EXPERIMENTING WITH OLD SATELLITE DISH

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macboney

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I am experimenting with reusing an old satellite dish to make a directional antenna for monitoring an 800 mhz P25 system. I live within 6 miles of three different tower sited for the same system and about 15 miles from a fourth. I get all kinds of gabled audio that I am guessing is coming from receiving two or more sites just milliseconds apart. (forgot the term) I am trying to get to the point of picking up only one transmitter site. I carefully measured the location of the center of 3 LNB so I could place an antenna at the same position. I then cut the LNB's off and a portion of the base they were mounted on. I then made a bracket to accept an NMO mount and placed an 800 mhz whip antenna on the bracket in the same location as the center LNB. I will drill completely throught the mouinting beam and run the feedline (LMR400) to the antenna and carry it below the mounting beam. Anyone have any ideas or suggestions good or bad? This is an experiment but if it works I will use it.
 

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prcguy

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In my experience you can spend a lot of good time working on this for little to no benefit. When you say 3 LNB it sounds like either an old DirecTV "Phase 3" Ku dish or a newer Slimline which usually has 5 LNBs and and designed for 5 Ku/Ka orbital slots covering 20 degrees of span in the sky. If so these were designed for much higher frequencies and the gain at 800Mhz will not be much. If its a Slimline dish only a portion of the dish is usable for one frequency since its designed to look at multiple locations at the same time with different feed positions.

If you did want to make it 800MHz and live with the small gain you probably want to use a two element beam pointing into the dish and not a whip. Feed placement is extremely critical and without some knowledge and equipment to test for gain and other things it will probably be just some metal in the air that won't do much.

I am experimenting with reusing an old satellite dish to make a directional antenna for monitoring an 800 mhz P25 system. I live within 6 miles of three different tower sited for the same system and about 15 miles from a fourth. I get all kinds of gabled audio that I am guessing is coming from receiving two or more sites just milliseconds apart. (forgot the term) I am trying to get to the point of picking up only one transmitter site. I carefully measured the location of the center of 3 LNB so I could place an antenna at the same position. I then cut the LNB's off and a portion of the base they were mounted on. I then made a bracket to accept an NMO mount and placed an 800 mhz whip antenna on the bracket in the same location as the center LNB. I will drill completely throught the mouinting beam and run the feedline (LMR400) to the antenna and carry it below the mounting beam. Anyone have any ideas or suggestions good or bad? This is an experiment but if it works I will use it. I tried uploading a picture but the file is too big for the server.
 

Ubbe

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In a sat LNB there's a tiny dipole antenna at the focus of the dish. When you are trying to use a 800Mhz antenna you should try to point the dish at a tower and then move the antenna both in depth and horisontal and vertically and also point the antenna up or to the side or in to the center of the dish. Having you arm in the way doesn't matter much to the signal hunting.

The signal bars and signal strenght indicators in the display are slow to update and it's much easier if you listen to the raw analog signal when trying to find the maximum signal level. Easier to do with a weak signal with noise in it, and you could also set a scanner to attenuator to reduce the signal level.

/Ubbe
 

macboney

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In my experience you can spend a lot of good time working on this for little to no benefit. When you say 3 LNB it sounds like either an old DirecTV "Phase 3" Ku dish or a newer Slimline which usually has 5 LNBs and and designed for 5 Ku/Ka orbital slots covering 20 degrees of span in the sky. If so these were designed for much higher frequencies and the gain at 800Mhz will not be much. If its a Slimline dish only a portion of the dish is usable for one frequency since its designed to look at multiple locations at the same time with different feed positions.

If you did want to make it 800MHz and live with the small gain you probably want to use a two element beam pointing into the dish and not a whip. Feed placement is extremely critical and without some knowledge and equipment to test for gain and other things it will probably be just some metal in the air that won't do much.
I am not after gain so much as directional receiving. I know a yagi will work, I just happen to have the pieces of what I am trying on hand. Don't want to spend the $$ for a yagi if I can help it. It is a Direct TV dish. Not sure how old it is, it was here when I bought the house a little over a year ago. Going by oxidized paint, it had been there for quite some time. Thanks for your reply. Will keep it in mind. If it doesn't work, I am mostly out of a little time. When you are retired like me, some things aren't a waste of time since I like to see what I can come up with. In many cases I have more time than $$$. Plus I like to bug the neighbors with what I "might" be doing. I do have a directional cell phone radome antenna that has an 850 +- element in it. IO might try that in place of the dish. Will be less wind load and weight up on a mast.
 
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prcguy

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You can easily spend a couple hundred $$ in labor on a project like this and a $20 Yagi off Fleabay will work much better.

I am not after gain so much as directional receiving. I know a yagi will work, I just happen to have the pieces of what I am trying on hand. Don't want to spend the $$ for a yagi if I can help it. It is a Direct TV dish. Not sure how old it is, it was here when I bought the house a little over a year ago. Going by oxidized paint, it had been there for quite some time. Thanks for your reply. Will keep it in mind. If it doesn't work, I am mostly out of a little time. When you are retired like me, some things aren't a waste of time since I like to see what I can come up with. In many cases I have more time than $$$.
 

macboney

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Thanks for the replies everyone. After experimenting with the "dish" I decided to back up and regroup. I am after a fairly compact with low wind load directional antenna. I found an antenna at 806-896 MHz Grid parabolic Antenna 12 dBi - Excel Wireless that should work. Attached is a picture as well. By being a grid it is much lighter, smaller and less wind load than a large yagi. It is 18" x 16" x 8" and made from aluminum. I was trying not sto spend much money but this looks like an ideal option for what I am attempting, which is directional use to get away from receiving 2 P25 antenna sites at the same time. Thanks again for your replies.
 

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prcguy

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Generally speaking, that type antenna or a corner reflector will have no better front to back ratio than a Yagi of similar gain. To me it looks to have more wind load than a Yagi of similar gain. You can get 800MHz Yagis fairly cheap and here is one with similar gain for $22 including shipping.


Thanks for the replies everyone. After experimenting with the "dish" I decided to back up and regroup. I am after a fairly compact with low wind load directional antenna. I found an antenna at 806-896 MHz Grid parabolic Antenna 12 dBi - Excel Wireless that should work. Attached is a picture as well. By being a grid it is much lighter, smaller and less wind load than a large yagi. It is 18" x 16" x 8" and made from aluminum. I was trying not sto spend much money but this looks like an ideal option for what I am attempting, which is directional use to get away from receiving 2 P25 antenna sites at the same time. Thanks again for your replies.
 

N9JIG

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I am all about experimenting with stuff on hand but I gotta go with the Yagi suggestions.

Also consider a "UHF" directional TV antenna, these are usually very good on 800 MHz., just mount them vertically instead of horizontally. These are often just horizontally mounted Yagis. They can be had dirt cheap as well.

A Yagi will have less wind loading issues than a corner reflector and is easier to aim visually.

We did some easy alignment of a PtoP link using Google Maps a few years back. We drew out our line on the map and looked for landmarks to aim at as we could not pick out the target tower visually from the source tower. We found a church steeple was right on the line so pointed it at the steeple and voila!
 
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