As frequency decreases the wavelength increases, requiring a taller antenna to make a given fraction of a wavelength. Construction costs and land area required increase with height, putting a practical limit on mast height. Masts over are prohibitively expensive and very few have been built; the tallest masts in the world are around . Another constraint in some areas is height restrictions on structures; near airports aviation authorities may limit the maximum height of masts. These constraints often require a mast be used that is shorter than the ideal height.