Definition
HH is the FAA facility class code for the most powerful category of Non-Directional Beacon. An NDB in the HH class transmits with 2,000 watts (2 kW) or more of output power, giving it the longest standard service range of any NDB class — typically 75 nautical miles at all altitudes between 1,000 and 18,000 feet above the facility.
Plain English
HH is a label that tells you a ground-based radio beacon is high-powered, so its signal reaches farther than weaker beacons. When you see HH next to a beacon on a chart or in a publication, it means that beacon puts out at least 2,000 watts and you can use it from a long way off.
Context Anchor
Seen in FAA abbreviation lists, facility information, and NOTAM-style references describing an NDB by power class.
Derivation
The double letter 'HH' is FAA shorthand within a four-tier NDB classification scheme: compass locator (lowest power), MH (medium-high), H (high), and HH (highest). The 'H' stands for 'High power'; doubling it to 'HH' marks the top tier — 2 kW and above.
Why Pilots Care
Higher power means greater signal range and reliability for ADF navigation and holding procedures.
Intuition Check
Do not read 'HH' as a callsign or station name. It is a power-class label, like a wattage rating, attached to the beacon's listing.
Example Sentence 1
The enroute beacon was listed as class HH, so the pilot expected a usable signal out to about 75 nautical miles.
Example Sentence 2
NOTAMs listed the HH facility as usable despite temporary reduced power testing.