Bill Turner wrote:
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

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

Very good.And if you turn the element the element it turns
the deflected missile or what ever impinged on it, in a different
direction. This is exactly why the question was stated the way that it
was, a little bit of knoweledge based on conventional
teachings(waves) that becomes distorted when people reverse the notion
that a yagi design is a subset of radiation. in the exploration of the
field and waves subject, When exploring radiation in its truest sense
you are dealing with the interaction of different fields which is not
predicated solely on element length
Since the yagi is designed for a specific purpose or parameters
one can then parrot other factors that are relavent only to this
particular design such as the idea element length determines what is a
director or a reflector such as a pin ball machine in a arcade which
generate sweeping terms or semantics.It is always better to pass on
accepted teachings in answer to any question than generating an answer
you think should have been asked on the assumption that the receiver is
not smart enough
to understand the correct response and is to be given a simplistic
response without caveates.
Art

Very good.And if you turn the element the element it turns
the deflected missile or what ever impinged on it, in a different
direction. This is exactly why the question was stated the way that it
was, a little bit of knoweledge based on conventional
teachings(waves) that becomes distorted when people reverse the notion
that a yagi design is a subset of radiation. in the exploration of the
field and waves subject, When exploring radiation in its truest sense
you are dealing with the interaction of different fields which is not
predicated solely on element length
Since the yagi is designed for a specific purpose or parameters
one can then parrot other factors that are relavent only to this
particular design such as the idea element length determines what is a
director or a reflector such as a pin ball machine in a arcade which
generate sweeping terms or semantics.It is always better to pass on
accepted teachings in answer to any question than generating an answer
you think should have been asked on the assumption that the receiver is
not smart enough
to understand the correct response and is to be given a simplistic
response without caveates.
Art

Bill Turner wrote:

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.


A reflector does not "reflect". It simply re-radiates with the correct
phase and level to null energy from the other element(s) in the
unwanted direction. This is why spacing and length is critical.

Try using your two element Yagi with a reflector lower in frequency.
The reflector becomes a director.

Now think of a director. The director removes signal from the rear
about the same as a "reflector" does. Change the length and it can
become a reflector.

Now think of a mirror or a screen that is not resonant.

A mirror or screen works on a different principle. It reflects. Once it
is a certain physical size, resonance does not matter. It can be 100
wavelengths across or 1 wavelength, and it still reflects. It will
reflect infared or ultraviolate, and a 100 foot screen will reflect 60
meters to 10 cm all the same if the mesh is small enough.

Thinking a reflector "reflects" and a director "directs" will doom you
to failure if you are trying to understand how a Yagi works.

73 Tom

wrote:

Thinking a reflector "reflects" and a director "directs" will doom you
to failure if you are trying to understand how a Yagi works.

73 Tom


Thanks for the nice synopsis.

I would add that, because of how the physics works, the pattern that
evolves doesn't happen during the first cycle, or the first few. It
takes a while, and the more elements involved, the longer it takes.

There were some nice online "movies" (simulations) of an antenna
"building" its near and far field patterns a year or so ago. I will
have to do some googling to find them again, they were quite interesting.

tom
K0TAR

Please allow me to describe the antenna I was talking about which is
"similar" to a dish for HF. It obviously is not a dish but it has the
appearance of a dish as it has no directors as it were
but multi reflectors.The reflectors tho straight are not in line but
form a parabolic shape behind a driven dipole. Using this as a
illustrative model it shows that
(a) multiple "reflectors" can be used to advantage and it some cases
can be shorter than the driven element thus illustrating that element
length does not determine reflector versus director.
And (b) it also illustrates how the primary radiated beam can be
lowered versus a yagi with the driver at the same height.
And (c) that a single reflector is not always the best choice
My intention was to describe an array that looked like a dish
physically but illustrated how multi reflectors can replace multi
directors to advantage. I apologise that I used the word "simulate"
when comparing it to a dish but as you can see I was describing what
had the "appearance" of a dish is actually an array which I used as a
illustrative model to describe or emphasise that element length,
position etc with the terms director and reflector
can mislead and it is always better to stay with the generated fields
aproach.
It was not my intention to bring true dishes into the post
Art
Art

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

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