A good choice for a antenna , the ANT751.
A good aim direction , 131 degree magnetic compass direction.
In between DC and Baltimore.
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For , WRC-DT REAL Digital UHF channel 48 , virtual number (4.1) NBC.
Most likely the situation is multipath , reflected reception off of trees and other objects including your own roof and house.
Try aiming the ANT751 antenna more towards DC or more towards Baltimore.
Can also raise or lower the ANT751 antenna.
Can also change the location of the ANT751 antenna.
Amplifiers will amplify multipath and can make reception worse.
As a Test also try removing the Preamplifier , disconnect the preamplifier , disconnect the power injector , disconnect the power supply , so that non of the preamplifier , power injector , power supply , is connected in any way.
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Digital Broadcast Tv Tuners can develop - Digital Glitches - that are not cleared out with a simple channel scan.
Do a Double Rescan.
http://www.dtv.fcc/rescan.html.
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If not able to lock in reception , then try this antenna , Clear Stream 2 Complete Vhf/Uhf antenna ,
http://www.antennasdirect.com/store/...VHF-Combo.html.
Aimed at a starting aim direction of 131 degree magnetic compass direction.
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As always , trees and tree leaves do a good job of , reflecting/multipath , absorbing , blocking , Digital Tv reception and so do buildings and other obstructions including your own roof and house.
It is best to install the antenna at a location that has the least amount to no amount of obstructions of any type or kind in the directions of reception including your own roof and house.
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Here are some places to buy antennas and etc. ,
http://www.solidsignal.com ,
http://www.amazon.com.