The delta band frequency was a frequency in which a carrier wave could be transmitted.

After traveling back to 2004 to locate rogue Xindi-Reptilians who were trying to destroy Humanity, T'Pol figured that the Xindi must have some kind of temporal beacon they needed to use to return to the 22nd century. However, for them to destroy the beacon, she and Jonathan Archer had to first find it. She suggested that if they modified their scanner to a delta band frequency, that they should be able to locate the signal from the temporal beacon. (ENT: "Carpenter Street")

During the tests to established a wormhole relay station in the Gamma Quadrant in 2371, the relay's transmission array sent out a test signal, which then created a carrier wave in the delta band frequency that was transmitted through the signalling array towards the wormhole. The test at that frequency, however, did not work, so they switched to a theta band frequency. (DS9: "Destiny")

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