Let \( H_{n}:\ \dfrac{x^{2}}{1+n}-\dfrac{y^{2}}{3+n}=1,\ n\in\mathbb{N} \). Let \( k \) be the smallest even value of \( n \) such that the eccentricity of \( H_{k} \) is a rational number. If \( l \) is the length of the latus rectum of \( H_{k} \), then \( 21l \) is equal to __________ .
