Superposition is the technique for analyzing circuits that have multiple sources: You simply determine the output with the inputs activated one at a time, and then add together all the results. (By the Rules of Superposition you may sum voltages or currents but you must not sum power.)
If half of the input power is forwarded to the load, the load voltage will be smaller than the input voltage by 0.707. (In an all-75W system, the power change is the square of the voltage change. The square root of 0.5 is 0.707.) Superposition says that if both inputs supply the same power, the voltage at the load will be 0.707+0.707=1.414. 1.414 squared is 2, which means there is twice the power at the load as was supplied at each input.
But you ask, “How can the power at the load equal the total input power if some of the power was reflected backwards?” The answer is that superposition applies to currents as well as voltages. The phase of the reflected currents is such that they subtract instead of add. The reflected currents will cancel each other out completely.
The doubling of the output power is equivalent to a 3 dB increase in the signal. If the combiner is 90% efficient then a 2.5 dB gain is seen. Note the dichotomy:
· If the antennas point in different directions, there is a 3.5 dB loss at the combiner.
· If the antennas point in the same direction, there is a 2.5 dB gain at the combiner.
This is a 6 dB swing. 3 dB of this is just the adding of the second antenna, but the other 3 dB is from the combiner becoming a much more effective device.
Avoiding a large loss at the combiner requires the complete cancellation of the reflected currents. This requires that both input signals be identical in amplitude and phase. This generally demands that the two antennas be identical, and that the two feedline delays be equal. Furthermore, both antennas must be in fields of equal strength, pointing the same direction, and must be positioned for proper phase. Making all this true for non-identical antennas is not practical.

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