line is not lossless even when perfectly matched at both ends. RF |
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