If i were to use a passive transmit coupler to connect two transmitters of equal power output to the same antenna, and connect one microphone to both transmitters, and then transmit on both at the same time on the same frequency, will that be the same as using one transmitter but with twice the power ?

Will a distant receiver see the two signals as one ? or as two separate signals just as if they were coming from completely separate stations ...

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    $\begingroup$ I answered Exactly that question in an answer to a different question of yours! $\endgroup$ Dec 6 '19 at 14:02

You certainly can, in theory, use multiple transmitters together to produce a signal of higher total power. However, there is a catch: the signals must be in phase with each other. Otherwise they will cancel each other out instead of adding, half the time.

To have the same phase, they would have to share the same RF oscillator (or be coupled to a master oscillator, perhaps), and the feed line lengths between each transmitter and the combiner must be equal. A more practical way to look at it is that you can build one transmitter which has many amplifiers that work together to produce a high power signal. This can even be a particularly useful technique if your amplifier components cannot tolerate the voltage, or heat, that would result from the total RF power you want to transmit — each individual amplifier works with a lower voltage and can have its own heat sink.

A related technique is beamforming, where you use multiple amplifiers and multiple antennas. The waves from each antenna interfere with each other, constructively or destructively; by controlling the amplitude and phase of the signals transmitted via each antenna, the antenna array can be “steered” to transmit signals in a desired direction, without need for any mechanical rotation.

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    $\begingroup$ If you rely on multiple transmitters, not only must they be in phase, but the signal propagation path lengths must be equal, to within a small fraction of a wavelength. Unless the transmitters share an antenna, this may be difficult or impossible to achieve in practice. $\endgroup$ Dec 6 '19 at 21:58
  • $\begingroup$ @PhilFreedenberg I understand the question to already be about combining the signals before the antenna. That said, I added a mention that the feed line lengths must be equal, which I didn't think to mention before. $\endgroup$
    – Kevin Reid AG6YO
    Dec 6 '19 at 23:20

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