[TenTec] "A low vswr will kill you"

[Sherrill WATKINS]

While I must agree with Dr. Johnson's statement that the mathematical definition of a resonate antenna pertains to the phase angle of the voltages and currents; I find this mathematical definition too abstract to understand for my limited education (feeble mind).  Since most hams, including me, have never understood higher math, I prefer to visualize what is happening by more simple concepts as graphical current and voltage waves. Since most ham's do not have discs or similar items at the end of their antennas , the graphical concept of current node at the ends and current loop at the center is quite valid and helpful to understand what is happening on a resonate antenna system.  As I stated in my previous post, in a resonant condition, a center fed wire antenna (with out discs, etc. the ends) is accompanied by a current loop or maximum at the center and zero current or a node at the ends.  (The voltage wave is just the opposite with voltage at a loop or maximum at the ends and zero at the center; i.e. the voltage wave is 90 degrees from the current wave.)  When the antenna is operated in the non-resonant condition, the current loop moves or shifts to other positions on the antenna, along with changes in the position of the voltage wave.  This may or may not be good or bad.  As an example, should a 160 or 80 meter center fed zep be operated on say, 15 or 10 meters, then because of the multiple current loops on each leg of the antenna,  the antenna will begin to act like two end fed, back to back long wires and will exhibit directivity and gain off the end of each wire. ( At this point, their may be a high vswr on the transmission line.)  This shift in the radiation pattern can be put to good use if one desires to work certain parts of the world where the radiation pattern is directed.   However, less I digress, other than possibly changing the angle of radiation of the antenna when it is operated in a non-resonant or harmonic condition,  ALL THE RF POWER DELIVERED TO THE ANTENNA TERMINALS IS RADIATED WHEN THE ANT
resonance does NOT have to be accomplished at the actual antenna itself but can be easily accomplished with a suitable tuner at the operating postion.  For an in-depth and thorough technical explanation of this fact, please read the articles that appeared in QST by Mr. Walter Maxwell,  W2DU, from about 1974 to 1977, entitled " Another Look at Reflections" or the book by the same name.  Now for another important point: If one is using a well designed antenna tuner, the fact that one may have a very high vswr on the transmission line is only important if one is using a high loss type transmission line, i.e. coax.  If very low loss line is used, such as open wire type, then a very high vswr (i.e. 20:1) is not significant, provided the tuner and transmission line are designed to handle it.  This is because the tuner, when tuned to resonance, will take all of the reflected power coming back down the transmission line toward the tuner, and reverse its direction and add it, IN PHASE, to the incident power going back up the transmission line to the antenna.  When it reaches the antenna, it flows out and along the antenna and is all radiated.   It is a basic fact that when the SYSTEM is resonant, regardless if the actual antenna is resonate or not , ALL the r.f. power is radiated by the antenna. (I am talking theory here and not anomalies such as loose or corroded connections, etc. that may absorb power and give a false low vswr reading.)  This can be confirmed by the use of forward and reverse reading watt meters inserted in the transmisson line between the tuner and the antenna. - 73's - Corn -k4own  


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