ATSC, which stands for Advanced Television Systems Committee, is the current set of standards for television transmission in the United States. It replaces the former NTSC standards and includes support for high-definition broadcasting. Though there is no specific "ATSC antenna," choosing a suitable antenna will greatly increase the quality of picture and sound received.
It is important to distinguish between the term "ATSC antenna" as a marketing phrase and as a definition. In reality, there is no such thing as a special ATSC antenna. A better phrase would be "an ATSC-enhanced antenna" or "an antenna designed for ATSC standards." In theory at least, any antenna can be used for receiving ATSC transmissions.
The most important aspect of ATSC when it comes to choosing an antenna is that it is a digital system. This plays a significant role when it comes to poor signal reception. Whereas analog broadcasts can be picked up through a weak signal, albeit with noticeably poorer picture quality, a digital broadcast from a weak signal may be unwatchable or even leave a blank screen.
One particularly useful source of information for choosing an ATSC antenna is AntennaWeb. This is an online resource created by the National Association of Broadcasters and the Consumer Electronics Association. By typing in your address and details of your building type, you can access a list of local broadcast towers and the type of antenna which will be best suited to picking up a strong enough signal.
The results from this resource use a standardized color coding system, with the system classifying antennae into six categories. These take into account whether the antenna receives signals from one direction only or are multidirectional, the size of the antenna, and whether it has a built-in pre-amplifier.
The color-coding system only applies to external, or "roof-top" antennae. There is no equivalent system for indoor antennae, since the strength of signal received in different rooms can vary immensely even within a building. This is because of issues such as the location of windows and the material used for construction. Instead of a categorized system, there is simply a quality mark known as the "Indoor Antenna Mark." This logo does not guarantee reception will be perfect, but shows an antenna meets an overall standard for indoor reception.
If you previously had good reception and it has now worsened, it may be worth rechecking your local broadcast coverage. The frequencies used for broadcast and the strengths of the signal available from each local tower may have changed since the switchover from analog to digital broadcasting. You may thus need to rescan your television or set-top box.