Which of these items would be useful for a hidden transmitter hunt?

In a hidden transmitter hunt, commonly known as a fox hunt, the primary goal is to locate a hidden transmitter. A directional antenna is particularly useful in this scenario as it helps to determine the direction of the signal being transmitted. By using a directional antenna, a participant can more accurately locate the source of the transmission by pointing the antenna in various directions. The stronger the signal received in a given direction, the closer the participant is to the transmitter.

The other items mentioned may have their uses in different contexts. A calibrated SWR meter is primarily employed for measuring standing wave ratio to ensure efficient antenna operation, rather than locating signals. A calibrated noise bridge is used for constructing antenna matching networks, and while it is valuable in other aspects of radio operation, it is not directly relevant to locating a hidden transmitter. Therefore, the directional antenna stands out as the most essential tool for successful participation in a hidden transmitter hunt.