Comparison of ScanEM-EC and the "industry-standard" E-Field Probes

As seen, the output of the passive probes is not only uneven, but also lacks sensitivity at lower frequencies (below 250MHz). A typical preamplifier will not compensate for the frequency response and will just amplify the noise. Should the device under test generate fundamental frequency and its harmonics, the visible ratio between them will be distorted and will let you believe that the output at lower frequencies is much lower than it actually is.