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