HF
| HF | |
|---|---|
| Full name | High frequency |
| Frequency range | 3–30 MHz |
| Wavelength | 100 m down to 10 m |
| Amateur bands | 80, 60, 40, 30, 20, 17, 15, 12 and 10 m; 160 m by custom |
| Operating | |
| Main propagation | Skywave, refracted by the ionosphere |
| Common modes | SSB, CW, FT8, RTTY, AM |
| US license | General and above for most of every band; small Technician segments on 80, 40, 15 and 10 m |
| Typical transceiver power | 100 W |
| Neighbors | MF below, VHF above |
HF, short for high frequency, is the part of the radio spectrum from 3 to 30 MHz, where wavelengths run from 100 meters down to 10 meters. It is also known as shortwave. For amateurs, HF holds nine bands, from 80 meters to 10 meters, and it is the long-distance part of the hobby: on HF, a modest station with a wire antenna can talk to other continents with no repeater, satellite or internet link in between.
The HF bands
Strictly, the 160-meter band at 1.8 MHz lies in the MF (medium frequency) range, just below 3 MHz. Hams group it with HF anyway, because the same radios, antennas and operating habits apply. The table lists the US allocations.
| Band | US frequencies | Character |
|---|---|---|
| 160 m | 1.800–2.000 MHz | Technically MF; winter nights |
| 80 m | 3.500–4.000 MHz | Evening and night; regional nets |
| 60 m | Four channels and a 15 kHz segment near 5.3–5.4 MHz | Shared with government users; regional contacts |
| 40 m | 7.000–7.300 MHz | Reliable day and night |
| 30 m | 10.100–10.150 MHz | CW and data only; 200 W PEP |
| 20 m | 14.000–14.350 MHz | The main worldwide daytime band |
| 17 m | 18.068–18.168 MHz | Small and uncrowded |
| 15 m | 21.000–21.450 MHz | Long distance when the sun is active |
| 12 m | 24.890–24.990 MHz | Best in the high years of the solar cycle |
| 10 m | 28.000–29.700 MHz | The widest HF band; Technician voice |
Thirty, 17 and 12 meters are known as the WARC bands, after the 1979 World Administrative Radio Conference that allocated them. By long-standing custom, contests stay off them.
Propagation
HF signals travel in two ways. Ground wave follows the earth's surface and fades out within tens of miles. Skywave leaves the antenna at an upward angle and is refracted back to earth by the ionosphere, layers of ionized gas roughly 30 to 250 miles up, landing hundreds or thousands of miles away. It can bounce off the ground and go up again, so several hops can carry a signal around the world. Between the end of the ground wave and the first skywave landing lies the skip zone, where a station may not be heard at all.
How well skywave works depends on frequency. Every path has a maximum usable frequency (MUF), above which signals pass through the ionosphere into space; the MUF rises in daylight and in the high years of the solar cycle. Meanwhile the lowest layer, the D layer, forms in daylight and absorbs the lower frequencies. Together these give the HF operator's rule of thumb: 20 meters and the bands above it are daytime bands that open widely when the sun is active, 80 and 160 meters come alive after dark, and 40 meters works around the clock. Low antennas on 80, 60 and 40 meters can also send signals almost straight up for NVIS coverage of a region a few hundred miles across, valuable for emergency communications.
HF is at the mercy of the sun. A solar flare can black out HF on the daylit side of the earth for minutes to hours, and a geomagnetic storm can disturb the bands for days. Noise is also much higher than on VHF: static from thunderstorms on the lower bands, especially in summer, and interference from power lines, appliances and household electronics.
US license privileges
The Technician license gives only a taste of HF: CW on parts of 80, 40 and 15 meters, and CW, data and single-sideband voice from 28.000 to 28.500 MHz, all at up to 200 watts PEP. The General license opens large portions of every HF band, including 160 meters and the 60-meter channels, and for many hams HF is the main reason to upgrade. The Amateur Extra class adds the rest, chiefly the lowest 25 kHz of 80, 40, 20 and 15 meters and additional voice segments. The maximum power is 1,500 watts PEP, except on 30 meters (200 W PEP) and 60 meters, which has its own lower limit.
Operating and equipment
Voice on HF is almost all single sideband: by convention, lower sideband below 10 MHz and upper sideband above it, with 60 meters an exception that uses upper sideband. CW fills the bottom of each band, and digital modes such as FT8, RTTY and PSK31 have their own small segments; some operators still enjoy AM. DXing, nets, ragchewing and Parks on the Air all depend on HF.
A typical station is an HF transceiver of about 100 watts, a feed line and an outdoor antenna. HF antennas are much larger than VHF ones: a half-wave dipole is about 33 feet long for 20 meters and 65 feet for 40 meters. Wire dipoles, end-fed wires and verticals are the common choices, often with an antenna tuner to cover several bands, and serious DX stations add a rotating Yagi on a tower.