  Suppose $\Lambda \subseteq \RR^2$ has the property that any two exponentials
with frequency from $\Lambda$ are orthogonal in the space $L^2(D)$, where $D
\subseteq \RR^2$ is the unit disk. Such sets $\Lambda$ are known to be finite
but it is not known if their size is uniformly bounded. We show that if there
are two elements of $\Lambda$ which are distance $t$ apart then the size of
$\Lambda$ is $O(t)$. As a consequence we improve a result of Iosevich and
Jaming and show that $\Lambda$ has at most $O(R^{2/3})$ elements in any disk of
radius $R$.
