Film-Tech Forum ARCHIVE: Thinkering with an old satellite dish.

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Posted by Randy Stankey (Member # 64) on 05-20-2005, 11:38 PM:

Okay, I've got an old "The Dish" satellite dish that somebody threw out lying around my garage. (Don't ask me why. I just do. [Wink]

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I've got some old, broken microphones from work that were going to be thrown out. The connectors and screens are all trashed up but the elements inside. are still good. We got some grant money and bought all new mics. These ones were lying around in the bottom of a drawer until, finally, they got tossed in the can. I scooped them up and they were lying around in my garage too. (You see a pattern developing here... Don't you? [Wink]

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So... I get a little energy and decide to tinker with something. ... "What should I fuck with today?" How about I take the elements out of those old microphones then rip out the LNBs from the satellite dish and put the mics in their place?

That's just what I did.

Behold! My latest creation! The Super Snooper 500!

I hooked it up to the portable sound system I borrowed from work. It works pretty well. However, it's not quite as selective as other parabolic shotgun mics I have seen. I'm thinking its because of the design of the dish.

If I remember my math correctly, shouldn't the focus of the parabola be where the pink star is?

So... This means I'm going to have to modify the dish. Doesn't it? (I'm already thinking about a way to modify it that doesn't seem too difficult.)

Why was the dish designed this way in the first place? Shouldn't the LNBs be at the focus of the parabola for best efficiency? Or, did they make this sacrifice in design so that the "shadow" of the LNBs wouldn't block those precious few bits of signal, thus allowing them to squeeze out a few more decimal points worth of gain?

Posted by Paul Mayer (Member # 355) on 05-21-2005, 12:56 AM:

The look angles on those "pizza pan" dishes are designed to be asymmetric. Run a beam trace from the LNB focus point and you'll see how the dish needs to be aimed.

I suppose LNB shadowing could be a reason for designing the dish this way. Another issue is to keep the dish near vertical when it is aimed at typical geosynchronous satellite look angles. Doing so minimizes snow buildup that would otherwise occur at the bottom of the dish. Said snow attenuates Ku-band signals quite well. Bigger professional dishes use the prime focus as in your drawing instead of an offset and have heaters installed to deal with any snow or ice buildup.

Posted by Randy Stankey (Member # 64) on 05-21-2005, 11:27 AM:

Hmmm... So this dish is designed to "bank shot" the signal?

Would it be better for me to get some 1/4"-20 threaded rod, etc. and remount the dish so as to aim it downward and put the mics in the sweet spot? Or should I just aim the dish lower than I think and take advantage of the dish's original design?

Posted by Bruce Hansen (Member # 281) on 05-21-2005, 07:21 PM:

Those DBS dishs are not ment to reflect sound. They are for 12 GHz RF, and are just a piece of a parabola.

Posted by Randy Stankey (Member # 64) on 05-21-2005, 11:10 PM:

I'm sure you're right, Bruce.

Even when I first looked at the thing before putting it all together I had some doubts about the shape of the dish.

It's hard to explain, sometimes, that the value of a project like this is in the act of performing the experiment. The judgement of success or failure doesn't necessarily hinge on whether the outcome is completely successful. The project is successful when you discover something or learn something you didn't know before.

As it stands right now, the only thing this project has cost me is a can of Krylon spray paint. (Which has been sitting on the shelf of my garage since last summer.) I've spent a total of two evenings working on it.

Right now, the thing works well enough to demonstrate the principle. It's not as selective as I want it. This, I'm sure, is because of the design of the dish. Just as you say, Bruce.

This evening, I took it outside again and aimed the front of the dish way down below where one would intuitively think the dish should be aimed. Yes, it DOES seem to use the "bank shot" principle. I was able to get it to work a bit better than I did yesterday simply by re-aiming the dish.

Now, I am thinking about whether I can find the focus in the center of the dish. If I can, I think I'll try remounting the microphones and/or the dish to see if I can improve its operation even more.

Posted by Bruce Hansen (Member # 281) on 05-22-2005, 06:14 PM:

Randy said:

quote: Randy Stankey
The project is successful when you discover something or learn something you didn't know before.

When Edison was trying to make a working light bulb, he had many failures before finding something that worked. After one of the failures someone said something about that experiment being a failure, Edison responded that it was not a failure, because it showed him another thing that would not work, which brings him closer to figuring out what will work.

Tinkering is fun, and often you end up learning something. Keep it up [thumbsup]

and then have a beer afterwards. [beer]

Posted by Dieter Depypere (Member # 3184) on 05-30-2005, 03:00 PM:

Cool stuff! Also wanna have one. But I think I'll rather watch TV with the satellite dish.
Nevertheless a good idea. [beer]

[Um... would this work backwards too? If there were speakers instead of micros? Would this be a cool sound system? Gonna try....]

Posted by Josh Jones (Member # 447) on 05-30-2005, 05:19 PM:

Actually, KU band is around 3.5-4.5 GHz. A dish will focus whatever energy at its focal point. This applies to light, radio frequency energy, and sound. I know of some net geeks that rebuild the feedhorns in these dishes and use them to pick up free wireless internet, courtesy of starbucks [Wink]

Posted by Bruce Hansen (Member # 281) on 05-30-2005, 06:23 PM:

C band is 4Ghz
Ku band is 12Ghz

BTW, the paint (or other coating) on an RF dish may not reflect sound as well as RF(and could absorb some sound).

Posted by David Stambaugh (Member # 1102) on 05-30-2005, 06:27 PM:

As of about 6 months ago, around here Starbucks' wireless Internet service was through T-Mobile and it wasn't for free. Has that changed? I admit I haven't checked it out recently.

Posted by Josh Jones (Member # 447) on 05-30-2005, 08:58 PM:

Spin it around the horizon, and you'll find something. It may be your neighbor's wireless lan. I was merely using coffee shops as an example, not a give all end all. I thought KU was lower than that, oops. Although that would explain why C band stuff is relatively unaffected by hail storms, but will wipe out KU material, unless you have a 12' dish [Wink]

Posted by Randy Stankey (Member # 64) on 05-30-2005, 10:47 PM:

I thought about the dish's surface in terms of sound reflectiveness.

The original paint was that rough, grey stuff that seems to come on all "The Dish" parabolics. I sanded it down with emery cloth so that it wasn't so rough and to prep for the Krylon I wanted to spray on it. (Krylon "Hi-Temp" barbecue grill paint.)

The surface isn't as smooth as it could be but it's a lot smoother than the original. It's a damn-sight cooler looking too! [Wink]

At this point I don't think the surface of th dish will be a problem reflecting sound.

My main concern is still getting the microphones at the main/center focus of the parabola.

A) I suppose I could do a lot of math and calculate where the focus should be.

B) I could paint the dish shiny silver and put it out in the sun. The point where a piece of newspaper starts to scorch and burn would be the focus point.

C) I could just use trial and error.

Truthfully, I think I'll just use "C". The only thing I have to do is figure out a way to mount the dish on its bracket or modify the position of the LNB(Microphone) so that they are in the focus zone.

Another thing is that the microphones are the bare elements from inside a regular stage microphone. Your garden variety dynamic microphone element. They are not very selective. I think I'm going to have to shield them so that sound from the sides can't hit the diaphragms diretly. A plastic "dog collar" around the mics, made from an empty Solo cup with the bottom cut out should do the trick. That, however, comes after I have found the central focus point.

Here's what I'm thinking on that front:

The LNBs are likely centered on the X axis of the dish but have been moved down the Y axis to take advantage of the "Bank Shot" effect. I THINK it would just be a matter of tilting the dish down on its mount so as to center the LNBs in the focus point. I may have to lengthen the LNB bracket as well.

I take the dish off its mount and put some 1/4-20 barrell couplers on the bolts. On the bottom, I put a couple of hinges then mount them on the bracket so the dish swivels downward. Then I get a couple pieces of 1/4-20 threaded rod and some nuts/washers. I lenthen the top bolts enough to swivel the dish down. Some lock washers and nuts will hold the thing in place.

The right diameter of PVC pipe will make the LNB arm "telescope" as needed. The afformentioned Solo cups go around the mic elements.

When I get the time to do it, I'll let you know how it comes out.

PS: I think satellite TV is cool but the idea of PAYING for television just creeps me out. I will probably never get "The Dish" or any other subscription TV service. Cable TV included unless it's just "Basic Cable". If I had the money to set up a real dish or modify a "The Dish" setup I might go for "Free-to-Air" satellite but that's about the extent of it.

The dish I used was on its way to the landfill when I grabbed it. (Hooray for me for recycling! [Wink]

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