Corn, I'm not a mathematician or engineer, but the math involved here is
nothing harder than simple high school trig and algebra and everything you
said below could have be said using the math in a couple of lines!!
However, you do show a talent as a writer and I enjoy reading your comments
when you aren't flaming someone.....keep up the good work!
73.....k6sdw
-----Original Message-----
From: Sherrill WATKINS [mailto:SEWATKINS@dgs.state.va.us]
Sent: Thursday, January 25, 2001 9:27 AM
To: tentec@contesting.com
Subject: [TenTec] "A low vswr will kill you"
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 TH!
E 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 PHAS!
E, 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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