Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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

Yagi Antenna Question
 

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