Distinguish between transponder bandwidth and channel bandwidth in a SATCOM system.

• A. Transponder bandwidth is the RF spectrum allocated to the transponder; channel bandwidth is the data-carrying bandwidth within that spectrum determined by modulation/coding.
• B. Channel bandwidth refers to the RF spectrum allocated to the transponder.
• C. Transponder bandwidth is the time interval; channel bandwidth is amplitude.
• D. Transponder bandwidth is in baseband; channel bandwidth is in RF.

Answer: (B)

In SATCOM, think of two related but distinct ideas: the RF footprint and the data-carrying width. The transponder bandwidth is the RF spectrum allocated to that transponder—the width of the passband on the uplink and downlink that the transponder can process. The channel bandwidth is the portion of that spectrum used by a single payload channel to carry data; this width is determined by the modulation and coding you choose, and it sets how much information that channel can carry within the transponder’s overall footprint.

For example, a satellite transponder might provide a 36 MHz RF footprint. Within that 36 MHz, you could place several channels, each occupying its own channel bandwidth (for instance, 4–8 MHz per channel, depending on the modulation scheme). The data rate you achieve on each channel depends on the spectral efficiency of the modulation/coding, not on the total transponder width itself. If you treated the channel bandwidth as the entire transponder spectrum, you’d miss how many channels can fit into that transponder and how their data rates scale with modulation. The other options mix up the concepts by describing time intervals or baseband vs RF, which aren’t the definitions used here.