As P.Gibson mentioned on his Vivaldi Aerial patent, this type of antenna has good performances in 2GHz-40GHz frequency range.
In a Vivaldi antenna, the slotline to free space transition is given by exponentially flared slotline.
In order to get efficient radiation in free space, the width of the open end should be greater than λo/2.
At 240MHz this dimension should be greater than 62cm, which makes the overall dimensions of the antenna to be great.
At these frequencies, patch antenna with air dielectric has the same great dimensions issues.
Increasing the dielectric constant of the substrate reduce the dimensions of the patch antenna, but the antenna still have narrow bandwidth.