Despite the fact that there is a perfectly good airport located between Koblenz and Cologne, which has a direct rail link to the city most visitors will come to Koblenz via the airport at Frankfurt Hahn. There is a simple reasons for this : it is bloody cheap.
Transport to and from the airport across perfect tank-driving country is provided by an effecient coach company (well would you expect anything less in Germany)
Return journeys run from the central station in Koblenz
There is a small airport near Koblenz called Frankfurt-Hahn.
You can get there with Ryanair fligths.
There is a bus between Frankfurt-Hahn and Koblenz train station for 12,25 Euro one way.
More information about the airport and a bus timetable under
Other possibilities are the airports Frankfurt and Cologne.