I had always thought that take off angle was
a function purely based on ht over ground
and nothing else.
When experimenting with my AO computor program
on colliear arangements I.E. without booms, I
am finding that the 'Take off' angle becomes lower
with increasing gain over a dipole.
The top edge of the leading lobe stays constant
with that of a single dipole but with the slight
lowering of the lobe angle as much as 1db of
increase in gain are obtainable at the LOWEST angle.
Does anyone know of a text book that discusses the
why's and wherefores of these lower angle gains
together with its 'known' limitations?
Is it possible that it is a parallelogram
resultant of increased vector value versus the
ground influence constant?

Regards
Art Unwin

It's really quite simple and fundamental. The "takeoff angle" (elevation
angle at which the pattern is maximum) depends on both the height and
the free-space vertical pattern of the antenna. Yagis end up having a
vertical pattern similar to a dipole in the forward direction because
the Yagi provides very little concentration in the elevation plane. Some
antennas do provide substantial concentration in the elevation plane,
however, such as a W8JK, or collinear as you mention. The elevation
patterns of vertically polarized antennas are further modified by the
different reflection coefficient encountered by vertically polarized waves.

Kraus has a good discussion of ground reflection coefficient in
_Antennas_. The vertical patterns reported by AO and similar programs
can be derived by hand from the free space pattern and reflections from
the ground using the reflection coefficients derived in Kraus.

Roy Lewallen, W7EL

Art Unwin KB9MZ wrote:
I had always thought that take off angle was
a function purely based on ht over ground
and nothing else.
When experimenting with my AO computor program
on colliear arangements I.E. without booms, I
am finding that the 'Take off' angle becomes lower
with increasing gain over a dipole.
The top edge of the leading lobe stays constant
with that of a single dipole but with the slight
lowering of the lobe angle as much as 1db of
increase in gain are obtainable at the LOWEST angle.
Does anyone know of a text book that discusses the
why's and wherefores of these lower angle gains
together with its 'known' limitations?
Is it possible that it is a parallelogram
resultant of increased vector value versus the
ground influence constant?

Regards
Art Unwin

Roy Lewallen wrote in message ...
It's really quite simple and fundamental.

Appreciate your response Roy, but the fact is the
matter is not simple to me. I am comparing horizontally
polarisation patterns in all cases thus I am having
difficulty with your explanation!
It comes to mind also that an antenna used for listening
( beverage ?) also comprises of stacked collinear
horizontally polarised radiators where the vertical
radiators appears to cancel
themselves out.
So it would appear to be a case where
a beam that is close to the ground ( coupled maybe to a
radiator other than the ground) is also capable of
decreasing the TOA even more than such an arrangement
at 1WL height.
Odd that you also brought into the picture the W8JK
antenna that also relies on critical coupling for
its extrorninary gain which you suggest also provides
for a low TOA when compared to the Yagi.
I will have to get the Kraus book from the library
for myself to read and hopefully there will be a
graph of some sort that will outline its advantages
and limitations.
In the mean time I will review VERT radiation patterns
of the examples that I chose in the initial post.
(Assuming that somebody does not come along and
triplicate the same thread) Since I see an advantage for
initial band openings without having to deal with the normal
early demise for stacked antennas that are not coupled.
Best regards
Art



The "takeoff angle" (elevation
angle at which the pattern is maximum) depends on both the height and
the free-space vertical pattern of the antenna. Yagis end up having a
vertical pattern similar to a dipole in the forward direction because
the Yagi provides very little concentration in the elevation plane. Some
antennas do provide substantial concentration in the elevation plane,
however, such as a W8JK, or collinear as you mention. The elevation
patterns of vertically polarized antennas are further modified by the
different reflection coefficient encountered by vertically polarized waves.

Kraus has a good discussion of ground reflection coefficient in
_Antennas_. The vertical patterns reported by AO and similar programs
can be derived by hand from the free space pattern and reflections from
the ground using the reflection coefficients derived in Kraus.

Roy Lewallen, W7EL

Art Unwin KB9MZ wrote:
I had always thought that take off angle was
a function purely based on ht over ground
and nothing else.
When experimenting with my AO computor program
on colliear arangements I.E. without booms, I
am finding that the 'Take off' angle becomes lower
with increasing gain over a dipole.
The top edge of the leading lobe stays constant
with that of a single dipole but with the slight
lowering of the lobe angle as much as 1db of
increase in gain are obtainable at the LOWEST angle.
Does anyone know of a text book that discusses the
why's and wherefores of these lower angle gains
together with its 'known' limitations?
Is it possible that it is a parallelogram
resultant of increased vector value versus the
ground influence constant?

Regards
Art Unwin

Art Unwin KB9MZ wrote:
Roy Lewallen wrote in message ...

It's really quite simple and fundamental.


Appreciate your response Roy, but the fact is the
matter is not simple to me. I am comparing horizontally
polarisation patterns in all cases thus I am having
difficulty with your explanation!

Ok, I'll try again. To determine the relative field strength at a
distant point at an elevation angle of, say 20 degrees, put the antenna
at the height you're interested in. Draw a line from the antenna to the
ground, at a downward angle of 20 degrees, then reflecting upward,
resulting in a ray going upward at an elevation angle of 20 degrees.
Draw another line from the antenna at an upward angle of 20 degrees. You
now have two parallel lines, a "direct ray" and a "reflected ray". At
some distant point, draw a line perpendicular to those rays. Measure the
distance from the antenna to the new line via each of the two paths, one
direct and the other reflected. You'll be adding these two rays, and the
difference between the two paths tells you the relative phases of these
two components you'll be adding. For example, if the antenna is a half
wavelength high, you'll find that at an elevation angle of 30 degrees,
the reflected ray travels exactly one wavelength farther than the direct
ray, so the two rays will exactly add in phase. At higher or lower
angles, they won't. When adding the two rays, you've also got to factor
in the free-space radiation pattern of the antenna to see just how much
the antenna is radiating at those angles (say, 20 degrees down and 20
degrees up from horizontal, for the pattern at 20 degrees). In the case
of a dipole, the free-space radiation pattern broadside to the antenna
is circular, so rays at all angles are equal. Thus, 30 degrees is the
"takeoff angle" for a dipole up a half wavelength. You do also have to
include a factor for the reflection coefficient of the reflected ray
from the ground. But for horizontally polarized waves at moderate to low
angles, it's very close to one. (But it's not, for vertically polarized
signals, so it should always be computed for vertical antennas.) This is
the way that AO, NEC, EZNEC, MININEC, and similar programs compute the
elevation pattern.

Now suppose that an antenna has a skinny elevation pattern in free
space. The W8JK is an example. At, say, 30 degrees up or down, the
signal is weaker than at 20 degrees. So the elevation pattern will be
correspondingly stronger than a dipole at 20 degrees relative to 30
degrees. This will lower the "takeoff angle" -- the elevation angle at
which the pattern is maximum.

These patterns can be pretty easily created with a calculator and either
some trigonometry or graph paper if you have the free-space pattern, but
modern programs can do the work for you.

It comes to mind also that an antenna used for listening
( beverage ?) also comprises of stacked collinear
horizontally polarised radiators where the vertical
radiators appears to cancel
themselves out.

No, the radiation from a Beverage is primarily vertically polarized, off
the end.

So it would appear to be a case where
a beam that is close to the ground ( coupled maybe to a
radiator other than the ground) is also capable of
decreasing the TOA even more than such an arrangement
at 1WL height.

I dunno. Look at the method I described, and try it on your theoretical
antenna to see if that's true or not.

Odd that you also brought into the picture the W8JK
antenna that also relies on critical coupling for
its extrorninary gain which you suggest also provides
for a low TOA when compared to the Yagi.

Egad, the magical "critical coupling". The W8JK has mutual impedance and
coupling between the elements like any other antenna. At 4 or so dB for
a couple of elements (if you keep losses down), I wouldn't call its gain
"extraordinary", either. It follows the same rules as all other
antennas, and its gain and other characteristics can be predicted with
great accuracy using the same ordinary methods used for all other antennas.

I will have to get the Kraus book from the library
for myself to read and hopefully there will be a
graph of some sort that will outline its advantages
and limitations.

It's described in _Antennas_, all editions I believe.

. . .

Roy Lewallen, W7EL

Roy Lewallen wrote in message ...
Art Unwin KB9MZ wrote:
Roy Lewallen wrote in message ...

It's really quite simple and fundamental.


Appreciate your response Roy, but the fact is the
matter is not simple to me. I am comparing horizontally
polarisation patterns in all cases thus I am having
difficulty with your explanation!

Ok, I'll try again. To determine the relative field strength at a
distant point at an elevation angle of, say 20 degrees, put the antenna
at the height you're interested in. Draw a line from the antenna to the
ground, at a downward angle of 20 degrees, then reflecting upward,
resulting in a ray going upward at an elevation angle of 20 degrees.
Draw another line from the antenna at an upward angle of 20 degrees. You
now have two parallel lines, a "direct ray" and a "reflected ray". At
some distant point, draw a line perpendicular to those rays. Measure the
distance from the antenna to the new line via each of the two paths, one
direct and the other reflected. You'll be adding these two rays, and the
difference between the two paths tells you the relative phases of these
two components you'll be adding.
O.K, so far
For example, if the antenna is a half
wavelength high, you'll find that at an elevation angle of 30 degrees,
the reflected ray travels exactly one wavelength farther than the direct
ray, so the two rays will exactly add in phase. At higher or lower
angles, they won't. When adding the two rays, you've also got to factor
in the free-space radiation pattern of the antenna to see just how much
the antenna is radiating at those angles (say, 20 degrees down and 20
degrees up from horizontal, for the pattern at 20 degrees). In the case
of a dipole, the free-space radiation pattern broadside to the antenna
is circular, so rays at all angles are equal.

Don't fully understand the circular part but let us press on..

Thus, 30 degrees is the
"takeoff angle" for a dipole up a half wavelength. You do also have to
include a factor for the reflection coefficient of the reflected ray
from the ground.
Understood
But for horizontally polarized waves at moderate to low
angles, it's very close to one. (But it's not, for vertically polarized
signals, so it should always be computed for vertical antennas.) This is
the way that AO, NEC, EZNEC, MININEC, and similar programs compute the
elevation pattern.
So far so good !

Now suppose that an antenna has a skinny elevation pattern in free
space. The W8JK is an example. At, say, 30 degrees up or down, the
signal is weaker than at 20 degrees. So the elevation pattern will be
correspondingly stronger than a dipole at 20 degrees relative to 30
degrees. This will lower the "takeoff angle" -- the elevation angle at
which the pattern is maximum.

Ouch. You jumped the Grand Canyon in two steps.
I need to think on that a bit more

These patterns can be pretty easily created with a calculator and either
some trigonometry or graph paper if you have the free-space pattern, but
modern programs can do the work for you.

It comes to mind also that an antenna used for listening
( beverage ?) also comprises of stacked collinear
horizontally polarised radiators where the vertical
radiators appears to cancel
themselves out.

No, the radiation from a Beverage is primarily vertically polarized, off
the end.
I was really thinking of a Franklin or Sterba thus the ?
So it would appear to be a case where
a beam that is close to the ground ( coupled maybe to a
radiator other than the ground) is also capable of
decreasing the TOA even more than such an arrangement
at 1WL height.

I dunno. Look at the method I described, and try it on your theoretical
antenna to see if that's true or not.

Odd that you also brought into the picture the W8JK
antenna that also relies on critical coupling for
its extrorninary gain which you suggest also provides
for a low TOA when compared to the Yagi.

Egad, the magical "critical coupling". The W8JK has mutual impedance and
coupling between the elements like any other antenna.



Wash my mouth out for saying critical coupling but having said that
I distinguish critical coupling from mutual coupling by the fact that
in critical coupling you have an INCREASE in current
bAt 4 or so dB for
a couple of elements (if you keep losses down), I wouldn't call its gain
"extraordinary", either. It follows the same rules as all other
antennas, and its gain and other characteristics can be predicted with
great accuracy using the same ordinary methods used for all other antennas.

Sorry, I don.t see it that way at all. W8JK has the arrangement of
critical coupling evidenced by its high gain that drops off quickly
as the coupling is changed. The increased current or coupling factor
is not evidenced with parasitic elements. I know all do not like to
hear such blasphemy which will create a howl but this is how the
original question came up i,e, critically close coupling between an
oversize
dipole by feeding a small dipole in close proximation which lowered
the TOA of the dipole alone ( yes, I added lumped constants to follow
the complex circuit aproach)
I will have to get the Kraus book from the library
for myself to read and hopefully there will be a
graph of some sort that will outline its advantages
and limitations.

It's described in _Antennas_, all editions I believe.

. . .

Roy Lewallen, W7EL
Thanks for going the extra mile with me Roy and for giving me
the time.
Will still have to lean on the library to get a copy for me to
look at to fill in the spaces especially the circular pattern
maybe I will just lay it down to the "curl" for the moment which will
apply a different "spin" to the subject ( Pun intended)
The Sterba antenna I will have to review since I see it as horizontal
ly polarised as in a double zep but my narrow education will have to
be
broadened.
Best regards
Art

Corrections:

The *magnitude* of the reflection coefficient for horizontally polarized
waves is very close to one. But the angle is 180 degrees. That is,
there's a phase reversal when the wave reflects. I was also incorrect in
saying that the difference in distances for a 30 degree elevation angle
for the half wave high antenna is exactly a wavelength. The distance
from the antenna to the reflection point is one wavelength, but the
difference in distances the rays travel to a distant point is exactly
1/2 wavelength. This can be seen by drawing a line perpendicular to the
direct and reflected rays as I suggested in my earlier posting, and
looking at the total distances traveled by both rays from their
intersection with it. Combined with the phase reversal, the 1/2
wavelength difference in distances results in complete reinforcement at
a distant point.

I apologize for the errors. Many thanks to John Farr for reminding me of
the phase reversal of the reflection.

Roy Lewallen, W7EL

Roy Lewallen wrote:
. . .
For example, if the antenna is a half
wavelength high, you'll find that at an elevation angle of 30 degrees,
the reflected ray travels exactly one wavelength farther than the direct
ray, so the two rays will exactly add in phase.

. . .

You do also have to
include a factor for the reflection coefficient of the reflected ray
from the ground. But for horizontally polarized waves at moderate to
low angles, it's very close to one.

. . .

Roy all of this has peaked up a new interest for me
when it appears that all already knew of this in detail.
I am still having diffriculty in reconciling it via AO
programming where vertical, cw and ccw radiation fields
now comes into being.( the relative patterns cannot be
overlaid as can std horizontal patterns )
I came across a rather large article on this matter in Radcom
March '98 ( Ala Antenna topicss publication from the RSGB)
which was sparked off by our old friend Lew McCoy,
where it gives further insights to Moxon's statement regarding
the use of a two element instead of a three element with a
adjustment in height. Probably jumping the gun but I ponder
the fact that this subject may well be related to the yagi/quad
debate but for now I have to read and reread what I have including
your comments until it becomes locked and intuitive.
Thanks again.
And now back to conjugate matching, reflections
and the like which have the interests of all (other than myself)
Regards
Art

Roy Lewallen wrote in message ...
Corrections:

The *magnitude* of the reflection coefficient for horizontally polarized
waves is very close to one. But the angle is 180 degrees. That is,
there's a phase reversal when the wave reflects. I was also incorrect in
saying that the difference in distances for a 30 degree elevation angle
for the half wave high antenna is exactly a wavelength. The distance
from the antenna to the reflection point is one wavelength, but the
difference in distances the rays travel to a distant point is exactly
1/2 wavelength. This can be seen by drawing a line perpendicular to the
direct and reflected rays as I suggested in my earlier posting, and
looking at the total distances traveled by both rays from their
intersection with it. Combined with the phase reversal, the 1/2
wavelength difference in distances results in complete reinforcement at
a distant point.

I apologize for the errors. Many thanks to John Farr for reminding me of
the phase reversal of the reflection.

Roy Lewallen, W7EL

Roy Lewallen wrote:
. . .
For example, if the antenna is a half
wavelength high, you'll find that at an elevation angle of 30 degrees,
the reflected ray travels exactly one wavelength farther than the direct
ray, so the two rays will exactly add in phase.

. . .

You do also have to
include a factor for the reflection coefficient of the reflected ray
from the ground. But for horizontally polarized waves at moderate to
low angles, it's very close to one.

. . .