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Reject power is the power returned from a device that combines RF fields. IN HD Radio you can combine the RF from two transmitters into one line. When you combine RF, most of it goes to the antenna, but by nature, a portion of it becomes rejected within the device that combines it. Reject power is essentially wasted power. You want as little of it as you can have, and you accomplish this by properly phasing the RF components into the combiner.
Reflected power is related VSWR. It is power reflected back off of the transmission system. The transmitter wants to send RF out, and it in a sense "looks" at the line. While most power goes out, items such as tuning devices, RF switches, and the length of feedline can cause a very minute amout of power to come back to the transmitter. However if a piece of the RF system fails (i.e. an antenna element gets shorted, a coax gets punctured or burns out), the impedance changes, a lot more power comes back to the transmitter. If the reflected power is too high, and is not shut down within seconds, you can suffer major damage to the transmitter (i.e. it can catch on fire).
Spacial combining is the method of using different antennas for the Main and HD signals, and using free air (the atmosphere) to "combine" the two signals. FM stations will mount an AUX antenna near their primary antenna, and feed just the AUX signal into it. There are rules for spacial combining however,
the aux antenna must be within a certain distance and height of the primary antenna, you can not use this method if your primary antenna is licensed directional, and you must license the HD antenna as an AUX antenna system with the FCC.
"To the optimist, the glass is half full. To the pessimist, the glass is half empty. To the engineer, the glass is twice as big as it needs to be."