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

A directional antenna is particularly useful for a hidden transmitter hunt because it allows the operator to determine the direction from which the signal is originating more accurately than a non-directional antenna. Directional antennas, such as Yagi or log-periodic, focus the radio frequency energy in a specific direction, which means that when you turn the antenna, you can gauge the strongest signal. This is especially helpful in locating the source of a transmission when multiple signals may be present, as it reduces interference and helps to pinpoint the transmitter's location.

The other items listed have their specific uses in amateur radio, but they are not as directly applicable to the task of locating a hidden transmitter. For instance, a calibrated SWR meter is primarily used for measuring the standing wave ratio to optimize antenna performance and is not generally employed in signal direction finding. Similarly, a calibrated noise bridge is utilized for determining impedance of circuits or antennas, which is not relevant in the context of hunting down a hidden transmitter. Therefore, in the context of the question, the directional antenna stands out as the most appropriate tool for this specific task.