ORIGINAL MESSAGE:

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



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Tom could have said "it reflects by radiating".

Semantics count here.

73, Bill W6WRT

Bill Turner wrote:
ORIGINAL MESSAGE:

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



*********** REPLY SEPARATOR ***********

Tom could have said "it reflects by radiating".

Semantics count here.

73, Bill W6WRT

That's an interesting point. Suppose you have a two-element driven array
with the elements spaced a quarter wave apart and fed 90 degrees out of
phase. This produces a cardioid pattern, which has a deep null. Is the
element toward the direction of the null "reflecting" and the other one
"directing"? If so, what are they "reflecting" and "directing"?

Each element intercepts considerable energy from the other and
reradiates it, if that makes a difference.

Here's another one: Build a 4 square array, assuming the ground is
perfect. (The EZNEC example file 4Square.EZ or demo equivalent
d_4Square.EZ can be used to illustrate this.) If you disconnect the
feedline to the rear array element and short circuit the feedpoint (by
deleting Source 1 in the EZNEC model), you'll still have a moderately
good directional pattern with about 15 dB front-back ratio. The rear
element is now a parasitic element, which we like to call a "reflector".
You've said it "reflects by radiating". Now connect the rear element
feedline as in the original antenna. The front/back ratio improves. But
the feedpoint resistance of the rear element is negative. This isn't
particularly unusual in driven arrays -- it means that the element in
question is absorbing power from the other elements and sending down the
feedline toward the source. The element is still radiating, because
current is flowing on it. But it's absorbing more power from the
surrounding region than it's giving back in the form of a field. (Again,
the excess is being sent back along the feedline to be used by the other
elements.) So, is that element now "reflecting"? If so, is it
"reflecting by radiating"?

Roy Lewallen, W7EL

ORIGINAL MESSAGE:

Roy Lewallen wrote:

That's an interesting point. Suppose you have a two-element driven
array with the elements spaced a quarter wave apart and fed 90
degrees out of phase. This produces a cardioid pattern, which has a
deep null. Is the element toward the direction of the null
"reflecting" and the other one "directing"? If so, what are they
"reflecting" and "directing"?



*********** REPLY SEPARATOR ***********

Trying to bridge the gap between engineering and English, I would
suggest this analogy:

A mirror reflects light energy fed to it, while a light bulb takes
electricity and turns it into light.

Either a mirror or a light bulb can be used to send light in a desired
direction, but only one is "reflecting" that energy in the usual sense
of the word. Likewise, only the mirror is "re-radiating" energy, much
like a yagi's reflector does.

The analogy is not perfect but that's what the words mean to me.

73, Bill W6WRT

Art Unwin wrote:
"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."

Glad to see you posting again, Art.

Kraus produced an organization chart of antennas on page 56 of the 3rd
edition of "Antennas". In the Kraus plan, the "Yagi-Uda" is among the
"End Fires".

The topic is: "Yagi Antenna Question".

Roy responded with:
"Suppose you have a two-element driven array with the elements spaced a
quarter wave apart and 90 degrees our of phase."

This driven antenna produces a nice null to the rear as a Yagi can, but
the Yagi is a parasitic array, not a driven array.

In this forum, a participant is free to take the discussion in any
desired direction and other participants are just as free to respond or
not any way they want to. It`s freedom of choice!

Best regards, Richard Harrison, KB5WZI

Why not cut to the chase? Antennas are based on Fields and
Wavres and not geometryas many seem to imply.The poster
stated "waves" since it is his starting point. he question he then asks
is in reference to element length relative to reflection and direction
which obviously eminates from those who are self taught around a
specific antenna ( yagi ) i.e vectors, rectection, defection,
reradiates e.t.c which some call semantics and is not how fiels and
waves are handled in general education. And their is good reason for
this, an element creats a field not a missile
that is reflected , deflected or independently deflected by individual
elements or sequentialy. What you really looking at is a reactionary
energy field formed by other elements that are impinged upon by the
initial energy field generated at the initial source.Thus the reaction
field generated by one or more elements to the impinging electrical
field is not based on element length but the field generated in
reaction by whatever is
in the field of reference which could be anything of any number, length
or material.IF the antenna is specifically a yagi you can ascribe to it
certain details as a subset to antenna education
and in general get away with it since the Yagi is in voque. In this
particular case the poster rightly starts off with the field aproach
but is confused by antenna education which revolves around a specific
antenna (yagi) whose design specifically
rebvolves around a singular design which allowed Tom to safely say
"that is how it is" thus avoiding reffering to true radiation
academics that revolve around fields and waves and where actual element
lengths can be viewed as academic. Would it not be better to respond
with an array example that could provide a shorter element by
reitterating what is taught in
accepted text books rather than concentrating what can be termed a
caveate in radiation in a similar way capacitance
is based on the premise of homoginous field e.t.c
This question is often asked and it is not thru ignorance but by
confusion generated by so called gurus who trot out an answer
that is close enough to the question askedand evading a corrective
response toi a question with thought that is not to be satisfied with
that is the way it is, a comment that is good for passing tests only
and not for furthering aducation.. Cherry pick all you want or give
answers to a question that you think should have been asked but that is
not how to perpetuate a title of a true guru
Nothing personal but the books that I have on antennas begin with field
and wave generation which individual arrays such as a yagi are descibed
as a subset and not the other way around.
Art

Tom, W8JI wrote:
"The shorter element doesn`t direct. It reradiates energy."

On page 905 of Terman`s 1955 edition he writes:
"If resonant at a higher frequency than is being transmitted, the
parasitic antenna acts as a "director" and tends to concentrate the
radiated field in its direction."

"Director" in quotation marks means: that is what they are called. Pity
the fool who argues with Terman!

Best regards, Richard Harrison, KB5WZI

I do believe that Tom is echoing what Terman is stating. Look again at
I totally disagree with the majority of this posting which
misrepresents what Terman actually said
into a self serving statement to give cover to an earlier misstatement.

Terman's statement
.. He states...........the parrasitic antenna acts as a "director"
He does not say it is a director and puts it into quotation purely that
is what others call it.
If he felt that it was self explanable he would have stated that it was
a director without the need for quotation marks and prefixes the term
with the word "acts" for clarification instead of the word "is" He then
follows on with his description that further explanion to emphasis the
need to place the term inside quotationas by adding his reasons
....."and tends to concentrate the radiated field in its direction"
Note he states "tends" rather than the word "directs "because as he
stated earlier
"it acts...." and not "is" and tjhen goes on to add the coup de gras by
stating what it dioes do......
"tends to concentrate the radiated field in its direction".
I view that asa very precise statement in describing what some call a
director as actually being a field with a tendency...e.t.c.
Frankly it reiterates what Tom said where one can be doomed if it not
described correctly
and it would appear that Terman had the term "director" very much in
mind when he described what others termed as a director. He certainly
was an amazing man who saw from the beginning
the need to refrain from the word "direct" or "director" as the field
generated does not warrent such an absolute word. This may appear to be
semantics as far as you may be concerned but the above analysis of what
he actually said provides a confirmation of what others were saying.
I would agreee howver with a small point that you reffered to and that
was regarding a fool who argues with Termam as one must first
understand what one actually read and convey the message to the brain
where the emphasis is to confirm what one wanted to read'
Art

I do believe that Tom is echoing what Terman is stating. Look again at
I totally disagree with the majority of this posting which
misrepresents what Terman actually said
into a self serving statement to give cover to an earlier misstatement.

Terman's statement
.. He states...........the parrasitic antenna acts as a "director"
He does not say it is a director and puts it into quotation purely that
is what others call it.
If he felt that it was self explanable he would have stated that it was
a director without the need for quotation marks and prefixes the term
with the word "acts" for clarification instead of the word "is" He then
follows on with his description that further explanion to emphasis the
need to place the term inside quotationas by adding his reasons
....."and tends to concentrate the radiated field in its direction"
Note he states "tends" rather than the word "directs "because as he
stated earlier
"it acts...." and not "is" and tjhen goes on to add the coup de gras by
stating what it dioes do......
"tends to concentrate the radiated field in its direction".
I view that asa very precise statement in describing what some call a
director as actually being a field with a tendency...e.t.c.
Frankly it reiterates what Tom said where one can be doomed if it not
described correctly
and it would appear that Terman had the term "director" very much in
mind when he described what others termed as a director. He certainly
was an amazing man who saw from the beginning
the need to refrain from the word "direct" or "director" as the field
generated does not warrent such an absolute word. This may appear to be
semantics as far as you may be concerned but the above analysis of what
he actually said provides a confirmation of what others were saying.
I would agreee howver with a small point that you reffered to and that
was regarding a fool who argues with Termam as one must first
understand what one actually read and convey the message to the brain
where the emphasis is to confirm what one wanted to read'
Art