In this article, a new configuration of the reltron oscillator (i.e., gridless reltron configuration with annular explosive emissive cathode) was proposed for long pulse and high-power RF generation. To investigate the electromagnetic (EM) and RF behavior, the structure was modeled in the CST Studio Suite and then simulated with the help of different solver modules of the CST simulation tool. The effect of various electrical parameters of the proposed configuration on the generated pulsewidth and RF power was also studied. For the optimized structure, the proposed configuration produced an RF signal of peak microwave power of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$>$</tex-math> </inline-formula> 190 MW with a pulsewidth of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula> s. These simulation results were compared with the experimentally reported gridded and gridless reltron, and it was noted that: 1) the proposed configuration produced a peak RF power similar to a gridded reltron with a lower operating voltage; 2) the pulsewidth of the generated RF signal was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 10 times higher than the gridded reltron; 3) obtained <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 10 times higher peak RF power than the gridless reltron; and 4) pulsewidth was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 0.2 times lesser than the gridless reltron. Eventually, the structure was benchmarked with the experimentally reported gridded and gridless reltron for its validation.