Art, not to split hairs, or separate bone from marrow, but a single
reflector, well dimensioned, and well phased provides more than -20 dB
null in the reverse direction. That is less than 1% of the radiated
energy goes 'backwards' [99% goes forward]. I'm not claiming the
multi-element solution for optimum null, but for us poor hams 1% seems
quite good enough.

art wrote:
I disagree unless yoiu are specipically adressing the yagi design
which
is an explanation in terms of vectors.However an element radiates a
field
not a vector. To 'maximise' the redirection of rear field generation
requires
multi "reflectors" or a dish to capture all the rear radiation. Tho a
dish is used
for micro wave frequencies it can be simulated by multi reflectors
aranged in
parabolic form. This method is not as mechanically feasable as the Yagi
but does
illustrate the effectiveness of a "refletor" versus a "director" in
terms of "efficiency"
or "effectivenes" ala, the two element yagi..when viewed as a mesh
cuircuit assembly.
and reradiator ((reflector) longer physical length is not a
necessity.as implicated by the Yagi inline design.

Art

wrote:

chris wrote:


Why (or how) does the
longer
element "reflect" the radio wave,

A reflector does not reflect anything. It reradiates.

When a element is self-resonant it reradiates with 180 degree phase
inversion.

Spacing from the driven element causes a phase delay. Lets say that
spacing is 90 degrees.


so, on many multi-element Yagi designs, the norm seems to be just one

reflector, and many director elements. Does this mean that the
reflector is more "effective" than the directors at modifying the
radiation field?

No.

Once something removes energy form the rear, there is no more energy to
excite and further elements. You can't excite additional reflectors
because there is no energy there to excite them, and so they become
useless hunks of metal without much current.


73 Tom

I read the question as referring to total radiation and not a specific
angle that you referred to.
With respect to a single reflector it is obviouslythe most significant
addition to a dipole (efficiency
addition if you like)and additional elements added follow a declining
level of addition ofredirected energy ( binomial) which I took as his
question. The other questions he raised were follow ups on the yagi
design which fits nicely to the vector style explanation without going
to deep into true antenna radiation thus raising the question of
reflector "length" It really is the same as explaning
"skin depth" without reference toi "volume" or capacitance that does
not refer to "skin depth",.
Most who ask a question may well accept "thats the way it is" but the
gentleman asked the question with respect to reflector length because
of yagi type explanations, which does not "fit"
with all arrays which are actually meshed cuircuits and more difficult
to address.
I have no quarrel with the choice of a two element antenna in the real
world because as Moxon states it is simpler and more rewarding to raise
a two element antenna than to make a three element antenna BUT
it depends on the end parameters that you require which if not stated
lead to perennial radio antenna arguements which plague forums.
Nuff said Art

I read the question as referring to total radiation and not a specific
angle that you referred to.
With respect to a single reflector it is obviouslythe most significant
addition to a dipole (efficiency
addition if you like)and additional elements added follow a declining
level of addition ofredirected energy ( binomial) which I took as his
question. The other questions he raised were follow ups on the yagi
design which fits nicely to the vector style explanation without going
to deep into true antenna radiation thus raising the question of
reflector "length" It really is the same as explaning
"skin depth" without reference toi "volume" or capacitance that does
not refer to "skin depth",.
Most who ask a question may well accept "thats the way it is" but the
gentleman asked the question with respect to reflector length because
of yagi type explanations, which does not "fit"
with all arrays which are actually meshed cuircuits and more difficult
to address.
I have no quarrel with the choice of a two element antenna in the real
world because as Moxon states it is simpler and more rewarding to raise
a two element antenna than to make a three element antenna BUT
it depends on the end parameters that you require which if not stated
lead to perennial radio antenna arguements which plague forums.
Nuff said Art