calling the longer element 'behind' the driven element of a yagi a
'reflector' is poor terminology. it does not actually 'reflect' it just
resonates at a slightly different point and the combination of phase and
distance from the driven element contribute to the gain in the 'forward'
direction and cancel out signals in the 'backward' direction. actual
reflection requires a conductive surface several wavelengths long in all
directions as is done on parabolic dishes for microwave use. even when you
look at vhf/uhf yagi's that have multiple reflectors they are still only
using the phase shift of the resonance of them to add or cancel energy from
certain directions. the smallest one that might be really called a
reflector is the 'corner reflector' antenna sometimes used at vhf and uhf.

" wrote in message
news:uEfjd.579123$8_6.52336@attbi_s04...
For ham radio it is conventional ( my thoughts) to see reflectors as
element/s placed behind
a feed element where. like other elements in an array receive radio
energy
and then reradiate a portion of the received energy.If one wants to
emulate
a point
radiation then many elements have to be arranged in parabolic fashion such
that all R.F.
received or transmitted meet at a common point.

Now I really know didly #####about dishes as my world is made up from
Yagis
but now
I am starting to think about these things which really raises questions
about what I had previously accepted
For instance,
1
Commercial dishes are not made of reflectors with a fraction of a wave
length ( surface is smooth)
So we have some how shifted from 'resonation' to a 'reflection' mode as in
light even tho a resonance
must still some how occur, but how and with what length reflectors (
multiple long wires?)?
2
If radiation/reception follows a common path then is the dual path
radiation
as seen with a Yagi (90 degrees apart)
changed ?
3
What would the result be if multiple electrical connection lines were made
between the horizontal reflectors (vert refl form)?

The emergency operation is now over after a lousy week in hospital but I
am
grounded for the rest of the year
( pain and infections and physical inabilities) books and the like is a
hard task but the laptop is at hand if I can find suitable links to study.

Plans are complete and parts available for a 20 metre dipole with eight
reflectors set in quasi parabolic form to deliver
14.5 dbi.BW 63 degrees which I hope then to reduce to 30 degrees for added
gain.
Feed dipole will be of variable length for different band use.
Isn't experimentation great when failure is not considered disasterous to
ones self esteem and resume?

Regards
Art

"Dave" wrote in message
...
calling the longer element 'behind' the driven element of a yagi a
'reflector' is poor terminology. it does not actually 'reflect' it just
resonates at a slightly different point and the combination of phase and
distance from the driven element contribute to the gain in the 'forward'
direction and cancel out signals in the 'backward' direction. actual
reflection requires a conductive surface several wavelengths long in all
directions as is done on parabolic dishes for microwave use.

Dave
So how does one "create" several conductive surfaces of what ever length
on a continuos electrical surface? My reflectors will consist of individual
elements tho I may well eventually connect them vertically at a later point.
If one should look at it in terms of phase and distance then how is the
length
of the phase changing element determined on a continuos surface?





even when you
look at vhf/uhf yagi's that have multiple reflectors they are still only
using the phase shift of the resonance of them to add or cancel energy
from
certain directions. the smallest one that might be really called a
reflector is the 'corner reflector' antenna sometimes used at vhf and uhf.

Since my wave length is long, does it really matter if the spacings between
reflectors
is :"open" for a small fraction of a wave length?
Can fields exist to deter penatration?

My cruising on the net has not provided one example of a "dish" style
antenna for H.F.
yet it would appear that there are advantages there if backyard space is
small,
especially for multi band use.
..
Surely, somewhere in ham radio history, a demarcation line for dish antennas
has been ascertained based
on advantages versus disadvantages ! If gain and f/b ( prime specs for the
amateur) can be manipulated
into higher extremes using dishes then is it reluctance to change or to lead
that dominates the hobby
based on the notion that all is now known?
There seem to be so many advantages with this aproach now that weight issues
are overcome
with foil coated fishing poles that I wonder what it is I am missing in a
much experimented field where
this portion has been pre dismissed or where coil dissertation is to prove
greater inroads in the hobby..
In this particular case I would welcome negative comments from those with
expertise in the field so
that these aspects can be given more thought,but then expertise is not a
real requirement since
so little is written.


Regards
Art



" wrote in
message
news:uEfjd.579123$8_6.52336@attbi_s04...
For ham radio it is conventional ( my thoughts) to see reflectors as
element/s placed behind
a feed element where. like other elements in an array receive radio
energy
and then reradiate a portion of the received energy.If one wants to
emulate
a point
radiation then many elements have to be arranged in parabolic fashion
such
that all R.F.
received or transmitted meet at a common point.

Now I really know didly #####about dishes as my world is made up from
Yagis
but now
I am starting to think about these things which really raises questions
about what I had previously accepted
For instance,
1
Commercial dishes are not made of reflectors with a fraction of a wave
length ( surface is smooth)
So we have some how shifted from 'resonation' to a 'reflection' mode as
in
light even tho a resonance
must still some how occur, but how and with what length reflectors (
multiple long wires?)?
2
If radiation/reception follows a common path then is the dual path
radiation
as seen with a Yagi (90 degrees apart)
changed ?
3
What would the result be if multiple electrical connection lines were
made
between the horizontal reflectors (vert refl form)?

The emergency operation is now over after a lousy week in hospital but I
am
grounded for the rest of the year
( pain and infections and physical inabilities) books and the like is a
hard task but the laptop is at hand if I can find suitable links to
study.

Plans are complete and parts available for a 20 metre dipole with eight
reflectors set in quasi parabolic form to deliver
14.5 dbi.BW 63 degrees which I hope then to reduce to 30 degrees for
added
gain.
Feed dipole will be of variable length for different band use.
Isn't experimentation great when failure is not considered disasterous
to
ones self esteem and resume?

Regards
Art

Art Unwin wrote:
"So how does one "create" several conductive surfaces of what ever
length on a continuous electrical surface?"

First examine the ways it has already been done and ask why? A
non-resonant reflector provides a much wider bandwidth than a resonant
reflector as used in an array like a yagi, for one thing.

Plenty of grid-dish, corner-reflector and similar non-resonant reflector
antennas are in use to reduce wind-loading. You have no need at lower
frequencies to use solid reflectors. Surface irregularities are not much
problem. The solid dishes at 2-GHz are built that way for convenience,
not because they need to be. The same is true at lower frequencies. At
higher frequencies, the conductors are so close together, there isn`t
enough reduction in wind-loading to be worth the construction effort
required with grid-reflectors.

Grid reflectors only need a collection of conductors parallel with the
radiating element(s) illuminating (driving) the antenna, and placed for
best results. Reflecting elemants in a grid reflector placed
perpendicular to the driven element don`t work. Curvature is used tn
dish reflector bars for focus. An angle is used in the corner-reflector
for more gain than a flat reflector. None of the reflectors needs
cross-connections between the reflector bars anymore than ground radials
need their far-ends interconnected. Such connections allow eddy currents
which only cause loss. The only current of value is that in the
direction of the ground radials or in the direction of the reflector`s
parallel conductors, as the case may be. So the eddy currents are bad.

Spacing between reflector bars is related to wavelength and capture
area.. Measure it in a good grid antenna and scale to your frequency.
Or, see Kraus for capture area etc.

Best regards, Richard Harrison, KB5WZI