.htacess - how to preserve %2B in query

Hi,
I'm quite a newbie and I've got a problem with my .htacess

It's simply like

RewriteEngine on
Options FollowSymLinks
RewriteRule ^site_(.*)\.htm$ /site.php?q=$1 [L]

And it work's fine - as long as there is no "%2B" (= "+") in the query
string. If there is e.g. "Adam+%2B+Eve", .htcaess converts it to "Adam Eve".

How can I manage to preserve the "%2B" resp. "+"?

Many thanks in advande

Frank