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DIELECTRIC : DEFINITION
di·e·lec·tric
Physics adjective
adjective: dielectric1. having the property of transmitting electric force without conduction; insulating.
noun
noun: dielectric; plural noun: dielectrics1. a medium or substance that transmits electric force without conduction; an insulator.
 
" Transmits electric force without conduction "
 
Dielectrics will not conduct electricity nor will they impede the transmission of electromagnetic waves. ( RF )
 
Electric Forces:
Electricity : A/C or DC Current ( Flow of Electrons )
Electro magnetics : Magnetic fields: Static or ocillating; RF
( Radio Frequency ): UHF, VHF, Shortwave, Microwave, etc.
 
Dielectrics Include: A vacume (best ), to varying degrees of dielectric properties: air, glass, ceramics, plastics, oils, etc.
 
Dielectrics used in communications industry include: Coaxial cable core, Various teflon and other plastic parts used in coaxial connectors, Dielectric greases, Hollow or solid; plastic or ceramic waveguides, Potting fill materials used inside various splitters and groundblocks, Snow cover materials for dishes, Rock camouflage
cover for dishes, etc.
 
Note on Dielectric Greases:
Often it is stated that using a dielectric grease on a coaxial or electrical connection will impede electrical throughput. If the dielectric is viscous, it will move out of the way of the electrical contact surfaces and occupy only space that was once occupied by air : A dielectric also.
 
Example: Common practice is to put grease ( a dielectric) on car battery terminals. The grease is extruded away from the electrical contacting surfaces when re-assembling, allowing electrical throughput, while  protecting the terminals from corrosion. 
 
 
 
 
Hope this information answered you questions.
 
 
 
 
 
 
 
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WIKIPEDIA Information
on "RF" Spectrum
LINK
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PAGE LINK
Dielectric Grease Filler for Weatherproofing Outdoor Coax Coaxial Antenna, Dish, LNB Connectors from Corrosion and Failure.