Folks,

I've got a question concerning Yagi antenna's: Why (or how) does the
longer
element "reflect" the radio wave, and the shorter element "direct" the

radio wave.

Also, 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?
Why are there no designs with just one director, and multiple
reflectors? Why does the shorter element apparently affect the field
more than the longer elements?

I've seen some interesting 3-D graphics of field strength
renderings for antenna's on the Internet, and some software can produce
some
nice looking 2-D graphics to help visualize the H and E fields. Does
anyone know of a site that let's one vary the element lengths, and
watch the effect on the 3-D graphic of the field strength vectors as
the element
lengths are varied?

Cheers!
Chris AI4MI

ORIGINAL MESSAGE:

chris wrote:

Also, 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?


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In a sense, yes it is more effective. Think about it this way: If the
reflector does its job, there is no more energy behind it to be
reflected any further. Just the opposite with a director. The more it
directs the energy, the more there is to be further directed.

When yagis first became popular there were designs published with two
or more reflectors. People soon realized the second one added little or
nothing and they disappeared.

I suppose if you want really exceptional F/B ratio you could use a
second reflector, but most people don't bother. One is enough.

Bill, W6WRT

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.

The phase to the rear direction would be -90+180=+90 degrees. Two
elements like this, when phased and in-line, fire in the direction of
lagging current. So the antenna fires towards the driven element.

and the shorter element "direct" the
radio wave.

The shorter element doesn't direct. It reradiates energy. When we
shorten an element and excite it with an external field, the current is
advanced some amount in phase besides having the 180 inversion caused
by reradiating an external field. Let's say we shorten it enough that
the phase advances 120 degrees from the shortening, and we have 45
degree spacing.

Now we have -45 (spacing delay) plus 180 flip (reradiation) = +135
degrees. To that we add another +120 because the element is short and
capacitive. That's +255 degrees. +255 is the same as -105 degrees, and
again since the elements fire in the direction of lagging currents the
driven element's energy is reenforced in the direction of the short
element.

Sorry you asked? Well, that's how it works.

Also, 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.

Why are there no designs with just one director, and multiple
reflectors?

Because it doesn't do anything when you put an element in an area where
there is no field to excite it.

Why does the shorter element apparently affect the field
more than the longer elements?

They don't. They just happen to be where energy has been concentrated,
and as long as they are being excited they can help shape the pattern.

73 Tom

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

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

Tom, W8JI wrote:
"A reflector does not reflect anything. It radiates."

Call a parasitic element anything you like, but the convention has
already set in.

Kraus tells the Yagi-Uda story on page 246 of his 3rd edition of
"Antennas". He writes:
"He (Uda) found the highest gain with the reflector about lambda/2 in
length (they must be near resonance to get excited properly) and spaced
about lambda/4 from the driven element, while the best director lengths
were about 10% less than lambda/3."

Uda`s reports were published between March 1926 and July 1929. There has
been much fine tuning since then.

On page 245, Kraus writes:
"When the parasitic element is inductive (longer than its resonant
length) it acts as a reflector. When it is capacitive (shorter than its
resonant length) it acts as a director."

Shortwave broadcast station I worked in about a 1/2 century ago used
parasitic arrays of horizontal antennas. They were called "curtains". We
did the adjustments of reflector phasings near the earth. The reflectors
had feedlines like the driven elements, but were connected to
short-circuit stubs instead of a transmitter. The shorting bar location
was adjusted for the proper phase lag behind the driven element. It`s
easier than trimming the reflector.

We hung sampling loops from the driven element and reflector and fed
them to an RCA WM-30-A phase monitor exactly as were used in medium wave
broadcast stations for maintenance of directional arrays.

You could have used such a phase monitor to check the phase difference
introduced by a mobile loading coil. It is an oscilloscope fitted with a
precision phase shifter which identifies which of the 4 quadrants the
phase difference falls in and the number of degrees.

The parasitic reflector performs the function of reversing the direction
of much of the energy traveling toward it. Someone in this thread said
it can be 99% effective. I also recall reading somewhere that if you are
constructing a 2-element parasitic array, you`ll get more gain from a
director than from a reflector. Our broadcast plant was behind our
reflectors so it made sense to protect it in spite of perhaps a slight
gain penalty.

Best regards, Richaed Harrison, KB5WZI

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.

A yagi works exactly as I described. It is nothing more than a
parasitically excited end-fire phased array. The beam forming mechanism
in a Yagi is nothing even remotely similar to the beam forming in a
wide area array like a dish or a broadside-collinear array.

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.

Not true.

The gain in a dish comes from the wide area of surface that is excited
in phase. The dish surface looks like multiple dipoles all excited in
exactly the same phase. Gain is not high because a reflector is "more
effective", it is high because a wide area of radiation (multiple
wavelengths wide) can be used to focus the forward beam.

This is why USIA Curtains for SW broadcast have substantial gain, as do
bedspring arrays at VHF and UHF. Dishes are much more closely related
to broadside-endfire arrays than any other antenna, and work on very
different principles than a Yagi.

The Yagi relates closely to an end-fire array, and that includes the
reflector.

This is why you do not see any yagis with multiple in line reflectors
and very few with trigional or sheet relectors, and why you do not see
dishes with directors. The workings
are entirely different.

73 Tom

Tom
I read as far as the word "tho a"
and you made my day, you confirmed what I suspected that all
antennas are based around yagis and not about antennas in general which
is exactly the point I made earlier.But your explanation tho correct
for a yagi is not all encompasing.
If you contend that your explanation over rules the mesh or field
aproach i.e a reflector must always be a longer radiator completely
avoids the essence of the first question i.e long versus shorter
reflectors. When aproached from a meshed
circuit point of view it can be seen that coupling of radiators behind
the dipole generator can easily produce an element(s) of a shorter
length.since as you stated a reflrctor does not reflect
but creates a reactive field in conjuction with other elements in
the immediate field to satisfy Newton's law
The rest of your reply I consider irrelavent to what was previously
stated and thus diversionary to the subject at hand.
If you read the initial post carefully you will note that his question
revolved around the length of reflector (s) which in essence calls for
a different aproach to the traditional format
based around a yagi .It is for this reason that such questions are
raised by those who apply deeper thought to the subject,
ask this question over and over again and cannot be explaned
unless coupling of individual fields are brought into play instead of
introducing personal rules similar to front to back ratios which
is a point ratio of energy efficiency rather than a overall array
energy efficiency. and so on.
Art