[en] A parametric instability has been observed in a cold low density plasma to be excited by the unidirectional lower hybrid wave driven by a multiple-ring slow wave antenna. The resonant decay waves are identified to be an ion-acoustic wave and a lower hybrid wave. The effect of non-monochromatic pumps on the threshold of parametric instability (PI) is investigated. When the power spectrum of the finite bandwidth pump produced by amplitude modulation is narrower than a resonance width γ₀ of the PI, this pump gives a lower threshold than that of the monochromatic pump. The multi-pump, whose spectrum is composed of multiple discrete components with a large (compared width γ₀) frequency separation between adjacent components, increases its threshold as the increase of the number of components. (author)