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news:1k%vd.198273$V41.135426@attbi_s52...

"hnkie" wrote in message
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"art" wrote in message
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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.
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.
Parabolics have a rolloff on the edge to decay the field to minimize the
back lobe.

(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.

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) One antenna has over a thousand elements.


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