Location of focus of feed horn?
 

On Fri, 3 Jun 2005 09:03:29 -0700, DaveC wrote:

What point is considered the focus point of the horn. The feed point for the
dish is calculated and known, but I'm not sure what point of the feed horn to
considered when aligning the horn to the dish.

Hi Dave,

A horn is an impedance matching device, not a lens. It is generally
pointed in the direction of desired maximum radiation. If you want to
employ what appears to be the equivalent to your application of focus,
then its "throat" is the term you should be interested in (at least
from the common literature on this subject - a subtle hint, in spite
of your protestations, on how to google this).

The plane of the throat is usually placed at the focus of the
truncated parabaloid (otherwise known as the dish) and orthogonal to
the parabaloid's major axis (the line that points in the direction of
the opening's maximum radiation). Hence both are collinear (and of
necessity, the horn blocks some of the available radiation), unless
this is an off-center feed. In such a case, the math becomes a little
more involved (but all structures conform to these usages).

73's
Richard Clark, KB7QHC

DaveC wrote:
Microwave corrugated horn at the end of the wave guide has multiple choke
rings.

What point is considered the focus point of the horn. The feed point for the
dish is calculated and known, but I'm not sure what point of the feed horn to
considered when aligning the horn to the dish.

I'm not going to use this horn, but simply want to determine where the focal
point of the horn is so as to place my own 2.4 GHz feed at that location.

This is on a surplus Direct TV feed.

Hi Dave.

W1GHZ has an application that will determine the focus of your dish for you.

However, I find it easier just to attach a few small mirrors on the dish
and point it at the sun to determine the focal point of your dish. See

http://www.qsl.net/va3rr/dualmode/dual_mode.htm

Now you just have to figure out where the phase centre of your feed is...

DaveC wrote:
On Sat, 4 Jun 2005 06:15:39 -0700, Russ Renaud wrote
(in article ):


W1GHZ has an application that will determine the focus of your dish for you.


I know the focus of the dish. Just need to know the point used as the feed
point.


Now you just have to figure out where the phase centre of your feed is...


It sounds like it is the entrance to the throat of the (original) guide where
the guide connects to the horn. According to this, you can lengthen or
shorten the guide at that point to change the "depth of radiation" (whether
the feed illuminates a small or large diameter of the dish), according to:

http://tinyurl.com/ar9es

I guess I wasn't able to divine your intent, then.

You say you have a surplus DirecTV "feed". You say you know the focal
point of your dish. You want to use your own 2.4 GHz feed. You
make mention of an adjustable feed - is it a chapparel-type feed because
I didn't think the DirecTV feeds are adjustable?

What kind of horn are you planning to use for 2.4 GHz? DirecTV dishes
are usually fairly small - the more efficient feedhorns are fairly
large, at least for 2.4 GHz if you have a .7 f/D DirecTV dish.

The simple answer is to place the phase center of your 2.4 GHz feed at
the focal point of your dish. What is the phase center of your 2.4 GHz
feed? Maybe this is the question you want to ask. If so, the answer is
probably here.

http://www.qsl.net/n1bwt/contents.htm

DaveC wrote:
On Sun, 5 Jun 2005 04:23:34 -0700, Russ Renaud wrote
(in article ):
No, I'm talking about replacing the feed with my own feed. "Adjustable" means
that depending on how much of the dish I want to illuminate, I can change the
length of the feed's waveguide to extend beyond the feed point as described
he

http://tinyurl.com/ar9es


What kind of horn are you planning to use for 2.4 GHz? DirecTV dishes
are usually fairly small - the more efficient feedhorns are fairly
large, at least for 2.4 GHz if you have a .7 f/D DirecTV dish.


I haven't decided, yet. Options are cantenna-type feed, or patch panel-type.


The simple answer is to place the phase center of your 2.4 GHz feed at
the focal point of your dish. What is the phase center of your 2.4 GHz
feed? Maybe this is the question you want to ask. If so, the answer is
probably here.


Well, this I think I know. I'm just trying to determine what the feed point
is for the *existing* feed. That gives me a point on the hardware to attempt
to duplicate with the new feed. This is different and easier than aiming for
a point in space (the dish's feed point).


http://www.qsl.net/n1bwt/contents.htm


Yes, a great resource. I discovered this last month. While it is my "bible"
for designing this project, it doesn't contain the answer to this question.

OK. I think I understand. Assuming you are using the DirecTv dish, and
you want to replace the DTV horn with your own horn or dish feed, the
'phase center' of the DTV horn is about .25 lambda from the mouth of the
horn.

So, you place the phase center of your replacement feed at the same
point as the DTV horn's phase center.

Again, assuming you are using the DirecTv dish, neither the cantenna
nor a patch antenna will provide ideal illumination for a DTV offset
dish, which has an f/D of around .7

If you are OK with linear polarization (is the antenna for WiFi use?),
I suspect a biquad would provide better illumination than a cantenna
or a patch. Not that the latter two wouldn't work, it's just they
aren't optimum for that f/D

Russ

"DaveC" wrote in message
news.net...
Thanks, Russ, for your observations.

If you are OK with linear polarization (is the antenna for WiFi use?),
I suspect a biquad would provide better illumination than a cantenna
or a patch. Not that the latter two wouldn't work, it's just they
aren't optimum for that f/D

Yes, WiFi. If need be, can't I just rotate the biquad feed by 90 deg. and
get
horiz. polarization?

Yeah, I see (via net search) that biquad fits nicely with a 0.7 f/D dish.

Thanks again,
--
Please, no "Go Google this" replies. I wouldn't
ask a question here if I hadn't done that already.

DaveC

This is an invalid return address
Please reply in the news group



Dave

Do you know of a web site where the offset dish f/D is defined? I am
familiar with the conventional (feed at the center) parabolic reflector f/D.
My question relates to my wondering if the TV satellite dish is a parabola
with the horn feed offset away from the focal point.

Jerry

DaveC wrote:
Thanks, Russ, for your observations.


If you are OK with linear polarization (is the antenna for WiFi use?),
I suspect a biquad would provide better illumination than a cantenna
or a patch. Not that the latter two wouldn't work, it's just they
aren't optimum for that f/D


Yes, WiFi. If need be, can't I just rotate the biquad feed by 90 deg. and get
horiz. polarization?

Yeah, I see (via net search) that biquad fits nicely with a 0.7 f/D dish.

Thanks again,

Yes. Just turn the biquad so the one quad is 'on top' of the lower quad
and you will have horizonal polarization. The pattern looks pretty
symmetrical in both the E and H planes

Best of luck

In article icHoe.9607$U_2.1496@trnddc06, "Jerry Martes"
wrote:

My question relates to my wondering if the TV satellite dish is a parabola
with the horn feed offset away from the focal point.

Jerry-

I understand there is a class of unsymmetric antenna where the feed is
actually at the focal point, but the reflecting surface is only a side
section of the whole parabola. Although it is physically "offset", it
functions as a parabolic antenna.

Fred

DaveC wrote:
On Sun, 5 Jun 2005 07:46:42 -0700, Russ Renaud wrote
(in article ):


So, you place the phase center of your replacement feed at the same
point as the DTV horn's phase center.


One more question:
For a bi-quad, is the phase center located at the plane of the element (the
bent wire)?

Chapter 6-2 shows the phase center of the NBS dual-dipole feed to be .25
wavelength behind the dipoles, and around the same for a quadloop feed.

I suspect the phase center for the biquad is something similar.