After extending in the obvious way the classic notion of a Heffter array H(n, k) to any group of order 2nk + 1, we give direct constructions for elementary abelian Heffter arrays, hence in particular for prime Heffter arrays (whose existence was already known). If q = 2nk + 1 is a prime power, we say that an elementary abelian H(n, k) is "over Fq" since, for its construction, we exploit both the additive and multiplicative structure of the field of order q.We show that in many cases a direct construction of a H(n, k) over Fq, say A, can be obtained very easily by imposing that A has rank 1 and, possibly, a rich group of multipliers, that are elements m of Fq such that mA = A up to a permutation of rows and columns. A H(n, k) over Fq will be said optimal if the order of its group of multipliers is the least common multiple of the odd parts of n and k, since this is the maximum possible order for it.We give an explicit direct construction of a rank-one H(n, k) -reaching almost always the optimality -for all admissible pairs (n, k) except for the demanding ones, i.e., those where n or k is a power of 2, or nk has only one odd prime divisor. A less explicit construction makes reasonable to believe that an optimal rank-one H(n, k) exists for any admissible pair (n, k).