How was the Borg Drone able to transport through the Enterprise's shields?

Question

In "Q Who," right after Guinan informs Captain Picard of the strength of the Borg, the captain orders the shields up. Several moments later, a Borg Drone appears in Engineering, having transported over from the Cube.

Do the Borg have the ability to transport through shields? Apparently not, or else they presumably could have simply beamed over Captain Picard in "Best of Both Worlds" instead of appearing themselves on the Enterprise bridge to fetch him. Perhaps there was a fluctuation or some such in the shields in "Q Who?," which the Borg were able to take advantage of.

Answer 1

The shields were up, but the Borg beamed straight through them.

WORF : He came right through the shields!

later

BORG : (interrupting) We have analyzed your defensive capabilities as being unable to withstand us. If you defend yourselves, you will be punished.

In the film "Generations", the Duras sisters use much the same technique to attack the Enterprise. By learning the precise frequency of the shields, they are able to hit the ship with both photon torpedoes and energy bursts from their disruptors. A more sophisticated enemy (such as the Borg) might have also used this knowledge to beam something through the shields.

In later episodes of TNG and Voyager, Federation ships use rotating frequencies to prevent the Borg from simply waltzing through their shields. The implication is that the Borg have technology that allows them to determine an enemy's harmonic frequency and match it instantly, possibly assimilated from the Vidiians, a Delta Quadrant species that seem to possess a similar capability (Voy : Fury)

By changing the 'shield harmonics' regularly, you gain at least a small measure of security from casual intrusion.