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When an intense relativistic electron beam is injected into an unmagnetized plasma, the plasma develops into a strong Langmuir turbulence state, and high-power broadband microwave radiation is emitted. The emission source has been confirmed to be the modulated beam interacting with the caviton fields. Total power of this radiation depends on the beam-to-plasma density ratio almost in the same way as the level of the beam modulation. The present paper indicates that by performing two and a half-dimensional particle-in-cell simulations, this dependence comes from the dependence of activity or the beam electron-plasma electron two-stream instability on the beam-to-plasma density ratio.