BibTeX does not have the right entry for preprints. This is a hack for producing the correct reference:
@booklet{EasyChair:6970,
author = {Aslam Chitami Priawan Siregar and Gatut Yudoyono and Yono Hadi Pramono},
title = {A Study of Silicon Effect as a Switch on Folded Dipole Antenna},
howpublished = {EasyChair Preprint 6970},
year = {EasyChair, 2021}}