"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 )