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

Walter Maxwell wrote in
:

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.

If you want more details on how the resultant reflection coefficients are developed I'll be glad to furnish
it.

Walt, W2DU