although the FCC states it may be possible to license additional Class A stations in Alaska. B (formerly II and III) — regional stations — 250 W to 50 kW, 24 hours. Stations on the AM expanded band, 1610 kHz to 1700 kHz, are limited to 10 kW days and 1 kW nights, non-directionally. Several expanded band stations operate DA-N or even DA-2 with up to 10 kW during all hours, after providing proof that such operations will not cause co- or adjacent-channel interference. If under 250 W at night, the antenna must be efficient enough to radiate more than 140.82 mV/m at 1 km. C (formerly IV) — local unlimited-time stations — 250 W to 1 kW, 24 hours. Class C stations that were licensed at 100 W are grandfathered. Rare Class Cs operate with directional arrays, such as KYPA and KHCB. D (formerly II-D, II-S, III-S) — current and former daytimers — Daytime 250 W to 50 kW, nighttime under 250 W or off-air. Field strength is limited to 140 mV/m (millivolts per meter) at 1 km. No new class D stations are licensed, with the exception of Class B stations that are downgrading their nighttime operations to Class D (i.e., less than 250 W). The station's daytime operation is then also reclassified as Class D. If a Class D station is on the air at night, it is not protected from any co-channel interference. TIS/HAR — travelers' information stations / highway advisory radio stations — Up to 10 W transmitter output power. Stations within US national parks are licensed by NTIA and not the FCC. Unlicensed broadcasting — (see low-power broadcasting) — 100 mW DC input to final amplifier with a maximum length radiator, no license needed, may be measured at edge of campus for school stations Notes: In the Western Hemisphere (ITU region 2), medium wave AM broadcasts are on channels spaced 10 kHz apart from 530 kHz to 1700 kHz, with certain classes restricted to subsets of the available frequencies. With few exceptions, Class A stations can be found only on the frequencies of 540 kHz, 640 to 780 kHz, 800 to 900 kHz, 940 kHz, 990 to 1140 kHz, 1160 to 1220 kHz, and 1500 to 1580 kHz. The exceptions are cited in relevant international treaties. While US and Canadian Class A stations are authorized to operate at a maximum of 50,000 watts day