Sec 1.4 Engineering an Antenna System. Engineering is the process of making workable compromises in design goals where theories and practical applications guiding different aspects of the design are in conflict, making it impossible to optimize all the goals. Good engineering is simply recognizing the correct choices in the compromises and relaxing the right goals, as in the spacecraft antenna design described earlier. We amateurs spend many hours building and pruning antenna systems. Wouldn't it be worthwhile spending some of that time learning how to engineer the design in order to make correct trade-off decisions among related factors instead of letting old king SWR dictate the design? First, we need to improve our knowledge of reflection mechanics and transmission-line propagation to understand:
(1) Why reflected power by itself is an unimportant factor in determining how efficiently power is being delivered to the antenna.
(2) The effect of line attenuation to discover why it is the key factor that tells us when and how much to be concerned with reflected power and when to ignore it.
(3) Why all power fed into the line, minus the amount lost in line attenuation, is absorbed in the load regardless of the mismatch at the antenna terminals.
(4) Why reflection loss (mismatch loss) is canceled by reflection gain through re-reflection obtained by the impedance matching device at the input of the line (Ref 19, Ref 2, and Ref 136).
(5) Why a low SWR reading by itself is no more a guarantee that power is being radiated efficiently than a high SWR reading guarantees it is being wasted.
(6) Why SWR is not the culprit in transmitter loading problems; why the real culprit is the change in line-input impedance resulting from the reflected power, and why we have complete control over the input impedance without necessarily being concerned with the SWR. T o o Lo w a n SW R Ca n Kill Yo u 1
(7) The importance of thinking in terms of resistive and reactive components of impedance instead of SWR alone, and why SWR by itself is ambiguous, especially from the viewpoint of the selection and adjustment of the coupling and matching circuitry of an external line-matching network.
Second, we need to become aware that with moderate lengths of low-loss coax, such as we commonly use for feed lines, loss of power because of reflected power on the HF bands can be insignificant, no matter how high the SWR. For example, if the line SWR is 3, 4, or even 5 to 1 and the line attenuation is low enough to ignore the reflected power, reducing the SWR yields no significant improvement in the radiated power because practically all the power being fed into the line is already being absorbed in the load (the antenna). This point has special significance for center-loaded mobile whip antennas, because of the extremely low attenuation of the short feed line, which is explained in detail later in this book.
Third, we should become more familiar with the universally known, predictable behavior of off-resonance antenna-terminal impedance and its correlation with SWR (Ref 2, Fig 2-7; Ref 7). This knowledge provides a scientific basis for evaluating SWR-indicator readings in determining whether the behavior of our system is normal or abnormal, instead of blindly accepting low SWR as good, or rejecting high SWR as bad. The following two examples emphasize the importance of this point by showing how easily one may be misled by a low SWR reading.
Chapter 1, Pages 5-6; Reflections III by M. Walter Maxwell
http://www.w3pga.org/Antenna Books/Reflections III.pdf
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