We present a new insight in the response of a CW NMR spectrometer for highly magnetized samples above 100 MHz. The spectrometer is a bridge made of a magic “T”. The output of the bridge is proportional to the reflection coefficient, Γ, of a resonant circuit, which is built with a coil containing the sample. The sensitivity of the reflection coefficient to the complex susceptibility of the sample, χ(ω) = χ′(ω) − jχ″(ω), depends on the quality factor, Q, of the circuit and filling factor, η. When the condition Qηχ″ ≪ 1 is not fulfilled, we show indeed that the use of a simple crystal detector, which is only sensitive to |Γ| gives rise to a strong nonlinear response of the spectrometer. Measurements of the complex value of Γ by means of phase sensitive detection allow to recover a linear behavior. We discuss and illustrate those issues with a few circuits we designed for our measurements on liquid3He with spin polarizations up to 15 %. A method is described to build in a reproducible and predictable way resonant circuits matched to 50 Ω in the frequency range 100 – 400 MHz with a quality factor as high as 1000 at 4 K.