Global coverage can be routinely achieved at high latitudes during the late fall and winter, by stations using modest power and common antennas.
With very high transmitting antennas or large vertically polarized arrays and full legal power, reliable worldwide communications occurs over nighttime paths.
Horizontally polarized antennas closer than a quarter-wave to earth produce predominantly high-angle radiation, which is useful for short-distance propagation modes, such as near vertical incidence skywave.
At higher latitudes, a noticeable skip zone sometimes appears on the band during nighttime hours in midwinter, which can be as much as 300 miles / 500 km, rendering communication with closer stations impossible.
During spring and summer (year-round in the tropics), lightning from distant storms creates significantly higher background noise levels, often becoming an insurmountable obstacle to maintaining normal communications.
Nearby convective weather activity during the summer months can make the band completely unusable, even for local communications.
In the winter months during the peak years of the sunspot cycle, auroral effects can also render the band useless for hours at a time.
As a result, authorities in the affected parts of the world restrict amateur allocations between 3.7 MHz and the top of the band.
It is common for illegal marine operations, generally using USB voice ("phone"), to occupy frequencies on the low end of 80 meters.
But depending on quality and condition of radio, audio characteristics, and proper adjustments the bulk of emissions on lower sideband will typically occupy 3.9970–3.9997 MHz.
All SSB transceivers have third- and fifth-order products of significant level, typically only 30–35 dB below PEP for third order intermodulation.
This means any operation above 3.998 MHz even lower sideband (LSB) comes with some risk of illegal emissions, even with good equipment.
Wide detection bandwidth, slow sweep rates, and common, loud, local ambient noise all mask the presumably weak emissions that survive a transmitter's internal filtering.
When it is night on both ends of the transmission path some broadcasters in Asia and Europe can be heard in North America between 3.9–4.0 MHz.