Why a floating cable radiates more noise at higher frequencies than grounded cables in Radiated Emission EMC?

 

The grounded shield can shunt the voltage/current generated in the shield by radiated RF energy, to ground.

The ungrounded shield obviously can't get rid of the voltage/current so it passes on through (basically being re-radiated on the opposite side of the shield.

 

Firstly, as Crutschow said, bur second - not always the low freq is more neglectible. For example, the magistral underground cable from 825 MW HES (what actually is second largest in Europe) somehow unhappy goes to the magistral lines straight beyound one three floors multiflat house. They claimed about strange illnesses and feeling bad all the time thus not knowing about that cable I begin to measure RF smog in their flat. Indeed, in the place of their bed instrument show 9 kV/m (at permitted 5) and 7000 microTeslas instead of permitted 100. But, just making a 5 meter distance from placing of that cable both field consistuents decreased under the normative values and in 10 meters (at flat outer walls) became neglectable. My only best councel was - just let sell this flat out.
Thirdly: Faraday Cage is wonderfully effective against to EMI comes INTO the ecranated structure. But Faraday Cafe principle DONT WORK COMPLETELY (!!) if we speak about EMI coming OUT of the ecranated box. In the last case only factor what stops anything is Earthing (grounding). Thus, the ground wire parasythic inductance, resistance and capacitance determines the EMI attenuation factor.
For those what meditates about opening mouth to protest: remember the Faraday Cup Principle: "any charge laid to electro-leading sphere`s inside wall will flow to outside wall let the inside wall potential be definite zero". (Nothing was said about immaginative outside charge going to inside to form the outside voltage define zero.) Summa summum: from inside to outside only parasythics of grounding wire will limit the attenuation whilst from outside to inside attenuation is definitive, principial and rather complete even without of any grounding, however grounding makes it more complete.