"Sonie" wrote in message
...

" wrote in
message
news:1k%vd.198273$V41.135426@attbi_s52...

"hnkie" wrote in message
...

"art" wrote in message
oups.com...
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 )


F/B looks too high, you should have some big splinters off the
backside.

Why? Do you have splinters in a parabolic dish?

You have individual elements, a parabolic has continous surface. Antennas
always have splinters, with just 5 elements and no blocking you probably
have major lobes on the back side, between elements.

Not so!
Small lobe yes. When you used the term "splinter" I thought it described a
large thin splinter or lobe, which is not the case. Tho the reflectors do
have a space
between them which would have a terrible effect where "reflection "
occurs.In the
case where definitive phase angles occur I doubt what you have referred to
is correct.

There will be places on the back side of your antenna that add in phase
with
two or more elements, which is a lobe or splinter.

see above

Parabolics have a rolloff on the edge to decay the field to minimize the
back lobe.

Yes I am hoping for that effect in conjunction with the narrowing of the
main beam.
Decay however is not the word I would chose. Redirect yes.

(with a dish it can be hard to get 30)
Gain looks to be high for a 1 driven and 5 reflectors.
If you picture a balloon squashed at one end more than
normal would not the gain increase?

What is the capture area? Each element captures RF energy and reflects it
back to the driven element in phase with the others.

Capture area? I don't know ( Now you can see the limits of my expertise)

Your idea has been tried very sucessfully at 450 to microwave, and they
now
use flat antennas with each element phase adjusted for delay (same as
curverature of a parabolic dish) .

Yes I certainly agree, but for some reason 440 seems to be a cut-off
frequency
and I do not know why. I did try using the director in a parabolic shape but
it was unrewarding. The delay thing you referred to may well be why the
reflectors deviated from a true parabolic shape.

One antenna has over a thousand elements

1000 elements could well be connected to what you referred to above with
respect to "blocking" where the spacing between elements must be
commensurate with the frequency used.

Regards
Art



Does model take into account mutual effects of the reflectors?
Yes
Art