On Mon, 21 Mar 2005 18:58:07 GMT, Nigel M wrote:
All parabolas have the same formula, simply different constants

Yes, but what is the difference (in gain terms) between one that looks
flat and one that looks pointy?

????????

They all look the same: Parabolas.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Mon, 21 Mar 2005 18:58:07 GMT, Nigel M wrote:
All parabolas have the same formula, simply different constants

Yes, but what is the difference (in gain terms) between one that looks
flat and one that looks pointy?

????????

They all look the same: Parabolas.

73's
Richard Clark, KB7QHC

Not even close. Prime focus parabolas ( and offset fed) are defined by their
F/D ( focal length to diameter ratio). A "deep " dish would have an F/D of
0.3 or less. Shallow dishes 0.6 or greater. The real problem here becomes
the ability to properly illuminate the dish with a feedhorn. Typical scalar
feeds will be efficient from 0.3 to 0.5 or so. Under illumination can mean
better G/T but you are likely not using the all the surface. Over
illumination results in seeing warm earth noise and degrading G/T.
Do a Google search of W1GHZ site- an excellent tutorial on passive
reflectors and feeds.

Dale W4OP

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

"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