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I have an old Yagi mounted on the roof and it does a pretty decent job of pulling in the DTV signals. Some channels, however, are spotty, with occasional drop-outs and I would like to use a preamp. The cable run from the antenna to the receiver is a 22AWG, 300 ohm Foam Twin Lead (the flat kind!). I have a 75 ohm adapter hooked to the end of it and the adapter is plugged into the receiver.

How can I hook-up a preamp with this setup?
 

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The preamp should go close to the antenna, otherwise you're amplifying all the noise that you pick up in your downlead. You can get a preamp with 300 ohm input. While you're doing this I would change your downlead to RG-6. A good preamp is going to have a 75 ohm output anyway. The RG-6 will give you a stronger signal and be less prone to picking up stray signals on the way down.
 

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If your antenna installation is very old, you should probably be changing the downlead anyway. Though twinlead has less attenuation initially, it deteriorates rather rapidly. UV, dirt, rain, and pollutants will cause the twinlead to need replacement every 3 or 4 years.


I think I would upgrade the whole system. Placing a preamp close to the STB would probably be pointless. The advantage to a preamp is that when placed at the antenna, the signal will be amplified before going through the loss associated with the feedline. Noise figure is the important spec. The noise figure of your STB is probably about as good as most preamps, so placing one there would not help, it would only open up the possibility of overload.


Bob Smith
 

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A point of correction: 75 ohm RG-6 does NOT maintain signal strength as well as 300 ohm twinlead. However, its greater loss isn't usually an issue (especially if the signal will be amplified at the antenna), and its coaxial nature renders RG-6 essentially immune to induced RF interferrence. I agree, go for the coax. I'd also consider replacing the antenna since "old" implies the possibility of terminal corrosion and attendant dropouts in a breeze. At the very least, maybe unscrew the wing nuts and hit the threads with a wire brush to expose bright metal. If you're lucky, maybe the manufacturer went to the expense of stainless steel or galvanized connections, but don't count on it. Rust on the antenna terminals is often evident after a year or less. If your reception is generally good on all the digital stations, don't go overboard with amplifier gain. You could end up at cross purposes with yourself by overloading your receiver's input stage. Too much signal strength will kill reception just as effectively as too little signal strength. Finally, if the dropouts are random and not wind related, they may be due to intermittant multipath when aircraft, large trucks, etc. happen to reflect an echo signal to your antenna. If this is the case, no amplifier will totally compensate.
 

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twin lead works great

as long as it is twisted every few feet



------------------

Studio Broadcast Engineer

KET
 

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Thanks for the correction. I always thought 300 ohm had higher loss than the rg-6. Live and learn
 
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