Here's a related question:

WHY do parasitic elements work the way they do?

Let's consider a two-element yagi with a driven element and a parasitic
"reflector", ie a parasitic element longer than a half wavelength.
(We could make the same arguments in reverse for a "director".)

The driven element radiates an electromagnetic field, some of which
impinges on the reflector. This causes a current to flow in the
reflector, and a voltage to appear across it. Since it is longer than a
half wavelength, it acts inductive, and the current LAGS behind the
voltage.

The reflector then radiates its own electromagnetic field in all
directions, some of which heads back toward the driven element.
(For simplicity, we ignore the mutual impedance effects and the new
current which is induced in the driven element.)

If the fields from the reflector and driven element are to be in phase
in the direction from the reflector towards the driven element, then the
radiated field from the reflector must be advanced in phase by how much
it lost traveling from the driven element to the reflector, plus another
same amount as it travels back. So the phase of the field radiated by
the reflector LEADS the phase of the driven element significantly.

Now the question is (assuming this is all right so far):
How do we explain the phase of the field radiated from the reflector, in
terms of the phase of the current and voltage in the reflector?


Bob W8ERD

Bob Dixon wrote:
Here's a related question:

WHY do parasitic elements work the way they do?

Let's consider a two-element yagi with a driven element and a parasitic
"reflector", ie a parasitic element longer than a half wavelength.
(We could make the same arguments in reverse for a "director".)

The driven element radiates an electromagnetic field, some of which
impinges on the reflector. This causes a current to flow in the
reflector, and a voltage to appear across it. Since it is longer than a
half wavelength, it acts inductive, and the current LAGS behind the
voltage.

The reflector then radiates its own electromagnetic field in all
directions, some of which heads back toward the driven element.
(For simplicity, we ignore the mutual impedance effects and the new
current which is induced in the driven element.)

You also need to ignore the fields from all other elements if present.
They can have a major impact on the overall field to the rear which the
reflector must attempt to cancel.

If the fields from the reflector and driven element are to be in phase
in the direction from the reflector towards the driven element, then the
radiated field from the reflector must be advanced in phase by how much
it lost traveling from the driven element to the reflector, plus another
same amount as it travels back. So the phase of the field radiated by
the reflector LEADS the phase of the driven element significantly.

But the purpose of the reflector isn't to make a field which reinforces
the driven element's field in the forward direction, but to make a field
which cancels it in the reverse direction. For this to happen most
effectively, the phase lag of the reflector current (relative to the
driven element current) and the distance between reflector and driven
element should add to 180 degrees. In practice, both the phase and
magnitude of the current induced in the reflector change with element
length. And in general, the farther you get from self-resonance, the
smaller induced current. So as you adjust the element length, by the
time you reach the optimum phase angle of induced current, its magnitude
is too small for good cancellation. A compromise is inevitably reached,
resulting in an acceptable but far from perfect front/back ratio.

Now the question is (assuming this is all right so far):
How do we explain the phase of the field radiated from the reflector, in
terms of the phase of the current and voltage in the reflector?

The magnitude and phase of the field are directly related to the
magnitude and phase of the current. The incremental longitudinal voltage
in the element can be ignored in calculation of fields. While it's
possible to base the field calculation on the longitudinal voltage
rather than the current, I don't believe I've ever seen this done.

Roy Lewallen, W7EL

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

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