Radomes are generally of honeycomb construction and erosion or pin holes in the outer skin can allow water to accumulate within the radome structure. Without regular inspection, including moisture testing and prompt repairs, moisture ingress into radome structure is inevitable. Such moisture attenuates the energy emitted by the radar and, since the transmission power through the radome is reduced, the returns are also weak. The effect is the same as having dirty glass on your car headlights; the lights are dim and you don't see so well. Being so close to the dish, any energy reflected directly back from the radome doesn't actually affect the display because the receiver circuits are disabled for a suitable time interval after each pulse is transmitted to prevent such effects.
At first, radar returns grow weaker and there is a loss of range; you don't see weather at the longer ranges and where the strength of weather returns gradually increase as they get nearer, this may be mistaken for a loss of STC in the receiver. As the water inside the radome structure builds up and output signal attenuation increases, the returns grow weaker and weaker until finally the radar becomes useless. Often, the radome delaminates and splits due to the water within the material freezing and forcing the outer layers apart but more usually the only symptom is loss of radar sensitivity.
Meters are available to measure the moisture content on radomes and should be used by your maintenance staff whenever pilots report weak returns. Unfortunately the current trend to combine the avionic trades has reduced overall skill levels, particularly in the Radio & Radar specialisation, and competence levels have declined noticably in recent years. From what I have seen and heard of recent radio examination questions, practical aspects have suffered at the expense of esoteric theory. Too often the response to a radar defect in the Tech Log is simply to "change the box" without bothering with the basics.
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Through difficulties to the cinema