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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