I have a Directv dish 35 inches wide x 20 inches tall 4inches deep.
I calculated the focus as F=D*2 / 16d where D =diameter and d=depth
F=18 inches Ok so far.
How do you find beamwidth? I think the 35" dimensiom would have a tighter
pattern than the 20 ", but how do you calculate,

Second, The feed antenna, There can be different feeds.
A simple monopole, dipole, biquad, patch, yagi, helix.
How does the feed antenna affect the gain of the dish, if at all?
Yea, it my be difficult to light the yagi or helix, so if you like throw
those out.
Thank You,
Mike

On Thu, 26 Jun 2008 07:35:31 -0500, "amdx" wrote:

I have a Directv dish 35 inches wide x 20 inches tall 4inches deep.
I calculated the focus as F=D*2 / 16d where D =diameter and d=depth
F=18 inches Ok so far.
How do you find beamwidth? I think the 35" dimensiom would have a tighter
pattern than the 20 ", but how do you calculate,

Second, The feed antenna, There can be different feeds.
A simple monopole, dipole, biquad, patch, yagi, helix.
How does the feed antenna affect the gain of the dish, if at all?
Yea, it my be difficult to light the yagi or helix, so if you like throw
those out.
Thank You,
Mike

You have a neat gadget here!
If you place a lamp (flashlight?) at the focus you can measure the
beam on a wall wth a tape measure.

Put a microphone at the focus and you can hear things you would not
believe...

Old ARRL antenna books, and authors like John Krause will be needed
to answer all of your questions.

John Ferrell W8CCW
Beware of the dopeler effect (pronounced dope-ler).
That's where bad ideas seem good if they come at you
fast enough.

"John Ferrell" wrote in message
...
On Thu, 26 Jun 2008 07:35:31 -0500, "amdx" wrote:

I have a Directv dish 35 inches wide x 20 inches tall 4inches deep.
I calculated the focus as F=D*2 / 16d where D =diameter and d=depth
F=18 inches Ok so far.
How do you find beamwidth? I think the 35" dimensiom would have a tighter
pattern than the 20 ", but how do you calculate,

Second, The feed antenna, There can be different feeds.
A simple monopole, dipole, biquad, patch, yagi, helix.
How does the feed antenna affect the gain of the dish, if at all?
Yea, it my be difficult to light the yagi or helix, so if you like throw
those out.
Thank You,
Mike

You have a neat gadget here!
If you place a lamp (flashlight?) at the focus you can measure the
beam on a wall wth a tape measure.

It grey, won't reflect light very well. Basically I wanted to know how the
beamwidth changed with the larger width dimension vs the smaller height
dimension

Put a microphone at the focus and you can hear things you would not
believe...

I already experienced the reflection. I had a radio playing on my right
side, I was moving the dish and got an increase in volume in my left ear. I
moved the dish back
to play around with the effect. Neat!

Old ARRL antenna books, and authors like John Krause will be needed
to answer all of your questions.

John Ferrell W8CCW
Beware of the dopeler effect (pronounced dope-ler).
That's where bad ideas seem good if they come at you
fast enough.

"amdx" wrote in message
...
I have a Directv dish 35 inches wide x 20 inches tall 4inches deep.
I calculated the focus as F=D*2 / 16d where D =diameter and d=depth
F=18 inches Ok so far.
How do you find beamwidth? I think the 35" dimensiom would have a tighter
pattern than the 20 ", but how do you calculate,

Second, The feed antenna, There can be different feeds.
A simple monopole, dipole, biquad, patch, yagi, helix.
How does the feed antenna affect the gain of the dish, if at all?
Yea, it my be difficult to light the yagi or helix, so if you like throw
those out.
Thank You,
Mike

The DirecTV dishes are not prime focus, bu rather offset. If it came with
the original horn mount, use it.
You can use:
http://www.w1ghz.cx/software/hdl_3b4.zip
to find the focal length,. Your formula assumes prime focus.

The gain of the dish is not a function of the feed, but the system gain is
because one must take into account surface illumination, feed efficiency
etc.

The subject of feeds for dishes is complex if you really want to optimize
G/T etc. Paul W1GHZ has a teriffic on-line text about parabolic surfaces and
feeds:
http://www.w1ghz.org/antbook/preface.htm

Dale W4OP

On 26 jun, 14:35, "amdx" wrote:
I have a Directv dish 35 inches wide x 20 inches tall 4inches deep.
I calculated the focus as F=D*2 / 16d where D =diameter and d=depth
F=18 inches Ok so far.
How do you find beamwidth? I think the 35" dimensiom would have a tighter
pattern than the 20 ", but how do you calculate,

Second, The feed antenna, There can be different feeds.
A simple monopole, dipole, biquad, patch, yagi, helix.
How does the feed antenna affect the gain of the dish, if at all?
Yea, it my be difficult to light the yagi or helix, so if you like throw
those out.
Thank You,
Mike

Hello Mike,

In case of uniform illumination, gain is maximum, side lobes are
maximum also and beam width is about 60*lambda/diameter (degrees).

You will certainly not reach this, for several reasons. The beam width
will be more, it can be twice as high (depending on the illuminator).

The maximum gain that you can obtain with a certain aperture antenna
is about 4*pi*A/lambda^2. In reality it will be less:

When you want uniform illumination, you will have "spill over" (part
of the radiated power will not reach the dish). To get most power onto
the dish, you must accept that the edges of the dish do receive less
power flux density.

In addition, the phase center of your illuminator will change versus
off-main beam direction. This causes phase differences so a non-planar
field. Also polarization may change for off-beam directions.

Other things are deviation from optimal parabolic shape, non correctly
positioned illuminator, etc. This will all reduce maximum gain.

Hope this helps a bit.

Best regards,

Wim
PA3DJS
www.tetech.nl

"amdx" wrote in message
...
I have a Directv dish 35 inches wide x 20 inches tall 4inches deep.
I calculated the focus as F=D*2 / 16d where D =diameter and d=depth
F=18 inches Ok so far.
How do you find beamwidth? I think the 35" dimensiom would have a tighter
pattern than the 20 ", but how do you calculate,

Thank You,
Mike

The DirecTV dishes are not prime focus, but rather offset. If it came with
the original horn mount, use it.

Yes, it is an offset feed and it did come with the mount and feed (LNA?)
Actually it came with two LNAs, but they didn't match each other. I don't
remember what junk pile this came from and don't have any info about it's
working condition.
I did measure the distance from the front of feed to the dish center and
it is 18" as calculated, however as I said I don't know if this worked as I
got it.

You can use:
http://www.w1ghz.cx/software/hdl_3b4.zip
to find the focal length,. Your formula assumes prime focus.

This link doesn't look right.

The gain of the dish is not a function of the feed, but the system gain is
because one must take into account surface illumination, feed efficiency
etc.

So I could use a biquad or patch as the feed antenna without much
difference?


The subject of feeds for dishes is complex if you really want to optimize
G/T etc. Paul W1GHZ has a teriffic on-line text about parabolic surfaces
and feeds:
http://www.w1ghz.org/antbook/preface.htm

Dale W4OP

I'll look at the w1ghz link.
I was suprised this dish is made of plastic. Does the EM wave reflect off
of the plastic or does the plastic have a coating?
Thanks, Mike

"Dale Parfitt" wrote in message
news:7xR8k.35$wR.10@trnddc07...

"amdx" wrote in message
...
I have a Directv dish 35 inches wide x 20 inches tall 4inches deep.
I calculated the focus as F=D*2 / 16d where D =diameter and d=depth
F=18 inches Ok so far.
How do you find beamwidth? I think the 35" dimension would have a tighter
pattern than the 20 ", but how do you calculate,

Second, The feed antenna, There can be different feeds.
A simple monopole, dipole, biquad, patch, yagi, helix.
How does the feed antenna affect the gain of the dish, if at all?
Yea, it my be difficult to light the yagi or helix, so if you like throw
those out.
Thank You,
Mike

The DirecTV dishes are not prime focus, bu rather offset. If it came with
the original horn mount, use it.

Hey I just found the dish I have it's at
http://www.solidsignal.com/satellite...enna_types.asp
Last dish on the right side of the page, it's called the International
Satellite Dish.
International 36". I note from the picture the feeds don't match, when I got
the dish
the position of the two feeds were reversed.
What can you glean fron this?
Thanks, Mike

amdx wrote:
"amdx" wrote in message
...
I have a Directv dish 35 inches wide x 20 inches tall 4inches deep.
=====================================
I was suprised this dish is made of plastic. Does the EM wave reflect off
of the plastic or does the plastic have a coating?
=============
It is possibly an aluminium foil reflector sandwiched between 2 layers
of plastic.
I have a 1 metre dia sat dish with the aluminium foil sandwiched between
2 layers of fibre glass/epoxy.

Frank GM0CSZ / KN6WH

It grey, won't reflect light very well. Basically I wanted to know how the
beamwidth changed with the larger width dimension vs the smaller height
dimension

Tape aluminum foil to it tobetter reflect light...
John Ferrell W8CCW
Beware of the dopeler effect (pronounced dope-ler).
That's where bad ideas seem good if they come at you
fast enough.

"Wimpie" wrote in message
...
On 26 jun, 14:35, "amdx" wrote:
I have a Directv dish 35 inches wide x 20 inches tall 4inches deep.
I calculated the focus as F=D*2 / 16d where D =diameter and d=depth
F=18 inches Ok so far.
How do you find beamwidth? I think the 35" dimensiom would have a tighter
pattern than the 20 ", but how do you calculate,

Second, The feed antenna, There can be different feeds.
A simple monopole, dipole, biquad, patch, yagi, helix.
How does the feed antenna affect the gain of the dish, if at all?
Yea, it my be difficult to light the yagi or helix, so if you like throw
those out.
Thank You,
Mike

Hello Mike,

In case of uniform illumination, gain is maximum, side lobes are
maximum also and beam width is about 60*lambda/diameter (degrees).

My dish is 35" wide x 20"tall
So if Lambda=4.89" Then 60 x 4.89 / 35 = 8.38*
and 60 x 4.89 / 20 = 14.67*
Then my beamwidth will be taller than it is wide.

You will certainly not reach this, for several reasons. The beam width
will be more, it can be twice as high (depending on the illuminator).

Yes. it's always something.

The maximum gain that you can obtain with a certain aperture antenna
is about 4*pi*A/lambda^2. In reality it will be less:

I was going to attempt this but I don't know how to calculate the area (A)
of my non circular dish. Any help there?

Thanks, Mike