Die Firma QuickerTek stellt externe Antennen für das 15″ und das 17″ Powerbook her:

Directional Antenna for Aluminum 15-inch and 17-inch PowerBooks
NEW! – the only external Apple Airport antenna for the 15″ and 17″ aluminum PowerBook G4s. It can easily double and triple the signal strength. Overwhelming customer demand from the expanding PowerBook wireless user market prompted the development of this new antenna design.
This new external aluminum PowerBook antenna includes detailed installation instructions that make it easy for most users to quickly improve wireless signal strength. QuickerTek also includes a CD containing software for wireless system performance measurement.

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These new wireless Airport antenna products are available for $89.95 for either aluminum PowerBook computer. PowerBook users can find QuickerTek antennas at resellers including MacAnywhere; TechnoWarehouse and SmallDog.