Question about dish's selectivity regarding reception of close satellites

[polgyver]

A few months ago I found interesting article by John Legon, mostly known as a mathematician researching Egyptian Pyramids.
John also is involved in satellite reception and devised some equations for finding parameters of unknown dishes.
I took a screenshot of part of his article.
What is puzzling for me is his statement that the left and right sides of a dish whose top and bottom were "truncated" for
making its shape more "modern" - that these sides enhance the dish's selectivity for receiving adjacent satellites.
I think that the only factor which determines the angle of a cone what the dish "sees" - is its focal length.
If the dish's focal length is relatively long, it will "see" just one bird. If its focal length is short, it will be vulnerable to adjacent satellites.
I am curious what other Members think about it?

 

[a33]

The phenomenon is called "beamwidth", and indeed seems to be dependent on width of the dish, not focal length.

An equation for beamwidth is e.g. here: Parabolic antenna - Wikipedia
I've once read an understandable explanation of it, but I don't recall where....

BTW John's article (from 2012 or thereabouts) is excellent. But he wasn't the first to discover the equation for the focal length of offset paraboloid dishes using the deepest depth as input; the french were earlier, I discovered.

Greetz,
A33

 

[mikekohl]

The phenomenon is called "beamwidth", and indeed seems to be dependent on width of the dish, not focal length.

An equation for beamwidth is e.g. here: Parabolic antenna - Wikipedia
I've once read an understandable explanation of it, but I don't recall where....

BTW John's article (from 2012 or thereabouts) is excellent. But he wasn't the first to discover the equation for the focal length of offset paraboloid dishes using the deepest depth as input; the french were earlier, I discovered.

Greetz,
A33

Here's an odd twist from the height of c-band terrestrial interference: study the placement of feedhorn scalar rings purposely slightly off the highest signal level, but also working in tandem with a perimeter screen mounted roughly perpendicular along the outer edge of the dish. Without an edge on the perimeter of the dish, interference from the side can enter the feedhorn or LNBF and sometimes completely wipe out desired signal. If you install a perimeter edge in this situation, it not only can block interfering signals from getting to sides of the focal point, but despite physically blocking a slightly larger outer surface area of the the antenna, you have reduced enough interference to result in a larger desired signal. Further playing with adjustable scalar rings and Chaparral GOLD RING add ons, or Deep Dish feeds such as those from ADL like their former RP-2 line (designed for 0.275 to 0.335 f/d ratios), can drop side interference further and result in more signal.
This is a 30 year old lesson that can be useful when encountering cell phone interference in the new C-band environment. In addition to, or instead of filtering, various screening methods can keep problem interference out of your dish's feed area. The lesson learned back then was that digital interference is sometimes impossible to filter out, and the better solution is to physically avoid it by careful placement and construction of dish sites. You might encounter situations where you wish to receive C-band frequencies on international satellites that may still be using frequencies that have been sold to cell phone companies in the USA. Such physical filtering could allow one to use "old" C-band feedhorns and LNBFs that may still cover down to 3400 MHz. A new feed or LNB/LNBF system has at least 500 MHz of spectrum above 3400 MHz filtered out, and it may not be capable of those lower frequencies, so it could be an alternate fix. Your mileage may vary.

 

[cyberham]

Your knowledge could be useful when I'm using my mini-BUD 1.2m dish with its wide beamwidth on C-band. Minimizing adjacent satellite interference is one way to improve reception and increase the number of receivable satellites.

Sent from my SM-G950W using Tapatalk

 

[polgyver]

seems to be dependent on width of the dish, not focal length.

Kindly, disagree...
When considering camera's lenses, things are obvious, that lenses with long focal length (telephoto) "see" much narrower field than wide-angle lenses (with short focal length).
Wikipedia, indeed, does not show in the equation any focal length.
I think there is a similarity to satellite reception : The focal cloud in front of a LNBF has certain size, dependent on frequency (for C-band - the wavelength of 4 GHz is about 7 to 8 cm or 3" : and for Ku-band the wavelength of 12 GHz is roughly 2,5 cm or 1"). It could be assumed that focal cloud for C-band could have diameter roughly 5 cm or 2" and for Ku-band - about 2 cm.
If we imagine a source of light of such diameter, located on LNBF "face" and shining towards dish, it seems logical that the angle of cone of light, reflected from the paraboloid, will depend on focal length. The farther the light from vortex, (the longer is the focal length) - the narrower the light cone.
cheers, polgyver

 

[a33]

Kindly, disagree...

Well, I guess you disagree with a lot of publications on the internet about satellite dish beamwidth, then.

The way I can understand it is as follows (though I by no means know if it is right or not):


First: Paraboloid mirrors have the purpose to sharply aim in one direction (parallel to symmetry axis of the (mother) parabola), I believe.
Spherical mirrors and lenses have the purpose to get a sharp image from everything at one distance, I believe.


Second: With lenses, there is the phenomenon of "depth of field" ("scherptediepte", in dutch). By changing the diafragm, to get a wider lens surface (aperture, dutch: "lens-opening"), you get the sharpness solely at the chosen distance, so the rest at other distances gets more fuzzy/blurred.
(And also more light comes through the lens!)

I think with a paraboloid mirror, a similar process takes place. The wider the dish surface, the sharper your focus is to the chosen direction, and the fuzzier the signal from outside that one direction becomes.
(And you also receive more satellite signal!)


Third: How do small focal length (wide-angle lens) and long focal length (tele-lens) come into this?
Well, for the aimed object there does not seem to be any difference in the resulting depth of field (DoF):

To make the comparison fair, I took two more shots. The first was taken at a focal length of 35mm and approximately 0.6m away from my focus point (still the eye of the nearest reindeer). For the second image, I moved the camera back, so it was 1.2m away from the subject. Then I zoomed into 70mm and framed the shot so that the head of the deer was approximately the same size and location as in the first shot. It turns out the DoF in both these images is the same.

Quoted from here: Understanding Depth of Field - A Beginner's Guide
I don't know how exactly to translate this quote to our satellite dish example, but I think the conclusion is relevant, and applicable to the parabolic dishes as well.


But I'm no expert on this. Or even worse: I am an absolute layman on this....
But I'm sometimes very interested in the (theoretical) background of these technical things...


Greetz,
A33

 

[polgyver]

But I'm sometimes very interested in the (theoretical) background of these technical things...

and so am I. Would be nice to have sort of "satellite reception primer" - something similar for Euclid's first geometry manual. Good source is writings of Paul Wade (can be found on Google).
My analogy of photo lenses was not finished, as I forgot to compare front face of LNBF to standard 24 mm x 36 mm film frame. Its diagonal is about 43 mm and it fits inside circle of 50 mm, and such a f=50 is a border between wide-angle lenses and narrow angle objectives.
Also my analogy is flawed as it compares to devices built of glass, since glass lenses introduce distortions, from which problems of "depth of sharp zone" follow...
Generally, a dish has 2 functions :
1. To concentrate, or condense, radiation falling on it to a much smaller circle which is LNBF "face" (this depends on ratio of surfaces)
2. To SELECT very narrow cone from wide angle of Clarke Belt (this depends on ratio between LNBF "face" dia to dish's focal length).
To illustrate my point, I "cooked up" an (annotated) sketch, below :

 

[Comptech]

I agre with your diagram, but won't work with sat. You left one important factor out. Polarity. The other sats on either side will have opposite polarity and hence interfere with the main one.

 

[Comptech]

Mike Kohl"s answer is the right one.

 

[Comptech]

I picked up a 12 foot paraclipse taken down. Have no intent of assembling it, but wanted the mesh just for this reason.

 

[a33]

The other sats on either side will have opposite polarity and hence interfere with the main one.

When you mean that the mirrored signal from the same satellite from the other side of the dish has a shifted phase, I can understand what you mean. Otherwise I don't; can you explain further?

I would guess that out of phase or more 'diffused' signal for "off-axis" satellite aiming would be greater for shorter focal length, than for longer focal length (assuming equal dish diameter).
That is because for a flatter dish (longer focal length), the parabolic curve is still rather like the spherical curve, so causing less 'off-axis parabolic aberration'.
That would be a counter-effect to what polgyver described above, I think.

But I have no idea how great these effects would be.

Greetz,
A33

 

[arlo]

When you mean that the mirrored signal from the same satellite from the other side of the dish has a shifted phase, I can understand what you mean. Otherwise I don't; can you explain further?

I would guess that out of phase or more 'diffused' signal for "off-axis" satellite aiming would be greater for shorter focal length, than for longer focal length (assuming equal dish diameter).
That is because for a flatter dish (longer focal length), the parabolic curve is still rather like the spherical curve, so causing less 'off-axis parabolic aberration'.
That would be a counter-effect to what polgyver described above, I think.

But I have no idea how great these effects would be.

Greetz,
A33

All of this rant and not a single mention of scalar theory. There are a gazillion Indian college papers posted on the 'net describing 'their' opinions of the function of them.
Going with the etched-in-stone instructions of how to locate them on a dish with f, fD...bla bla bla crap.
Well I went that route. on a 12 footer with button hook feed. C-Band with a side-car Ku LNBF (no scalar on that one).
At first my signal sucked. In and out, left and right with the C-Band LNBF for what seemed hours over the course of days.

Then I said screw it and started moving the scalar in and out in small increments. And then the LNBF.
The sweet spot for the scalar was significantly closer towards the dish center (in optical terms) than all of the science extrapolated.
I came to terms that my calculated distance made the scalar "over illuminate" the dish.
Yet further away from the sweet spot signal peaking on the LNBF was lower. Days I tell you.
And then Squeeze Box came on the radio. Hah.
It makes me wonder if guys with triangular dish edge support rods to mount the scalar in a fixed position, determined from hard calculations, don't realize that moving the scalar in or out slightly can give several more dB of signal.
I understand optics well. I understand Newtonian telescopes better. And I just happen to love photography and have the fixer stains to prove it. Yup. Homie still does film.
From the above. A parabola is a parabola. Period. Anything less (or more) and you're having to load up the shuttle and head out to Hubble.
I happen to have a "shallow focus" dish. The calculations for f and fD didn't mean squat. It got me in the ballpark.
December 2, 2021. And that's my rant

 

[a33]

The sweet spot for the scalar was significantly closer towards the dish center (in optical terms) than all of the science extrapolated.
I came to terms that my calculated distance made the scalar "over illuminate" the dish.

I would say the parabolic form does not lie. So I'd be (scientifically) interested whether you based your calculated distance on just ONE measurement of the parabolic shape, that is depth at the center of the dish, as many people do; or whether you based it on multiple measurements of the parabolic shape at various points, so (also) on depth measurements further away from the center?

It is quite possible that the central area of the dish has another focal length than the outer area; so that just using the central depth measure is not the best procedure to use.
Just a hypothesis, though. That's my rant.

How to test such a hypothesis? Two procedures to calculate focal length from multiple depth measurements at prime focus dishes are here: Calculating focal length of a PrimeFocus dish that has a hole in the middle
When the resulting focal length outcomes have more error than normal measuring error, the cause would likely be parabolic shape error. I guess not something to be happy with....

Greetz,
A33

 

[polgyver]

It is quite possible that the central area of the dish has another focal length than the outer area

For sure, this is the case if the dish is a part of a sphere (like, Arecibo - this info I got from Paul Wade articles). Not sure if there were trials to combine more paraboloids of rotation into one shape, though...
If there is just one paraboloid, it has only one focal spot.
This thread got enriched by more info about other reception problems and possible solutions, but still needs the conclusions about values-parameters needed to calculate Angle of View of a dish, Theta.
From Wikipedia, we need to input coefficient "lambda" (wavelength in cm), multiplied by somewhat arbitrary factor of "70" and all - divided by the dish diameter "d". I did not like that the formula omitted "f", the only factor defining parabola. Maybe it is somewhat contained in this "70" ?
As an alternative, I tried to use another formula :
Theta = arc tg (0.76 lambda / f
The coefficient 0.76 I found somewhere on Internet, it helps determine active focal cloud diameter in front of LNBF.
Both equations give very similar results.
My 4' Ariza dish has horizontal dia = 120 cm, and "f" = 72 cm.
From Wiki formula, it has Theta (Ku) = 1.46*, and for C-band Theta=4.37*
Using arc tg and "f", it is - respectively - 1.5* and 4.5*
A few photos/scans follow :

 

[a33]

For sure, this is the case if the dish is a part of a sphere. (....)
If there is just one paraboloid, it has only one focal spot.

But of course, I was mentioning this for a paraboloid dish that was intended as a paraboloid, but was manufactured badly.
In that case, just one depth measurement is not the way, to get a meaningful idea of where to put the feedhorn...

But, back on topic, as you wish...

Greetz,
A33

 

[arlo]

But of course, I was mentioning this for a paraboloid dish that was intended as a paraboloid, but was manufactured badly.
In that case, just one depth measurement is not the way, to get a meaningful idea of where to put the feedhorn...

But, back on topic, as you wish...

Greetz,
A33

99.99% of new people in the sat hobby who come across a dish for the taking may have all of the hardware to reassemble it on site.
Or in the case of the cheap ones tek2000 sells (sold) that have spotty at most and poorly translated instructions.
So where do you run to? Google a quick "how to" to find out.
Agreed?
You run string. You take measurements. You put the thing together. You might head out to the hardware store for hardware and conduit. Smash the ends in a vice and drill holes and bend a few angles.
Stick in your feed and hook it up and go signal searching.
And like me, you find the best signal and strengths and bolt the lnbf down and head inside.
Agreed? I think you will.
Not much is said about heading to the health food store for a bottle of einsteinium-Q10 first.
The calcs are based on a true parabola.
You do state some interesting reading. Above a guy like me for the most part.
All I'm getting at is that a person should experiment and veer from all of the hard calculations, engineer for a little wiggle room, and not assume anything. Especially when it comes to scalar placement. If you use one. In the case of a prime feed antenna. That 'should' be a true parabola.

 

[polgyver]

I think that the only factor which determines the angle of a cone what the dish "sees" - is its focal length.

This was my initial suggestion submitted in my original post. I asked for other Members' opinion, and they stated that the dish's selectivity depends on its diameter: the formula (equation) in Wikipedia for "angle of view" = theta, contains no focal length as a factor. I disagreed, but recently realized that I was only partially right:
The angle of view - indeed - depends ALSO on the dish's diameter, but - not exclusively: it depends on its focal length, too.
For explanation, the reverse process could be used : the LNB could be assumed as a source or radiation so the dish could truly ILLUMINATE the Clarke Belt, as if it were driven by B.U.C.
Using some idealization, first we can imagine that in the focal spot there is ideally small source of radiation, sort of geometrical, dimensionless point. If the paraboloid is also ideally correct, then the radiation reflected from dish would take shape of cylindrical beam, going to infinity, all rays - parallel.
In reality, there is no dimensionless spot - there is certain Focal Cloud, its size depending on used frequency of radiation.
I memorized from Internet article, that "conventional" diameter of focal cloud can be derived as a product of wavelength and certain factor or coefficient = 0.76. For 12 GHz, the wavelength = 25 mm, (1") thus FC diameter = 19 mm or 3/4".
If such a Focal Cloud radiates energy, then - due to its size - reflection from dish will be a cone, not a cylinder, as every point on dish's surface will reflect the energy according to laws of optics, and the radiation rays will spread out.
The farther the dish's reflecting spot from its Focal Cloud - the narrower the radiated cone.
Reversing the settings from radiation (illumination) to reception should also be true.
Thus John Legon was correct : wider parts of truncated dish increase its selectivity.
A sketch of drawing follows: