# Using Pre-amp with 811 and duplexer



## DanC-P (Jan 20, 2005)

Hi,

My 811 arrives tomorrow and I am trying to figure out what I can do to maximize the number of OTA channels I can pull in. I have an old UHF antenna on the roof that the satellite guy who came to install my 61.5 dish set up to work on the same cable as the satellite dish using a duplexer. I don't really get how the duplexer set-up works... My question is this -- if the antenna isn't getting a strong signal, can I use a pre-amp between the connection to the 811 and the duplexer in my room, or could that screw up my dish reception? I hope that makes sense...

Thanks in advance for your help!

Dan


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## Jason Nipp (Jun 10, 2004)

Makes perfect sense Dan, and hey welcome to DBSTalk.

As far as Diplexing with a preamp installed, sure it will work and I used to have my OTA setup this way. I did change that configuration though. I hold an opinion that it is best to leave out the extra devices to avoid insertion losses and failure potentials. Your pretty much in the city and I live in the boonies, so I would think your signals would be strong. But to maximize your reception I would make sure your antenna is in decent shape and that the cables are in reasonable shape with no corrosion or cracking. Check the fittings... is it 200 or 75 ohm, I would make sure it is matched to the preamp.

Hope this helps,


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## DanC-P (Jan 20, 2005)

Thanks Jason -- forgive my newbieness but I'm not sure what you mean by "fittings" and how I can determine the "ohms".

Thanks!

Dan


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## Jason Nipp (Jun 10, 2004)

DanC-P said:


> Thanks Jason -- forgive my newbieness but I'm not sure what you mean by "fittings" and how I can determine the "ohms".
> 
> Thanks!
> 
> Dan


Fittings are pretty much your coonection points...for wiring, such as an RG-59 coax connection.

200 vs 75 ohm....Does your antenna bring a coaxial cable connection into your house 75ohm or does it bring a flat 2 wire ribbon wire into your house 200 ohm. Now I am talking delivery method, not origination.


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## DanC-P (Jan 20, 2005)

With the diplexer set-up that I have both satellite and antenna come into the house on an RG59 coax cable, which is then split into two RG59 cables at the diplexer. So that means I am 75 ohm?


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## Jason Nipp (Jun 10, 2004)

DanC-P said:


> With the diplexer set-up that I have both satellite and antenna come into the house on an RG59 coax cable, which is then split into two RG59 cables at the diplexer. So that means I am 75 ohm?


Correct. But let me make the suggestion of replacing the existing single RG59 run with a RG6u run that is swept up to 2.3GHz. This grade of cable is reccomended for DBS (satellite) applications.


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## KKlare (Sep 24, 2004)

The impedance of "free space" or wires well separated is 277 ohms (from memory, a long time ago.) To match that in the old days twin lead was used and is actually less than the stated 300 ohms as it would have to be less than 277 due to the dielectric.

Coaxial cables typically range from 25 to 125 ohms with 50 (RG58) and 75 (RG59) popular. The impedance depends on the log of the ratio of outer and inner diameters and on the dielectric constant of the filler. Higher impedances typically have less dielectric loss but more resistive loss. The least dielectric loss is for vacuum or air, the most for solid polyethylene, and foam is in between. RG-59 is solid poly and thus not as good for high frequencies as RG-6, which is also 1/4 inch with foam and thus a larger center conductor to compensate and is OK up to a few hundred feet. RG-11 is 1/2 inch cable with 75-ohm impedance and thus 4 times the center conduction. The greater wire thickness is why it is harder to bend. Never kink any coax.

Impedances of 75 and 300 are easily matched with a two-turn transformer. At the same time it changes the balanced 300 antenna twin-lead into an unbalanced 75 ohm coax.

Hope most of that is correct.
-Ken


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## Jason Nipp (Jun 10, 2004)

Good stuff Ken, good stuff.


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