Asking for Advice for class project

[Agnapot]

Hi there everyone! I just signed up to this forum because you all seem like a knowledgable bunch and I had an idea that I needed some perspective on.

I'm a high school math teacher in Los Angeles and I was thinking about a project for my Pre-Calculus class. They are studying conic sections soon, and parabolas are a part of that. I wanted to give them some hands-on experience with what a parabola can do, with the idea of a focus, etc, and I thought it would be neat to have them build a satellite dish that could receive some FTA signals. Now, these are likely to be fairly crude dishes, and I'm not expecting quality signal, just something that can be pointed at a satellite and receive *some* signal, for that "Wow!" factor.

I have some schematics for some dishes, but basically I was wondering if there was a particular focal length that was necessary, or a particular dish depth, or if basically any dish will do that is big enough. I'm thinking between 80-160 centimeters, and I'm not sure if the Ku or C band would be a better choice, or which one would be more "forgiving." For example, would tin foil be fine as the reflector?

I was also going to purchase a few things for the project, so all the students had to make was the dish and to place a pole at the focal point that I could attach the LNB. These were what I was going to get, and again, any advice here would be most appreciated.

This KU band LNB: Orbital Tracker Universal LNB Single KU-Band .2 MPEG 4
and/or This C Band LNB: NEXspark C-Band LNB LNBF DMS BSC421 Single Digital Satellite HDTV FTA 13K 65db | eBay

This simple receiver: Smart Digital Satellite TV Receiver DVB-T2+DVB-S2 FTA 1080P Decoder Tuner MPEG4 691110811008 | eBay

And maybe this signal meter to test and to have as a backup incase the signal doesn't actually make TV happen so they can see *something* is happening: satellite-signal-meter-and-finder

So, do you think this would be a possible project? Or am I misjudging the challenges here? I have a while to put this project together, so there's time to test stuff out, but any advice would be greatly appreciated. Thank you!

 

[comfortably_numb]

I think its a wonderful idea, and I applaud you for considering it. All educational positives aside, I hope it maybe brings attention to satellite engineering to a whole new generation. I myself was in middle school when I started fiddling around with satellite.

I'll defer to the experts on your other questions

 

[ken2400]

Allow 3 weeks for delivery DVB-T2 - Wikipedia appears to be what the rest of the world uses for OTA?

 

[Titanium]

Welcome Agnapot

Sounds like a fun project. Might also place a microphone in place of the LNBF to demonstrate the focal point and importance of proper feed placement for collecting the reflected sound and beamwidth as the reflector is pointed towards and away from a sound source.

For the reflector size that you are planning to construct, I would recommend an easy to handle size of 70-100cm for KU-band. While C-band frequencies are more forgiving for surface accuracy and aiming, a much larger reflector would be needed. Foil smoothed over a non-reflective surface would work fine.

To use an off the shelf KU-band LNBF, the FD ratio should be in the .5-.6 range. Probably be easiest to construct a prime focus type rather than an offset, but maybe an extra points project for construction of an offset type in the typical range of 24-26 degrees would add an extra design and aiming challenge.

The dish would need to be supported rather than handheld to lock onto a satellite signal from 22,300 miles distance.

I would purchase a Maverick MK1 LNBF ($9-$14 on Ebay). This is a quality, single LO 10750 LNBF with great performance.

The receiver should be fine for experimentation. Might be nice to have a brand and model that some of us have worked with before in the event that you need assistance in setting the menu parameters.

A word of caution... Receivers shipped from China usually are defaulted to a 50hz output frequency and a resolution that North American TVs often do not display. They often do not include North American visible satellite lists. If you purchase this model, be prepared to set-up the receiver and manually input satellites and transponders. This just adds one more level of difficulty if you don't have previous FTA set-up experience. Would be good to enquire with any seller if the STB default is 60hz and if it includes a North American satellite and current transponder lists.

The meter will give a good indication of when the dish is aimed at any signal source, but not useful for identifying or optimizing. Use the meters in the STB installation menu to identify the satellite and optimize the feedpoint FL (Focal Length) and skew polarity settings.

Goid luck and we are here to help you be successful with your project!

 

[. Raine]

If you use shiny foil for the reflector, you should probably paint it to make it not reflect sunlight. A shiny surface will focus the sun's rays and burn whatever is in the focal point.

Great project and welcome to Satellite Guys!

 

[primestar31]

I would ask any of your students if they or their parents have an older FTA satellite receiver or equipment in the closet, that they could donate to this project. That could save a few dollars.

If so, they are likely older units that only receive DVB-S signals, but you could aim at 97w, and there's literally HUNDREDS of open DVB-S KU channels up there!

 

[comfortably_numb]

you could aim at 97w, and there's literally HUNDREDS of open DVB-S KU channels up there!

And most of them are very high power and easy to tune

 

[raydio]

We have a good family friend here in the Albuquerque area who is a school teacher and he is constantly spending his own money for various supplies and other things for his classes and students. I applaud teachers these days, one for being in the profession at all because there are other jobs that pay way more, and two for being selfless by spending their own money for the benefit of their students. If you like, I would like to donate a FTA receiver to your effort. It is certainly not the best, but is setup with NA satellite list and is also setup for a standard Ku band LNBF.

If this is something that you are interested in, PM me and I will ship to you no charge.

This satellite receiver requires a KU or C Band dish installed for it to work. Includes remote control that can be programmed to turn TV on/off. Our High Definition DVB S2 satellite receiver is designed for North America and set to NTSC and 1080p capable. It has fast, accurate, blind scan. Receive hundreds of Free To Air channels and supports a maximum of 5000 channels. Remote control, batteries, HDMI cable, RCA Cables, and user manual are included. Ultra-Connectivity Includes an RF output for connecting to older style televisions. The RF output is also good for watching TV in two rooms with one receiver. Simply run a coaxial cable from the RF output to the other room and watch TV on channel 3. Use the included HDMI cable for the best output, up to 1080P. The unit also has an optical audio output for use with HIFI sound systems. Connect a USB thumb drive and start recording live TV or schedule an event to record so you can watch it later. With a thumb drive you can pause live tv and resume later. It is also useful to back up your satellite settings and channel list.

 

[comfortably_numb]

I'd be willing to donate an LNB. I have an extra Maverick in the closet you can have

 

[a33]

To use an off the shelf KU-band LNBF, the FD ratio should be in the .5-.6 range. Probably be easiest to construct a prime focus type rather than an offset, but maybe an extra points project for construction of an offset type in the typical range of 24-26 degrees would add an extra design and aiming challenge.

I believe an LNBf for Ku-band is usually made for offset dishes, with 'typical' illumination angle of about 70 degrees and f/D-ratio of about 0.6; however... this f/D ratio uses focal distance of mother PrimeFocus dish, and width of offset dish, so the meaning of that value is not directly clear.
See for instance this post and thread: Inverto black ultra feedhorn .. F/d ?? - SatsUK

So the "f/D"of a Ku-LNB would not be a directly usefull value, as the f/D for C-band feedhorns is: you'd best focus on the illumination angle.
It seems (see the above link) the Inverto Black Ultra has a illumination angle of 84 degrees. (The "standard" for Ku-dishes and LNBs is not very standard, it seems...)

Point of this being: You should design the dish (width and depth) so that it matches the illumination angle of the LNB!

Nice parabola calculations are needed for that... (I did some studying on parabola calculations recently, for making a satellite dish specs calculator. Interesting stuff!)

Greetz,
A33

 

[Titanium]

Point of this being: You should design the dish (width and depth) so that it matches the illumination angle of the LNB!

I proposed the off-the shelf KU LNBF for the prime focus mount as it will be much less expensive and the performance loss of an offset feed will be negligible for this proof of concept experiment. I would agree that a proper feed would be used to optimize the system, but this experiment is using a free or $20 receiver, $10 LNBF, $6 meter and aluminum foil...

 

[a33]

I would agree that a proper feed would be used to optimize the system ...

Do we misunderstand each other?
I don't say that they should acquire an LNB with a proper illumination angle.
My comment was meant exactly the other way round: If you have an LNB with known illumination angle, build a dish to that illumination angle.

It's not so difficult with the parabola equations, to design a specific f/D (and thus specifying illumination angle).
Then, if precision of the school-built parabolic mirror might be an issue, at least the subtended angle fits, and doesn't cause extra issues!

For the rest I can support your idea of a prime focus dish, for Ku.
However, I just saw this: FTApinamar: Antenas Offset (1)

Greetz,
A33

 

[SatPhreak]

I built a 100cm dish about 10-11years ago. It was a100 cm prime focus dish with a f/D of 0.6 to match a standard Ku lnbf. It was made of 12 segments out of plywood. It was covered in steel window screening. It did get a couple of transponders on 97w, but not very well. I would use 16, or 20 segments to get a better parabola. I would also cover the segments in plywood covered in tin foil to get a better signal than steel window screen. Maybe even a 120 cm dish would be better(the bigger the more signal).

If I can find the website the dish building instructions were posted on I'll post it,

Sent from my P00A using the SatelliteGuys app!

 

[SatPhreak]

The forum was vetrun .aus but they are no more. From what I found they suffered a a irreplaceable hard drive failure and are no longer around.

Sent from my P00A using the SatelliteGuys app!

 

[a33]

The forum was vetrun .aus but they are no more.

Was this one also on vetrun? Scratchbuilt Wooden Offset Satellite Dish
(There's also a topic about that on satelliteguys.)
BTW: They used Parabola Calculator version 2.0, it seems. The calculator-part for (wifi-)offsetdishes has as (incorrect) assumption, that the vertex is always exactly at the bottom of the dish .(They wrongfully mention that as 'result' of the calculation, instead of as assumption. Aouch!)
So the focal point calculation often isn't right, you'll have to use another calculator for that!

Greetz,
A33

 

[SatPhreak]

Was this one also on vetrun? Scratchbuilt Wooden Offset Satellite Dish
(There's also a topic about that on satelliteguys.)
BTW: They used Parabola Calculator version 2.0, it seems. The calculator-part for (wifi-)offsetdishes has as (incorrect) assumption, that the vertex is always exactly at the bottom of the dish .(They wrongfully mention that as 'result' of the calculation, instead of as assumption. Aouch!)
So the focal point calculation often isn't right, you'll have to use another calculator for that!

Greetz,
A33

Yeah that looks similar, but newer. I built mine in the summer of 2008.

Sent from my LG-M153 using the SatelliteGuys app!

 

[Agnapot]

Hey All! This is super helpful information, thank you! And thanks for your warm welcomes.

It sounds like this would be a feasible project, so I think I'm going to start writing it up. I'll share what I make here for the curious!

A couple things:

1. For those offering to donate stuff, THANK YOU! I will be messaging you shortly.

2. Am I correct in thinking that, for an f/D of 0.6 this would mean, for a 100cm diameter dish the focus would be at 60cm above the vertex? So, parabola formula is y = 1/4p x^2 and p would be 60 for the focal length, so y = 1/240 x^2 as a reasonable formula? Basically multiply the diameter by .6 to get the p aka focal length? Or is this oversimplifying? (This is for a prime focus dish, I'm pondering an offset option, but have to look into the challenges for the kiddos)

 

[a33]

Am I correct in thinking that, for an f/D of 0.6 this would mean, for a 100cm diameter dish the focus would be at 60cm above the vertex? So, parabola formula is y = 1/4p x^2 and p would be 60 for the focal length, so y = 1/240 x^2 as a reasonable formula? Basically multiply the diameter by .6 to get the p aka focal length?

You've got three YES's .
Though personally I would choose something like f/D=0.65, matching the illumination angle of 84* of he Inverto IBU LNB. Alas, the illumination angles of Ku-LNBs are rarely given in the specs.

Or is this oversimplifying?

No, it's not. The calculations are not that difficult; the practice is. I guess for a Ku-dish, you need to focus on millimeter precision.
But I've no experience with that, myself.

(This is for a prime focus dish, I'm pondering an offset option, but have to look into the challenges for the kiddos)

Some thoughts:
If you're planning to build the dish on 'ribs', for a PF dish you'd need ribs for 360*, for an offset you'd need longer ribs, but fewer (assuming you center the ribs at the vertex).
For calculation of an offset dish, it is easiest to calculate for PF dish, but build only part of it. That way you don't get the typical elliptical form of an offset dish, but that doesn't really matter for its function (In Italy many offset dishes have the form of a diamond/square that sits on a corner).
Remember that on a PF dish, the LNB is exactly in the middle of the signal way, so taking away signal. And an LNB takes most of its signal from the middle part of the dish, less at the sides of its illumination angle.
A diameter of 100 cm or 110 cm or 120 cm really makes a difference in reflecting surface. More surface means you need more material, but the amount of work often doesn't increase likewise, when you are at it..

But, of course, these are just thoughts, from behind a desk.

Greetz,
A33

 

[a33]

BTW,
And if you want an offset dish (with vertex at bottom of offset dish) with illumination angle of 84*, so f/D-equivalent of 0.65, you'd have to do some trigonometric calculations.

But those will be a piece of cake for you and your class, I guess, when you are already learning about conic sections .

greetz,
A33