There is a joke, or saying, when it comes to designing an oscillator or amplifier:

“amplifiers oscillate, and oscillators amplify”

The statement describes the fact that, when attempting to design one, there are many pitfalls that cause you to have the other happen.  Usually when designing an amplifier oscillations are your enemy, and you tune them out with bypass capacitors and work to ensure the stability criteria is met.  When designing an oscillator, on the other hand, the circuit can readily swing into a stable state if you don’t have a very good design, and end up as just an amplifier.  Granted, with proper design and construction this is avoidable, but when you are building one for the first time or just quickly building one, this can happen.

I propose a similar saying for antenna design, although to make this work, i am going to use “radiator” instead of “antenna”:

“Radiators couple, and couplers radiate”

This only applies, really, to antennas with more than one port, and really can apply to any microwave circuit.  But this way the phrasing works out much better.  But basically says that usually when designing an antenna, you are battling this coupling between ports, trying to get efficient radiation.  Yet on the other hand, when designing a microwave circuit, one of your enemies becomes radiation, and you work to keep that to an absolute minimum, because unwanted radiation is unwanted loss.