On Mon, 21 Mar 2005 23:36:02 GMT, "Dale Parfitt"
wrote:

Not even close.
....
The real problem here becomes
the ability to properly illuminate the dish with a feedhorn.

Hi Dale,

What feed horn? Let's stick to what is and not what might be. A
dipole is perfectly capable of seeing any surface generated even if it
is not particularly the most optimized focus.

Besides, this is hardly on the scale of 10-24GHz and all such
discussion presents Point Locus Parabolic Reflectors. The original
poster is seeking a obtainable goal, not a theoretical maximum. The
discussion of his provided links show a simple achievement of 10dB
which is not shabby by any means.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Mon, 21 Mar 2005 23:36:02 GMT, "Dale Parfitt"
wrote:

Not even close.
...
The real problem here becomes
the ability to properly illuminate the dish with a feedhorn.

Hi Dale,

What feed horn? Let's stick to what is and not what might be. A
dipole is perfectly capable of seeing any surface generated even if it
is not particularly the most optimized focus.

Besides, this is hardly on the scale of 10-24GHz and all such
discussion presents Point Locus Parabolic Reflectors. The original
poster is seeking a obtainable goal, not a theoretical maximum. The
discussion of his provided links show a simple achievement of 10dB
which is not shabby by any means.

73's
Richard Clark, KB7QHC

Hi Richard,
I have worked with parabolas for years- the current dish is a 14' w/ 0.36
F/D and illuminated with a scalar feed. A dipole is perhaps one of the worst
feeds for a parabola. If you're going to put the surface up why not take
full advantage of it? It takes little if any additional work to properly
illuminate it.

Dale W4OP

On Tue, 22 Mar 2005 02:37:15 GMT, "Dale Parfitt"
wrote:

I have worked with parabolas for years- the current dish is a 14' w/ 0.36
F/D and illuminated with a scalar feed. A dipole is perhaps one of the worst
feeds for a parabola. If you're going to put the surface up why not take
full advantage of it? It takes little if any additional work to properly
illuminate it.

Hi Dale,

Well, I described how to build the reflector, you can describe how to
build the horn. Myself, I think that at 900MHz that is where the
trouble is going to start as the horn will almost certainly shadow the
reflector that already gives him 10dB gain.

Now if Nigel is trying to stretch Wi-Fi into Wi-Max, then such an
investment may be opportune.

73's
Richard Clark, KB7QHC

"Nigel M" wrote in message
...
In rec.radio.amateur.antenna, Dave Platt wrote:

In this particular case, because the feed has an omnidirectional
pattern, it seems to me that there would be a definite advantage to
using a relatively "deep" and thus somewhat "pointy" parabolic
section, in which the focal point lies a fair distance back from the
forward-most edges of the actual reflector.

This is what I thought, but then I thought that this may make the
positioning of the focus much more critical.

My reason for posting was really to see if anyone had seen any research
on the optimum "depth" in this application.

I've been thinking some more about parabolas in general. It dawned on me
that the difference in shape isn't a change in formula as such, just the
range of co-ordinates that you use.

I reckon I can use Excel to draw one, but I'm not sure about finding the
focus, other than by measurement.

The practical field method.
You can find the focus my placing small mirrors at dishes edge and aiming
toward a light source.

gb

On Tue, 22 Mar 2005 11:06:06 GMT, Nigel M wrote:

I reckon I can use Excel to draw one, but I'm not sure about finding the
focus, other than by measurement.

Hi Nigel,

You aren't going to find a simpler method than the one I described and
Dale has already given you the range to match dimensions against.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Tue, 22 Mar 2005 02:37:15 GMT, "Dale Parfitt"
wrote:

I have worked with parabolas for years- the current dish is a 14' w/ 0.36
F/D and illuminated with a scalar feed. A dipole is perhaps one of the
worst
feeds for a parabola. If you're going to put the surface up why not take
full advantage of it? It takes little if any additional work to properly
illuminate it.

Hi Dale,

Well, I described how to build the reflector, you can describe how to
build the horn. Myself, I think that at 900MHz that is where the
trouble is going to start as the horn will almost certainly shadow the
reflector that already gives him 10dB gain.

Now if Nigel is trying to stretch Wi-Fi into Wi-Max, then such an
investment may be opportune.

73's
Richard Clark, KB7QHC
Hi Richard,
Depending on surface size, that may be correct. An offset dish and an 0.7F/D
horn would avoid that problem. Without doing the math, the now defunct- and
available fro free, Primestar 1M offset dishes may be an excellent solution.
Then again, if he only needs 10dB, a small loop yagi could serve with a lot
less surface area.

Dale W4OP

On Tue, 22 Mar 2005 22:06:24 GMT, "Dale Parfitt"
wrote:

Depending on surface size, that may be correct. An offset dish and an 0.7F/D
horn would avoid that problem. Without doing the math, the now defunct- and
available fro free, Primestar 1M offset dishes may be an excellent solution.
Then again, if he only needs 10dB, a small loop yagi could serve with a lot
less surface area.

Hi Dale,

But this does not answer the problem of the horn design.

As I see it, a simple waveguide (no flared horn) itself will run
something like 6" x 12". About the size of a shoe box. Adding the
flare will probably quadruple that area. Now, to build an offset dish
will force a rather more difficult reflector design project. The
irony is that simply turning the horn around and using it would
probably do just as well, but unfortunately be more cumbersome to
construct and use than the simple design already linked to.

As for the Primestar dish, that is undoubtedly a Point Locus Parabolic
Reflector design that would clash with the geometry of its 10 times
larger intended application. This does not bode well when the appeal
is efficiency based.

Even the style of the classic pringles-can-array is probably simpler
to achieve than horns combined with offset dishes. [However, probably
closer to Crisco Can sized weighing in at several pounds.]

Quick Moral, The first 6 to 10 dB is the easiest gain to achieve.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Tue, 22 Mar 2005 22:06:24 GMT, "Dale Parfitt"
wrote:

Depending on surface size, that may be correct. An offset dish and an
0.7F/D
horn would avoid that problem. Without doing the math, the now defunct-
and
available fro free, Primestar 1M offset dishes may be an excellent
solution.
Then again, if he only needs 10dB, a small loop yagi could serve with a
lot
less surface area.

Hi Dale,

But this does not answer the problem of the horn design.

As I see it, a simple waveguide (no flared horn) itself will run
something like 6" x 12". About the size of a shoe box. Adding the
flare will probably quadruple that area. Now, to build an offset dish
will force a rather more difficult reflector design project. The
irony is that simply turning the horn around and using it would
probably do just as well, but unfortunately be more cumbersome to
construct and use than the simple design already linked to.

As for the Primestar dish, that is undoubtedly a Point Locus Parabolic
Reflector design that would clash with the geometry of its 10 times
larger intended application. This does not bode well when the appeal
is efficiency based.

Even the style of the classic pringles-can-array is probably simpler
to achieve than horns combined with offset dishes. [However, probably
closer to Crisco Can sized weighing in at several pounds.]

Quick Moral, The first 6 to 10 dB is the easiest gain to achieve.

73's
Richard Clark, KB7QHC

At least on the last paragraph- agreed.
On the Primestar, I have sued this on 10GHz with a feed designed for high
F/D with very good efficienvy. Being offset, the feed does not eclipse any
portion of the dish.
There are a couple of new designs for feeds out- one is a square guide
that does not require a scalar for choking off edge currents. My version at
1296 has an inside dimension of 5.75" and would then be 1/2 this for 2.4
GHz- or about 6 " SQ inches of occupied space. A 1M dish would have approx
1200 SQ", so even in a prime focus configuration, the feed blockage is a
non- issue.
In the final analysis though, I'd go with a parasitic design- bogner or
loop yagi. Too high a gain can also be a pointing issue problem.

Dale W4OP

Nigel
I have little experience with dish antennas but I do know that to provide
the 'new' it is
one percent insparation and 99 percent persperation, so how about this
aproach:
Determine frequency of use.
Then draw a graph of frequency versus reactance change from a zero point
indicating resonance
of the prime frequency.
This curve is parabolic and would be an ideal starting point as there would
seem to be a connection
If one would move to far away from the point of resonance I would suggest
that the dish it's self
would rise in temperature as current starts to flow instead of a pure 'loss
less' reflective action.
Regards
Art




"Nigel M" wrote in message
...
I've been looking to make a linear parabolic reflector for Wi-Fi, I've
found quite a few sources, such as:

http://www.genericgeek.com/index.php?q=node/280

http://www.freeantennas.com/projects...te2/index.html

Those I've found all give templates for a parabola, but without any
explanation as to why they have chosen that *particular* parabola, or
the formula used to draw it. As a result, the drawings are a bit
"sketchy" to say the least!

I know radio amateurs are often knowledgeable on antennae, so I thought
this was a good place to ask. I'd like to know a bit more theory, and
the pros and cons of various parabolic shapes.


--
Nigel M
"Occam's razor is not always sharp"