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Additive manufacturing ( AM ) techniques are used increasingly for the direct fabrication of microwave devices , such as graded index lenses and dielectric resonator antennas , which have spatially-varying dielectric properties ( i.e .
 Additive manufacturing, AM, fabrication, microwave, as, dielectric, dielectric
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
Answer:
 Additive manufacturing, AM, fabrication, microwave, as, dielectric, dielectric