"Roy Lewallen" wrote in message
...
A plain dipole 76 feet above perfect ground has a gain of about 8.6 dBi -
surely you've modeled one with AO. So the gain of your antenna over a
dipole is about 5.3 - 5.5 dB. (This would be a more meaningful way to state
gain.)

W4RLN ???? said he prefered dbi if I remember correctly. Can't remember the
reason why

This is just slightly more than the gain from a three element Yagi, and
less than a typical four element beam.

Difficult to compare Roy as the top reflector position moved beyond the
feed point albiet also much higher.

The F/B ratio and bandwidth might be better than that of a typical three
or four element Yagi (or they might not be -- a Yagi can be adjusted for a
wide range of characteristics),


Well I thought the F/B was exceptional, especially because it stayed
relatively constant
over the band width as did the gain. I especially liked the fact that the
impedance
was in the 50 ohms vicinity where optimum characteristics of an yagi antenna
usually drives
the impedance to a low level. Force 12 emphasises that to get optimum gain
one must accept
unrealistic impedance values among other things
Can you point to a 4 element yagi where its characteristics are better in
every way?
( there must be something where this excels. grin)



but the gain is certainly not exceptional

I agree, but because it is relatively constant across the band I would
consider it very good.
On the other hand the elements are all straight from end to end on that
model. I may
be optimistic but I am anticipating up to 2 db increase in gain when I
reduce the beam
width to 60 degrees from 70 degrees by shaping the elements into a quasi
parabolic shape.
All in all it seems it will perform quite well and does put into focus the
question of not only
having more than one reflecter but having a reflecters that also provides a
consistent 50 ohm plus impedance.!
This is just an experiment that I am sharing with the antenna group in an
area where I have not seen
any examples in print and not something that one can say... 'Dah Da the
yagi replacement !'
But it does show a method where antenna attributes can be made consistent
across the band
which can be wider than the norm. Comparisons with the Yagi is also
difficult to make because
where a yagi is at a constant height in this case certain elements have an
increased height.

Thanks for the comments, tho I must say I do view the model with more
enthusiasm than you do,
but I do defer to your broader experience and knoweledge.

Best Regards
Art

..

Roy Lewallen, W7EL

art wrote:
In the past I mentioned that I was modelling an antenna for 20 metres.
This antenna became a single driven element with five reflectors which
emulate a dish in it's crudish form not only in position of reflectors
but also the elements were not shaped at this time.
The following table follows what one would get for a reflective dish
used on frequencies in the giga hertz range where the bandwidth is
broader than
that obtained with the yagi format PLUS relevant atributes stay more
constant over a frequency range than that obtained with a super gain
type array.
The band width is wider than a typical yagi in that the major portion
of the radiation is to the front this.This band with is expected to
narrow and to increase gain when individual elements are shaped to a
parabolic form and
it is hoped that forward side lobes will not occur as is normal for a
long boom multi element yagi form as used on 440 Mhz.
The driven element is at a height of 76 feet over perfect ground.

Normally one would say that if results seem out of the ordinary one
should question it, so comments are requested regarding my explanations
which
lead me to think that they really are what one should expect.



Freq 14.0 14.1 14.2 14.3 14.35

Gain dbi 13.86 14.05 13.99 13.9 13.87

F/B 30 25.6 27.47 33.17 32.5

Zr 45.9 44.5 43.8 43.2 42.8

Zi -12.87 -6.52 0.6 7.84 11.54

SWR 1.32 1.2 1.14 1.25 1.34


I do not advocate the replacement of the normal yagi with the above
because of building difficulties tho it is my intention to build it.

Regards
Art

(Modelled using AO PRO with 80 segment/half wave and set up by a
consumate error generator Grin )

Roy
After I mused a while on your post I then accepted it for the
unbiased
analysis that it was rather than a desired reflection of my own
thoughts.
It then drove me to consider chosing gain as the main criteria, after
all my interests
are talking to my mates in the U.K., but at the same time carrying on
with my main thought of moving away from the long boom analogy and its
accompanying gain aproach.
By adding a mast of 30 feet to accommodate additional elements rather
than extending boom length I was able to narrow the beam width to some
where in the 57 degree region but my excellent F/B that I obtained
earlier went way down. This was done by constructing elements in "V"
form with no restriction on alignment with respect to the driven
element other than the center of the elements must be close to the mast
i.e
to take advantage of low torque requirements.
With six elements on the 30 foot mast on the tower not only did the
gain increase
to 16 dbi ( others please take note of prior posts on this thread) but
the 'take off angle'
dropped to pretty close to !0 degrees ! This aproach for somebody who
wants to work DX seems a good way to go.
I now intend to pursue the aproach of letting the elements deviate
from the "V" shape to what I expect to be a parabolic shape, to
determine whether the extra work is worth while, as well as the
"ommission" of the most upper most element for when heavy static
occurrs.
What I really do find interesting is that one CAN get away from the
boom length aproach when seeking gain as well as obtaining a reasonable
impedance.
I also intend to attempt a tri band aproach as I move along, which may
take a while as my program has provision for a limited number variable
dimensions to optimize at one time and the number of dimensions in the
next model will certainly exceed 40 at a minimum, thus modelling cannot
be achieved in a single shot.
My present model, which moves me towards gain, seems to exceed what
performance I can expect compared to high F/B and is certainly opening
fresh avenues as to what type of antenna I will construct after the new
year.

Comments solicited from all........
and happy hollidays

Art

The elevation angle at which radiation is maximum (the "takeoff angle")
can be influenced by narrowing the free-space elevation pattern of the
antenna. A Yagi provides considerable directivity in the horizontal
direction, but not very much in the vertical direction. The elevation
pattern of the forward lobe of a Yagi doesn't look much different from
the elevation pattern of a dipole. Therefore, the elevation patterns of
a Yagi and dipole are very similar (except for very long Yagis, which do
begin to have some noticeable directivity in the elevation pattern). One
consequence of this similarity is that the gain of a Yagi over a dipole
in free space is about the same as the gain of a Yagi over a dipole at
any elevation angle when the two are mounted at the same height -- a
single number is adequate to describe the gain of a Yagi over a dipole,
no matter where the two are mounted (as long as they're at the same height).

There are some antennas that do compress the vertical pattern. One
notable antenna in this category is the W8JK. The result is that the
"takeoff angle" is lower for the W8JK than for a dipole at the same
height, and the gain of a W8JK compared to a dipole is different at
different elevation angles. I haven't looked at one in a long time, but
believe the "Lazy H" antenna also compresses the vertical pattern
somewhat, resulting in a lower "takeoff angle" than a dipole when
mounted over ground.

It's not too surprising that Art's antenna has the ability to compress
the vertical pattern somewhat, for the same general effect as the W8JK,
due to its physical geometry. This general characteristic of a lower
"takeoff angle" can be an advantage for working DX, so it might make the
constructional difficulties worthwhile for some people. The reduction in
F/B when maximizing gain also isn't surprising. I don't believe there's
any fundamental reason for this to always happen, but it's all too often
the case. Most arrays, both parasitic and driven, have relatively poor
F/B when adjusted for maximum gain. It becomes up to the user to decide
on the optimum tradeoff for his particular application.

Roy Lewallen, W7EL

art wrote:
Roy
After I mused a while on your post I then accepted it for the
unbiased
analysis that it was rather than a desired reflection of my own
thoughts.
It then drove me to consider chosing gain as the main criteria, after
all my interests
are talking to my mates in the U.K., but at the same time carrying on
with my main thought of moving away from the long boom analogy and its
accompanying gain aproach.
By adding a mast of 30 feet to accommodate additional elements rather
than extending boom length I was able to narrow the beam width to some
where in the 57 degree region but my excellent F/B that I obtained
earlier went way down. This was done by constructing elements in "V"
form with no restriction on alignment with respect to the driven
element other than the center of the elements must be close to the mast
i.e
to take advantage of low torque requirements.
With six elements on the 30 foot mast on the tower not only did the
gain increase
to 16 dbi ( others please take note of prior posts on this thread) but
the 'take off angle'
dropped to pretty close to !0 degrees ! This aproach for somebody who
wants to work DX seems a good way to go.
I now intend to pursue the aproach of letting the elements deviate
from the "V" shape to what I expect to be a parabolic shape, to
determine whether the extra work is worth while, as well as the
"ommission" of the most upper most element for when heavy static
occurrs.
What I really do find interesting is that one CAN get away from the
boom length aproach when seeking gain as well as obtaining a reasonable
impedance.
I also intend to attempt a tri band aproach as I move along, which may
take a while as my program has provision for a limited number variable
dimensions to optimize at one time and the number of dimensions in the
next model will certainly exceed 40 at a minimum, thus modelling cannot
be achieved in a single shot.
My present model, which moves me towards gain, seems to exceed what
performance I can expect compared to high F/B and is certainly opening
fresh avenues as to what type of antenna I will construct after the new
year.

Comments solicited from all........
and happy hollidays

Art

Roy,
Thanks for the pertinent points raised in your post.
I might add that since the element centers are close to the mast
I have avoided the installation of a boom at the top of the mast
as one would do to lower the take off angle by the use of a stacked
beam. This 'top beam' provides major instability when using a single
mast. With me using my normal fishing pole elements the arrangement
will help quite a bit in mast mechanical terms. Since it is element
height over the feed element that, in the main, is creating a
compressed lobe (as I believe your post infers) I will take a different
tac with respect to the driven element, where it will be the only
element shaped in parabolic form and all others will
remain in a "V" configuration( straight portion for clamping at the
center). The parabolic shaped driver would then hopefully allow the
addition of staggered elements for other bands. The mast used will be
a 2 inch diameter fibre glass to prevent vertical radiation mutations.
Regards
Art