The dependence of the first critical field of long linear and annular Josephson junctions on the width of the surrounding stripline, the so called idle region, is investigated experimentally. The stripline modifies the effective Josephson length λeff in the junction. The experimental data are compared with the theory by Caputo et al. [J. Appl. Phys. 85, 7291 (1999)] and good agreement is found. The dependence of the first critical field on the width of the surrounding stripline can be well explained using the same λeff for both annular and linear junctions.