  A Sidon set is a set of integers containing no nontrivial solutions to the
equation $a+b=c+d$. We improve on the lower bound on the diameter of a Sidon
set with $k$ elements: if $k$ is sufficiently large and ${\cal A}$ is a Sidon
set with $k$ elements, then $diam({\cal A})\ge k^2-1.99405 k^{3/2}$.
Alternatively, if $n$ is sufficiently large, then the largest subset of
$\{1,2,\dots,n\}$ that is a Sidon set has cardinality at most $n^{1/2}+0.99703
n^{1/4}$. While these are only slight numerical improvements on
Balogh-F\"uredi-Roy (arXiv:2103:15850v2), we use a method that is logically
simpler.
