Walter Maxwell wrote in
:

On Sat, 07 Apr 2007 05:03:51 GMT, Owen Duffy wrote:

Walter Maxwell wrote in
m:

On Fri, 06 Apr 2007 23:03:42 GMT, Cecil Moore
wrote:

MRW wrote:
Any comments? Really, what I'm trying to understand here is: if
constructive interference does any good in radiowave propagation.
I was thinking that with an increase in amplitude the signal would
be able to travel a little further, but the signal received may
not be accurate in terms of the information it is conveying.

Antenna gain over isotropic is an application of
constructive interference. The constructive
interference must be balanced by an equal amount
of destructive interference elsewhere to avoid
violating the conservation of energy principle.

This is what I've been trying to persuade the 'anti's' that whenthe
radiation fields from two vertical dipoles superpose at some point
in space, where their magnitudes are equal and are 180° out of
phase, the wave cancellation resulting from destructive interference
produces a null in a predetermined direction, and thus prevents
those fields from propagating any further in that direction. At the
precise instant the null is produced, the constructive interference
following the principle of energy conservation yields an increase in
the field strength in directions away from the null direction. This
explains the concept of antenna-pattern modification, and
contradicts the notion that the two fields just plow through each
other with no effect on either.


Walt, this seems inconsistent with the approach that I believe you
seem to use in analysing waves in transmission lines where you seem to
want to not only deal with the forward and reverse waves separately
(ie to not collapse them to a resultant V/I ratio at a point), but to
deal with multiply reflected waves travelling in the forward and
reverse direction (which is only necessary in the transient state).

Owen

Owen, it appears that you've misinterpreted my approach. In developing
a condition for impedance matching, such as adding a series or shunt
stub at the proper place on a transmission line, the object has always
been to generate a new reflection at the stub point of the opposite
phase to that appearing on the line at the stub point. Thus when the
stub reflection and the load reflection superpose at the stub point,
the resulting reflection coefficients of voltage and current form
either a virtual open circuit or a virtual short circuit. These
conditions are produced because when the load impedance is greater
than Zo, the resultant reflection coefficient angles at the stub point
are 0° for voltage and 180° for current, establishing a virtual open
circuit at the stub point to rearward traveling waves. When the load
impedance is less than Zo, the resultant reflection coefficient angles
are 180° for voltage and 0° for current, establishing a virtual short
circuit at the stub point for rearward traveling waves.

Hi Walt,

I read the above, and I think I can see what you are getting at, however
I think it is flawed.

If you were to try to extend this method to explain the common two stub
tuner (where the length of the stubs is adjustable and the distance
between them is fixed), you will have to deal with a situation where the
load end stub junction does not present the "virtual o/c or s/c" you
describe, your "total re-reflector concept" and you come to need to
calculate the situation on the source side of the load end stub (possibly
by conventional methods?).

Walk your explanation around a Smith chart, and explain why, if the
principles on which your explanation are based are correct, why energy
fills a 3/4 wave hi Q coaxial resonator rather than being blocked by the
virtual s/c or o/c at the first voltage minimum or current minimum.

Someone else persuing the theme that reflected waves always travel all
the way back to the source, seems to come to a position that some kinds
of matching produce a complementary reflected wave, and that really there
are two (or more) reflected waves, its just that they have zero net
energy. Some of us would accept that if the resultant is zero, there is
no wave. Otherwise, you would see a multitude of net-zero waves all
around us to complicate every analysis.

These "new" and alternative explanations are questionable and don't seem
better than the conventional explanations of a transmission line that are
set out in just about any reputable transmission lines text. What
advantages do these explanation have, who are they targeted at? Is the
"total re-reflector" concept to appeal to a dumbed down audience who can
get their mind around a bunch of words that describe specific situations
in a simple and appealing way, but an incorrect explanation nonetheless?

I think it is a real challenge to teach people a simple explanation of
what happens without telling them convenient lies that have to be
unlearned to develop further. The "reflected wave is (always) dissipated
in the PA as heat" is an example of one of those convenient lies.

Owen