Die Schilde haben ein in gleichbleibenden Intervallen wiederauftauchendes LOCH? Der Beam-Vorgang dauert immerhin mehrere Sekunden ...

Meine Informationen stammen aus der Technical Library des Daystrom Institute (erst "Sci-Tech", dann "Force Fields"):
For over a century after the invention of the shield it was impossible to use transporters to beam to or from a shielded location, but to an extent this limitation has now been circumvented. In general sensor and weapon windows are insufficient to allow beaming; whilst technically there is nothing to prevent a ship opening a window in its own shields of sufficient size to allow transport, in practice such windows are almost always large enough to be detected and exploited by enemy vessels and it is far simpler just to drop the shields briefly altogether. The more modern Starfleet shield designs have now reached a point at which transporters can be operated via a large wide frequency window which is briefly opened over the hull emitters. This gives greater flexibility in using the transporter during high threat situations, but it remains a somewhat risky proposition - should an enemy score even a near miss on such a window the effects on the ship would be considerable.
Allerdings gibt der Autor auch zu, daß alles außer dem ersten Satz reine Spekulation ist ... was wiederum auch irgendwie passend ist: SF - Speculative Fiction ...