KDIA (1640 kHz) is a commercial AM radio station licensed to Vallejo, California, and serving the San Francisco Bay Area. It is owned by the Salem Media Group and broadcasts a Christian talk and teaching radio format. Salem also owns KFAX, which airs a separate schedule of Christian programming. The radio studios and offices are on Liberty Street in Fremont.

KDIA transmits 10,000 watts. By day, it is non-directional, using one of the KKSF towers in Richmond on San Francisco Bay. At night, it is directional, using a four-tower array on Noble Road in Vallejo.

KDIA originated as the expanded band "twin" of a station on 1190 kHz.

In 1979, a World Administrative Radio Conference (WARC-79) adopted "Radio Regulation No. 480", which stated that "In Region 2, the use of the band 1605-1705 kHz by stations of the broadcasting service shall be subject to a plan to be established by a regional administrative radio conference..." As a consequence, on June 8, 1988, an ITU-sponsored conference held at Rio de Janeiro, Brazil adopted provisions, effective July 1, 1990, to extend the upper end of the Region 2 AM broadcast band, by adding ten frequencies which spanned from 1610 kHz to 1700 kHz.

While the Federal Communications Commission (FCC) was still making U.S. preparations to populate the additional frequencies, known as the "Expanded Band", John R. Quinn, president of station WJDM in Elizabeth City, New Jersey, arranged to have a provision added to the Communications Act of 1934 in late 1991, mandating: "It shall be the policy of the Federal Communications Commission, in any case in which the licensee of an existing AM daytime-only station located in a community with a population of more than 100,000 persons that lacks a local full-time aural station licensed to that community and that is located within a Class I station primary service area notifies the Commission that such licensee seeks to provide full-time service".

Although this addition was designed to aid WJDM's efforts to receive a fulltime authorization, KXBT on 1190 in Vallejo also qualified to take advantage of this provision, as it was limited to daytime-only operation, and required to sign-off at night, because it was located within protected nighttime coverage area of a Class I "clear channel" station, KEX in Portland, Oregon, and there were no unused fulltime assignments available on the AM and FM bands in the congested Bay region. On March 19, 1996, it was authorized to also broadcast, on 1640 kHz, as the second U.S. station, following WJDM, authorized to operate on an expanded band frequency.

On March 22, 1996, the FCC issued an updated list of expanded band allotments, which now assigned KXBT to 1630 kHz, so transmissions were switched to that frequency. On March 17, 1997, the FCC released a final revised roster of eighty-eight expanded band assignments, with KXBT designated to move back to 1640 kHz. The expanded band operation was now treated as being a separate station with its own unique call sign, and a construction permit for it was assigned the call letters KDIA on April 17, 1998.

The FCC's initial policy was that both the original station and its expanded band counterpart could operate simultaneously for up to five years, after which owners would have to turn in one of the two licenses, depending on whether they preferred the new assignment or elected to remain on the original frequency. However, this deadline has been extended multiple times, and both stations have remained authorized. One restriction is that the FCC has generally required paired original and expanded band stations to remain under common ownership.