Ionospheric phase errors degrade high-resolution radio images below 100 MHz, and they differ significantly from the tropospheric errors which dominate at high frequencies. The ionosphere is so high (~400 km) and the VLA primary beam is so wide (~0.2 rad) that the intersection of the beam with the ionospheric screen is larger than the "isoplanatic patch" size, a phase coherent region on the sky. Antenna-based calibration techniques developed at higher frequencies cannot be used because ionospheric phase errors vary significantly across the field-of-view of each antenna. This paper describes the "field-based calibration" technique adopted for the 74 MHz VLA Low--frequency Sky Survey (VLSS) being made with the 10 km "B" configuration. This technique is useful for a range of array sizes but fails on baselines longer than the linear size of the isoplanatic patch, a few 10s of km at 74 MHz. Implications for designing larger low-frequency arrays are discussed.