Plain-language answer: The lowest usable frequency is the lowest frequency expected to deliver an acceptable signal-to-noise ratio over a defined path.
Why it matters in the antenna
A lower band may refract but remain unusable because of D-layer absorption, atmospheric or local noise, antenna loss, or insufficient power.
What to verify
Compare neighboring bands over the same path and record signal plus noise, not signal alone.
The common mistake
Assuming the lowest frequency that returns is automatically best.
Operating boundary
Use the exact documented component, geometry, frequency, power, duty cycle, clearances, and feed/return system. Begin tests at low power and stop for heating, arcing, damaged insulation, or unstable supports.
