wilkinson resistor
FANT, true, but some explanation to your points:
1-to increase the output power: I saw many systems that use inphase combining without a resistor, but usually those systems are intelligent (adaptive, that is) so they automatically adjust for temp and aging variations.. though not perfect, but still supply adequate combining gain without being much in-coherent.
2-to have a " failure resistant " system. Here is a good trick (one of my 'grandma' secret recipes
:
You have your wilkinson, without the resistor ofcourse, but instead of having the arms of the combiner quarter-wave length, make them HALF-wave length 50Ohms lines. Here, when both amplifiers are operating, they combine as usual, since having half-wave instead of quarter wave does not do any difference (since it is just operating as a matched 50ohm line independent of length). When an amplifier fails (or better, somebody pulls it out in system level), the impedance at the floating arm is open-circuit, and the half-wave arm translates it as is (open circuit also) so the other arm does not see any "loading" due to pulling the other amplfier off.
I know this works best if you pull the amplifier completely (i.e. hot-swapping systems), but can still also provide some help if an amplifier fails, specially if its drain is damaged and thus acts as an open circuit, or having some simple intelligence to open circuit the failing amplifier.
This worked for me without any trouble for a hot-swapping system. I saw other systems trying to solve this using "REALLY" expensive techniques. In our case, it is just a nother piece of extra coax length
.
My point is: there are systems out-there that do use wilkinson (or modified wilkinson, as described above) without need for the extra high-power resistor.
Adding a little bit of intelligence is not that epxensive (you can use very simple techniques, 55 timers...PICs..etc), but can add a lot to the system.