Ian White GM3SEK wrote:
Cecil Moore wrote:
I'm not reclassifying anything. The differences between traveling-wave
antennas and standing-wave antennas have been known for many decades.
Oh good! Exactly where do *you* draw the line between them; and why?
Please justify this by giving examples of two antennas that are very
close to your chosen line, but on opposite sides.
Glad to oblige. The two classical examples are a 1/2WL dipole
vs a terminated rhombic. The differences are obvious. The ends
of the standing-wave 1/2WL dipole are open-circuited so forward
waves undergo a total reflection. Ideally, the traveling-wave
rhombic is terminated in its characteristic impedance so
reflections are eliminated.
The equation for the current in a 1/2WL dipole is roughly
proportional to cos(x)*cos(wt). The equation for the current in
an ideal rhombic is proportional to cos(x+wt) where w=2*Pi*F.
For anyone with a math background, those differences are more
than obvious and I pointed that out years ago.
Then please justify the difference between your two different
classifications of current.
I don't have to justify that, Ian. Mathematics automatically
justifies it for me. If you would simply take the time to understand
the difference between cos(x)*cos(wt) and cos(x+wt), you would
understand it also.
The current in an ideal rhombic is 100% forward current proportional
to cos(x+wt). The current in a 1/2WL dipole is the sum of two
currents. The forward current is roughly proportional to cos(x+wt)
just as it is in the rhombic. The reflected current is roughly
proportional to cos(x-wt) and when those two traveling-wave currents
are added the resultant standing-wave current is proportional to
cos(x)*cos(wt), a completely different kind of current as is obvious
from their different equations.
The purists may take me to task for using a cosine function instead
of a sine function or using '+' for forward waves and '-' for
reflected waves but it doesn't change the conceptual conclusion.
Unfortunately, there is a difference in sign conventions between
optics and RF. Such mundane differences do not change the concepts
involved.
--
73, Cecil
http://www.w5dxp.com