Here is what you need to know to get the most optimal setup from broadcast tower to your TV:
Broadcast Towers: In a local market it is easy to assume all the channels come from the same location. This is not always the case. Just because a channel is considered to be a major network it does not mean it’s at the same location as other towers, the same distance away, or operating at the same broadcast power. Go to TV Fool, put in your address and see where all the towers are. It will help you to optimally point your antenna.
Antenna: Signals in the USA and Canada have all been switched from analog to digital. Digital signals are line of sight, meaning the antenna needs to face the broadcast tower. On paper the signal travels in a straight line, but it’s often reflected by objects such as trees, buildings, and hills and also atmospheric conditions. Take that into account when pointing your antenna. Also, your attic is considered an obstacle.
Cables: For digital signals you should be using RG6 coax cable. If the cable run comes close to electrical wires or appliances then you should use Shielded RG6. A big mistake some homeowners make is using the old RG59 cabling originally installed by a cable TV company.
Splitters/Combiners: When you introduce a splitter into the coax cable you will lose 3.5 decibels of gain. Basically if you have a 100% signal the two outputs will only have 50% signal each. Also, when using splitters it is recommended that the system is balanced. That means each split run will be the same length. You should also be terminating any unused lines.
Cable Run: The longer the cable the more signal loss you will experience. Make your cable runs as short as possible and never exceed 100 feet or you could lose the signal completely. A mistake people make is they have a 80 foot of cable run in the wall and then add a 25 foot pre-made coax cable from the wall port to the TV. Your entire run needs to be under 100 feet.
Tuners: TV tuners are all different. Some amplify the signal and some don’t. The Tablo also additionally splits the signal either 2 or 4 ways depending on your model. Your objective should be to get the strongest signal before the tuner comes into play.
How to improve a signal: Amplifiers are a good way to compensate for the signal loss from splitters and long cable runs. You have to insert an amplifier in the right place though. Amplifying a signal after it as been degraded will only make things worst. Think of a radio station that has a lot of static. Turning the volume up just makes the static louder. Pre-Amplifiers will boost the gain at the antenna. They will not magically make your antenna pull in stations from farther away, but they will improve weak signals your already receiving. A pre-amplifier consists of two parts, a amplifier that gets installed at the antenna and a power inserter that gets installed down the line near an electrical outlet. Both parts need to be installed before the splitter on the line. Another type of amplifier is a Distribution amplifier. This is just basically a amplified splitter. You may find you need both types or just one or the other.
Notes: TV tuners can only handle so much. If a signal is too strong because of a powerful, close by broadcast tower and/or an over amplified signal, the signal may cut out all together. Since at least one of your devices gets all the channels we can assume the antenna picks them all up. I would try a distribution amplifier first and see if that helps. If not follow all my suggestions above.