Skywave Deck

01 · Propagation

Ground Wave, Sky Wave, Skip

HF communication over long distances depends on refraction of the transmitted signal by the ionosphere. A station one thousand miles distant can be received at greater strength than a station nearby, and a given station is received on different frequencies as solar illumination changes.

The Skip Path

A single refraction from the F-layer carries a signal hundreds to thousands of miles beyond the horizon. Ground wave attenuates within approximately 100 miles. The aircraft is drawn above true scale for legibility; the blue tick at the ground line marks cruise altitude to scale.

☀DayThe D-layer forms at low altitude and absorbs the low bands (below approximately 7 MHz). The higher bands (15/20 m, 20 MHz WWV) refract well. Usable frequencies rise with daylight.

☽NightThe D-layer dissipates. Low bands (75/40 m, 5/2.5 MHz WWV) propagate over long distances. The highest bands frequently penetrate the F-layer without refraction.

Airborne Reception Characteristics

Cruise altitude (FL410, approximately 12 km) has negligible effect on the geometry above. The ionospheric layers lie at 150–400+ km. Skip geometry at cruise altitude is essentially identical to that of a ground station. Three characteristics differ:

✈Unobstructed horizonNo terrain, structure, or vegetation shadows the aircraft antenna's low-angle radiation in any direction. Ground stations are subject to such shadowing and depend on higher-angle, shorter-hop propagation.

📡Electrically short antennaThe aircraft's leading-edge or tail-cap antenna is electrically short at the low end of HF. A quarter wavelength at 3 MHz is approximately 25 m, longer than the airframe. Every airborne HF installation therefore requires an antenna coupler, described in Section 03.

∅No ground waveGround-wave propagation requires a conductive surface path along the earth. An aircraft in flight receives sky wave only. The short-range coverage available to a ground station through ground wave is absent at altitude.

Auroral Propagation

The growl or warble received on HF near the aurora is a documented and named propagation phenomenon, distinct from atmospheric noise.

Mechanism: the aurora consists of moving curtains and patches of ionization in the E-layer, each drifting and pulsing independently. A signal refracted by that continuously changing surface arrives by many slightly different and shifting paths at once. Each path carries its own small Doppler shift, and the overlapping copies beat against one another in real time. Amateur operators log such a contact as “5&9 aurora” and describe the sound as a rough mechanical flutter, at times low enough in rate to be perceived as a buzz or growl.

Concurrent effects: auroral and polar-cap absorption attenuate the low end of HF. The 2–5 MHz range is affected first and most severely, and recovers last; the upper 20s of MHz remain usable longest. The flutter itself is best documented at the high end of HF and into VHF. A disturbed polar ionosphere therefore produces a weaker low-band signal and a distorted high-band signal through two separate mechanisms.

Real-time indicator: the planetary Kp index (spaceweather.gov). Kp 0–3 is quiet. Kp 5 (NOAA G1) indicates degraded polar and high-latitude paths. Kp 7 and above (G3) indicates significant degradation extending well beyond the polar regions.

Polar-route significance: geostationary SATCOM has a permanent coverage gap above approximately 80–82°N, where the satellite lies below the horizon. This gap is fixed by orbital geometry and independent of space weather. HF's polar limitation, Polar Cap Absorption, is driven by solar proton events; it is intermittent, forecastable, and centered on the same polar cap. A strong solar event can therefore remove the HF backup in the same region where SATCOM coverage is already absent. Polar dispatch incorporates space-weather forecasts for this reason, and oceanic NOTAMs flag HF as degraded near the pole under such conditions.

02 · Modulation

Sideband, Decoded

USB, LSB, and AM differ in one respect: the fraction of the transmitted signal that carries information. Every entry in this reference depends on that distinction.

The Three Basics: Amplitude, Wavelength, Frequency

A radio wave is fully described by three quantities: amplitude, wavelength, and frequency. Sideband describes which frequencies are present in a signal and the amplitude carried by each.

↕AmplitudeThe strength of the wave, measured as its height above the centerline. In AM, amplitude is the single dimension that carries the audio (see the envelope diagram below).

↔Wavelength (λ)The distance the wave travels in one full cycle, measured in meters. This is the “m” row beneath the MHz axis in Section 05.

↻Frequency (f)The number of full cycles per second, measured in Hz (or MHz, one million cycles per second). Every tuned frequency in this reference is stated in this unit.

All radio waves travel at the speed of light. Wavelength and frequency are therefore fixed in relation to each other: λ (m) = 300 ÷ f (MHz). Higher frequency corresponds to shorter wavelength, the inverse curve on the axis in Section 05. 2 MHz is a 150 m wave; 30 MHz is a 10 m wave. Neither figure describes amplitude. Amplitude is an independent quantity, and it is the quantity AM modulates.

Carrier frequency and waveform are two different kinds of plot. A frequency is a position on the spectrum. The plots directly below run frequency left to right: a spike marks the position of a signal's energy on the spectrum and carries no information about the instantaneous voltage. A waveform is the second kind of plot, voltage against time, the trace an oscilloscope draws and the view shown in “Two Views of the Same AM Signal” below. An unmodulated carrier is the simplest waveform: one pure sine wave, repeating without change of shape. Modulation impresses a second signal, the audio, on that carrier. The frequency plot below shows the cost: energy spreads from the single carrier spike into sidebands on either side. The two plots describe one event on two axes.

From Carrier to Single Sideband

Modulating a carrier with audio produces a mirrored copy of the audio spectrum on each side of the carrier. Those mirrors are the sidebands. SSB transmits exactly one of them and suppresses the carrier and the other sideband.

Terms, Precisely

Carrier. The pure tone at the tuned frequency. Unmodulated, it carries no information; it is the signal on which modulation is impressed.

USB

Upper sideband. The audio spectrum, translated to lie above the carrier frequency.

LSB

Lower sideband. The same audio spectrum, mirrored below the carrier. It duplicates the information in the upper sideband.

Purpose of suppression. AM expends most of its transmitter power on a carrier that carries no message, plus a second sideband duplicating the first. SSB suppresses both. Nearly all transmitter power goes into the one information-bearing sideband, which yields approximately 3–4× the effective range per watt in approximately half the bandwidth.

~6 kHz

SSB

~2.7–3 kHz

Two Views of the Same AM Signal

In the time domain, AM appears as a fast carrier wave whose height traces the slower audio tone: the envelope. In the frequency domain (the panels above), the same signal is a spike flanked by two sidebands. The two views describe one signal. The top and bottom of the waveform are the positive and negative swing of a single voltage, and both sidebands are present in every instant of it. USB and LSB are separable only in frequency.

WWV's use of AM: a plain AM signal demodulates on the simplest receiver. No local oscillator is required to replace a suppressed carrier. For a public time-and-frequency broadcast, that simplicity and compatibility with every receiver in service outweigh the additional bandwidth. Every other service in this reference (military, utility, oceanic, amateur) addresses a narrower audience and uses SSB.

The LSB-below-10 MHz / USB-above convention is an amateur-service convention of long standing, adopted so that two amateur stations default to the same sideband without coordination. Government and military services do not follow it: HFGCS operates USB on every channel, including 4724 and 6739 kHz. COTHEN and the oceanic ARINC families operate USB throughout. WWV and WWVH operate AM on every frequency. Outside the amateur bands, sideband is determined by the service, and each entry in this reference states it.

Aural identification: a steady one-second tick and a voice reading the time identifies AM and the WWV family. A burst of warbling tones followed by clipped voice identifies USB and, in nearly every case, a utility or military network completing an ALE handshake.

03 · Equipment

The HF-9000 Installation

On the Embraer Pro Line Fusion flight deck, the HF-9000 is controlled through an MFD software page. The rendering below reproduces HF1 CONTROL as displayed on N406AD: a one-page window overlaid on the Tuning menu. The “2 / 2” indication in the corner belongs to the underlying Tuning menu.

HF1 CONTROL, As Displayed

The window is an interactive reproduction of HF1 CONTROL and matches the aircraft display. The dashed box beneath it contains simulator-only controls that have no counterpart on the aircraft. No control on this page reaches a radio.

M TUNING 2 / 2

HF1 CONTROL

10.0000

POWER

MODE

EMISSIONS

HF PRESET FREQUENCIES

Simulator controls — not on the real HF1 CONTROL screen

enters a frequency into the tuning window, as the aircraft scratch pad does

receive audio for the tuned frequency. The first PTT engagement on a new frequency initiates the coupler tune (the tone). The next engagement transmits. On 5616, that transmission initiates the exchange.

Frequency library — audio available in the simulator10 frequencies

MHz	Station	Recording	Selection
2.5000	WWV Fort Collins CO	Off-air, Dec 8 1991. Second ticks and the voice time announcement.	Preset 1
3.3300	CHU Off airOttawa ON	NRC's own recording of the final minutes of service, Jun 22 2026, 3.33 MHz. Bilingual voice announcement and second pips.	Scratch pad entry: 3.33 or 3330.
4.6250	UVB-76 “The Buzzer”	Off-air, May 13 2018, via the Twente WebSDR. Buzzer tone only.	Scratch pad entry: 4.625 or 4625.
5.0000	WWVH Kauai HI	Off-air, 1980s. Station ID sample.	Preset 2
5.6160	Gander Radio ScriptedNAT-B family	Scripted exchange. N406AD check-in: position report, SELCAL check BG-PR, primary and secondary frequencies. Transcript below.	Preset 5. The first PTT engagement tunes the coupler; the second initiates the exchange.
7.8500	CHU Off airOttawa ON	Same Jun 22 2026 recording.	Scratch pad entry: 7.85 or 7850.
10.0000	WWV Fort Collins CO	Same Dec 8 1991 recording.	Preset 3. Initial frequency on load.
14.6700	CHU Off airOttawa ON	Same Jun 22 2026 recording.	Scratch pad entry: 14.67 or 14670.
15.0000	WWVH Kauai HI	Same 1980s station ID.	Preset 4
20.0000	WWV Fort Collins CO	Same Dec 8 1991 recording.	Scratch pad entry: 20 or 20000. No preset holds this frequency on N406AD.

On the air, WWV and WWVH share 2.5, 5, 10, and 15 MHz. The simulator assigns each recording its own frequencies so that one station is audible at a time. CHU ceased transmission on June 22, 2026; its three rows carry NRC's recording of the final minutes of service. The 5616 kHz exchange is the one scripted item: no licensable off-air recording of a NAT check-in exists, so the exchange is constructed from the published procedure. The frequency is silent until the aircraft transmits. The first PTT engagement tunes the coupler; the second initiates the exchange, which runs from the aircraft's initial call. Every other frequency in the 2.0000–29.9999 MHz range is silent. The keyed tone is available throughout that range.

5616 kHz, as scripted

N406AD Gander Radio, Gander Radio, November four zero six alpha delta, position, on five six one six.

Gander November four zero six alpha delta, Gander Radio, go ahead.

N406AD November four zero six alpha delta, position. Five zero north zero five zero west at one four two two. Flight level four one zero. Estimating five one north zero four zero west at one five zero five. Next five two north zero three zero west. Request SELCAL check, bravo golf papa romeo.

Gander November four zero six alpha delta, Gander Radio copies. [position report read back] SELCAL check coming.

SELCAL BG, then PR. Two simultaneous tones per pair, one second each, 0.2 second between pairs.

Aircraft Aural voice message, no chime: SELCAL, SELCAL. Voice message 88 in the AOM aural warning table (9-15-45, Rev 04): incoming communication from HF or VHF. The HF SELCAL CAS message and the aural reset on PTT engagement.

N406AD SELCAL checks okay, November four zero six alpha delta.

Gander November four zero six alpha delta, roger. Primary Gander five six one six, secondary eight eight six four.

WWV, WWVH, CHU, and UVB-76 audio are off-air recordings from the Shortwave Radio Audio Archive, via the Internet Archive, CC BY‑NC 3.0. WWV: Dec 8, 1991, 5000 kHz, Rockford IL (rec. Myke Dodge Weiskopf). WWVH: circa 1980s station ID (rec. via SRAA). UVB-76: May 13, 2018, 4625 kHz, via the University of Twente WebSDR (rec. Pedro Calhau). CHU: Jun 22, 2026, 3330 kHz, NRC's own recording of the final minutes, made outside the Ottawa station (via Ronald McKinnon and SRAA). The 5616 kHz exchange is scripted: two synthesized voices through an SSB-bandwidth filter, SELCAL tones generated from the ICAO tone table, and a third synthesized voice for the aircraft's SELCAL aural. The keyed tone is synthesized: a steady tone for the duration of PTT engagement, matching direct observation on N406AD.

#	Control	Function
1	Frequency field	Direct frequency entry, displayed to four decimals in MHz. The bowtie cursor indicates the digit group that the CCD or line-select keys edit next.
2	POWER	LO / MED / HI transmit power selection. Output figures are given under “Transmitter Output Power” below.
3	MODE	SIMPLEX: one frequency for transmit and receive; the mode for every service in this reference. DUPLEX: paired transmit and receive frequencies, used on some oceanic channels. EMER / MAR: emergency and maritime calling conventions. SIMPLEX is the mode for receive-only monitoring.
4	SQ — squelch	Squelch threshold, stepped 0–3. Higher settings mute progressively weaker and noisier signals.
5	EMISSIONS	LV / UV / AM: the HF-9000 designations for lower sideband voice, upper sideband voice, and full-carrier AM. These are the three modes described in Section 02.
6	EDIT	Reprograms a preset channel. Adjacent to TEST on the display; the two functions are independent.
7	TEST	Initiates the radio self-test.
8	HF Preset Frequencies	Scrollable preset memory. On N406AD, presets 1–4 hold the WWV/WWVH frequencies listed in this reference.

Presets on N406AD. Presets 1 through 4 hold the WWV/WWVH frequencies listed in this reference: 2.5 / 5 / 10 / 15 MHz, tagged UTC1–4. Preset 5 is a NAT oceanic backup at 5.616 MHz. The time-standard frequencies therefore require no manual entry.

Terminology: EMISSIONS on the HF1 CONTROL page reads LV / UV / AM. Elsewhere in this reference, and in Section 02, the same three modes are written LSB / USB / AM.

Frequency notation: the frequency field displays four decimals in MHz; 10 MHz is 10.0000. Outside the amateur bands, this reference states frequencies in kHz, the utility and military convention. Entry into the frequency field requires division by 1000: 8918 kHz is entered as 8.9180. Amateur frequencies are stated in MHz and require no conversion.

Transmitter Output Power

POWER	Output	Notes
LO	≈10 W PEP	Figure reported in Collins Aerospace product literature. The N406AD Pro Line Fusion AFM (Fig 5-91, HF Control page) documents the LO/MED/HI selection and does not publish per-step output; that figure is carried in the equipment-level manual.
MED	≈50 W PEP	Same source as LO.
HI	175 W PEP	Confirmed in the Collins instruction book. 200 W PEP on aircraft fitted with the higher-power receiver-transmitter (HF-9087D/F) and the matching HF-9012D control head; that is a different unit, and there is no fourth switch position.

For scale, 175 W is roughly one tenth to one third of the power of a domestic microwave oven, and it carries a voice signal thousands of miles. SSB concentrates transmitter power in the single information-bearing sideband (Section 02), and sky-wave refraction provides the distance. A broadcast AM station requires kilowatts to cover a metropolitan area with a signal intended for every receiver in it. An aircraft HF installation closes one link to one ground station that is listening for it.

Two radios, one antenna: HF1 and HF2 each have a dedicated antenna coupler. The two couplers are mounted together and feed the same antenna lead-in. An interlock between them prevents simultaneous transmission: a PTT engagement on HF2 while HF1 is transmitting is refused, and the display reads “other system keyed.” The radio that is not transmitting is disconnected from the antenna for the duration of the other radio's transmission and receives nothing. A coupler retune produces an interval of silence of approximately one second, and up to six seconds on a frequency the coupler has not previously matched. A preset or recently used frequency retunes almost immediately. A faint, distorted trace of the other radio's transmission on a nearby frequency is normal bleed-through and does not indicate a fault.

The tone on PTT engagement is a steady single tone. It is present for the duration of PTT engagement and ceases on release. This is confirmed by direct observation on N406AD. The antenna coupler performs its impedance-matching sequence concurrently. The aircraft antenna is electrically short at HF, so the load it presents to the transmitter differs from the 50-ohm load the transmitter is designed to drive; without correction, part of the transmitter power reflects back from the antenna and is lost. The coupler corrects this by switching a network of inductors and capacitors in steps while a discriminator measures the standing wave ratio, the proportion of power being reflected, and stops when that ratio reaches its minimum. The sequence produces no audio in the flight crew's headset. The audible tone is the keyed tone itself, for the length of the transmission. This is standard behavior for a keyed HF transmitter.

04 · Propagation Indicator

The Solar Flux Gauge

WWV broadcasts the Solar Flux Index at 18 minutes past each hour; WWVH broadcasts it at 45 minutes past. It is the most direct real-time indicator of which part of the HF spectrum is propagating.

SFI < 70: low bands propagate. WWV on 5 / 2.5 MHz; amateur 40 m / 75 m. The high bands are unlikely to be open.

SFI 70–150: normal conditions. The frequencies in this reference propagate as listed.

SFI > 150: high bands propagate. WWV on 15/20 MHz; amateur 15 m/10 m.

When HFGCS 11175 kHz is weak in daylight, 8992 kHz commonly carries the same traffic at usable strength. An amateur band without audible voice traffic may still be open. The FT8 digital segment decodes signals below the threshold of voice reception and indicates whether the band is propagating.

05 · Spectrum Allocation

2–30 MHz At A Glance

Every category in this reference, positioned by frequency. Each mark carries its exact figure. Dense clusters (COTHEN, HM01) are drawn as bands; the reference tables in Section 07 list every discrete channel.

One channel

Primary channel, labeled

Frequency range, several channels; listed in Section 07

06 · Day and Night Frequencies

Frequency Selection by Time of Day

Each service is received on a different frequency before and after sunset. This table restates Stages 1 and 2 of the monitoring sequence (Section 09) as a lookup.

Category	☽ Night frequency	☀ Day frequency
WWV / WWVHNIST time & frequency standard	5 MHz2.5 MHz in the late night hours	15 MHz20 MHz near solar peak
HFGCSUSAF global communications, EAMs	8992 kHznight primary	11175 kHzprimary day channel
Oceanic ATCPosition reports & clearances	3–6 MHzNAT-B 2899 / 5616	8–11 MHzCAR-A 8918, NAT-A 8906
COTHENCBP/DEA interdiction traffic	5732 / 5909.5lower channels active	8912 / 10242highest daytime activity
Amateur RadioAmateur service, SSB & FT8	7.200 / 3.98540 m & 75 m open	14.300 / 21.30020 m & 15 m open
Numbers & CuriositiesIntelligence traffic and unidentified signals	4625 kHzUVB-76, stronger after dark	HM01 slotsoff air; status at priyom.org

07 · Reference Tables

Services by Category

WWV & WWVH

NIST time and frequency broadcasts, continuous: minute tone, second ticks, and a propagation report at 18 (WWV) or 45 (WWVH) minutes past each hour. The reference station for every other entry.

Station	Frequencies (kHz)	Mode	Best time	Signal	Notes
WWV Fort Collins, CO	2500·5000·10000·15000·20000	AM	10 MHz all-around · 15/20 day · 2.5/5 night		Male voice announcement, “At the tone…”; second ticks; 1000 Hz minute tone; 440 Hz at minute 2; geophysical alert at minute 18; marine warnings at minute 8.
WWVH Kauai, HI	2500·5000·10000·15000	AM	15 MHz often best from the West		Female voice announcement, which distinguishes WWVH from WWV; 1200 Hz minute tone; 440 Hz at minute 1; geophysical alert at minute 45; storm warnings at minute 48.
CHU Ottawa, ON (NRC)	3330·7850·14670	AM (voice) + digital code	—	Off air	Transmission ceased June 22, 2026 after more than 90 years of service, an NRC budget decision. The format was a bilingual English/French voice announcement, alternating by the second, with a digital time code beneath the tone. The frequencies are silent. The simulator in Section 03 carries NRC's recording of the final minutes.

Shared frequencies: WWV and WWVH both transmit on 2.5/5/10/15 MHz. Under favorable night conditions both voices are received on the same frequency. CHU, formerly the stronger time signal in the Northeast, ceased transmission on June 22, 2026. WWV and WWVH are the only time signals remaining in this reference, and they carry the SFI report.

WWV — :02 tone · :08 storm · :18 SFI

WWVH — :01 tone · :45 SFI · :48 storm

HFGCS — “Skyking”

The US Air Force High Frequency Global Communications System. Emergency Action Messages in NATO phonetics, transmitted at any hour and received readily throughout CONUS. USB on every channel, including 4724 and 6739 kHz.

kHz	Role	Best time	Notes
4724	Primary	Night	Simulcast EAMs, frequently with a characteristic echo between sites.
6739	Secondary	Day / night
8992	Night primary	Night	Strong EAM channel; the alternate when 11175 is quiet.
11175	Day primary	Day	The most monitored HFGCS channel.
13200	Secondary	Day
15016	Secondary	Day
6712	Croughton (UK) discrete	—	Weak from CONUS.

Traffic format: EAMs are coded strings read in NATO phonetics, frequently simulcast from several sites at once, which produces the echo. “Skyking” traffic has priority over all other traffic and interrupts it: “Skyking, Skyking, do not answer…” followed by a time and authentication group. Ground stations (Andrews MD, Offutt NE, Beale AFB CA, Elmendorf AK, Hickam HI, Salinas PR) ceased self-identification around 2013 and use the collective call “MAINSAIL.” Strong reception zones: Pacific Northwest / Montana and Cape Cod / Northeast.

Oceanic ATC — ARINC

New York Radio and San Francisco Radio handling position reports, oceanic clearances, and SELCAL checks with aircraft over open water. Air traffic control traffic, unencrypted. USB on every family, including the 2–3 MHz NAT-B and CAR-A channels.

Family	Frequencies (kHz)	Stations	Notes
NAT-A	3016·5598·8906·13306	Gander, New York, Santa Maria, Shanwick	Southern NAT routes
NAT-B	2899·5616·8864·13291·17946	Gander, New York, Santa Maria, Shanwick	Central / northern routes
NAT-E	2962·6628·8825·11309·13354	New York, Santa Maria
CAR-A	2887·5550·6577·8918·11396·13297·17907	New York, Panama, Piarco	Caribbean — strong from CONUS
CAR-B	3455·5520·6586·8846·11330·17907	New York, Panama, Paramaribo	Caribbean
Pacific (SF Radio)	2932·5547·5628·6655·8843·8915·10048·10057·11282·13300·13354	San Francisco / Oakland	CEP / CWP / NP families — Western CONUS

Commonly active (CONUS reports): 6577 · 6586 · 8879 · 8891 · 8906 · 8918 · 11279 · 11336 · 11396 · 13297 · 13306 kHz. Traffic peaks with the daily traffic flows: eastbound in the evening (UTC), westbound at midday (UTC). Authoritative frequency charts: radio.arinc.net.

COTHEN & SHARES

Customs Over-The-Horizon Enforcement Network: CBP, Coast Guard, ICE, DEA. The highest volume of federal HF voice traffic remaining in CONUS since the USCG voice guard was discontinued.

Net	Frequencies (kHz)	Notes
COTHEN (USB / ALE)	5732·5909.5·7527·8912·10242·11494·12222·13312·13907·14582·15867·18594·20890·24838.5	Active daily 8912 & 10242 carry the highest daytime activity; the lower channels are active at night.
FEMA / SHARES (USB)	5211 (night) · 10493 (day) · 6809 · 5821 · 14396.5 (SHARES primary) · 5236 (secondary)	Intermittent: nets and exercises. Many newer channels are undisclosed.

Traffic format: continuous ALE handshakes, short bursts of warbling tones as stations link automatically, followed by USB voice: aircraft interdiction calls and marine patrol traffic.

Amateur Radio

Licensed operators on SSB and FT8. These are the most consistently present signals on the HF spectrum under any conditions, and they serve as a reference for whether a band is open.

Band	SSB calling	FT8 (digital)	Notes
75 m	3.985 LSB	3.573	Evening / night, regional
40 m	7.200 LSB	7.074	Reliable day & night
20 m	14.200 / 14.285	14.074	Best all-around DX band
17 m	18.130	18.100
15 m	21.300 / 21.385	21.074	Daytime, better in high sunspots
10 m	28.400 (29.600 FM)	28.074	Daytime, sporadic-E openings

Scheduled nets: Maritime Mobile Service Net, 14300 kHz USB, daily approximately 1600–0200 UTC, near-continuous. Pacific Seafarers Net, 14300 kHz, approximately 0300 UTC. County Hunters Net, 14336 kHz, daytime when 20 m is open. 3905 Century Club, approximately 3905 kHz LSB, evenings.

Numbers & Curiosities

Cold War-era stations and one-time-pad intelligence traffic. Legal to receive and impossible to decode. The rarest receptions in this reference.

Station	Frequencies (kHz)	Mode	Status	Notes
HM01 Cuban DGI	9065·9155·9240·9330·10345·10715·10860·11435·11462·11530·11635·12180·13435·14375·16180·17480	AM + digital	Off air	Transmitter failed 23 Aug 2024. A few seconds of its audio appeared within Radio Habana Cuba broadcasts as late as Feb 2026, which indicates that the transmitter remains connected. No return to service has occurred. Current status is published at priyom.org.
UVB-76 “The Buzzer”	4625	USB	Rarely received	Continuous buzz tone with occasional Russian voice and number groups. Rarely received in CONUS; reception is propagation-dependent and more likely at night.
The Pip ENIGMA S30	5448 day · 3756 night	—	Active	Continuous marker beep, approximately 50 per minute; occasionally interrupted by a spoken Russian “for” and a callsign test message. Russian Southern Military District, Rostov-on-Don. One of several channel-marker beacons of the same type as UVB-76.
Squeaky Wheel ENIGMA S32	5473 day · 3828 night	—	Active	Same family and district; a creak/squeak marker tone in place of a buzz or beep. UVB-76, the Pip, and Squeaky Wheel are logged together as the “marker trio.”
S06 “Russian Six”	variable, published schedule	USB + carrier	Reported active	Russian male voice reading paired 5-digit groups: the message traffic itself, for which UVB-76 is the channel marker. Sites reported at Moscow, Orenburg, and Chita.
V02a “Atencion,” Cuban DGI	7555 (LSB)	LSB	Discontinued	Opened with the spoken word “Atencion,” Spanish female reader. Superseded by HM01 in 2012. Recordings of this station were entered as evidence in the 1998 Cuban Five espionage trial. A separate station from ENIGMA S06, with which it is frequently conflated.
E11 Polish military intelligence	seasonal rotation	USB	Reported active	English, Eastern European-accented female reader; transmissions of approximately 3 minutes 10 seconds. Transmitter reported near Chotomow, north of Warsaw. On one logged occasion a Windows XP shutdown chime was transmitted after sign-off.
E10 Attributed to Mossad	multiple simulcasts	USB + carrier	Discontinued	Female reader transmitting NATO-phonetic letters in place of digits; one of the few major stations that used letter groups. Attributed by the monitoring community to Israeli intelligence, never confirmed. Last reported March 2011.
The Koreas V15 (North) · V24 (South)	V15: 657·3320·6400 · V24: 4900–6310 (9 channels)	Voice, AM	Dormant	V15 carried its groups within ordinary Pyongyang domestic broadcasts; not heard since March 2020. V24 was the South Korean counterpart, with K-pop interval tunes, also silent since 2020. V24 is frequently mislabeled as North Korean in informal accounts. It was operated from South Korea.
Lincolnshire Poacher / Cherry Ripe ENIGMA E03 / E03a	5422·6485·8464·11545·…	AM	Discontinued	The most widely documented numbers station: RP-accented reader, interval signal looped from the folk tune of the same name, traced to an MI6 site at RAF Akrotiri, Cyprus. Off air since 2008. Cherry Ripe was its Pacific counterpart (Guam, then Australia), off air since 2009.
Swedish Rhapsody ENIGMA G02	historical, ~5733–11525	AM	Discontinued	German “Achtung!” opener over a music-box interval tune, generated by an East German Stasi voice synthesizer. Declassified Polish files confirmed the operator as Polish intelligence. Operated into the late 1990s; a brief English-language revival ended by 2007.
Yosemite Sam vs Cuban Jammer two different things	3700–10500 · 5980–13820	Clip / DSB noise	Correction	“Yosemite Sam,” a looped cartoon audio clip heard Dec 2004–Feb 2005, traces to a New Mexico test facility. The Cuban jammer is a separate broadband noise signal that remains active on Radio Martí frequencies.

Encryption: genuine numbers traffic uses one-time pads, which are mathematically unbreakable without the pad. The content is unrecoverable. The reception itself is the object of monitoring.

Status reliability: these stations are monitored entirely by volunteers. Stations cease and resume transmission without notice. “Active” in this table means most recently reported. It carries no schedule information. The live schedule at priyom.org supersedes every status shown here.

08 · Discontinued Services

Discontinued Services

Discontinued

Service	Frequencies (kHz)	Off since
New York VOLMET	3485·6604·10051·13270	~2018
Gander VOLMET	3485·6604·10051·13270	12 Jun 2025 (NAV CANADA 2025-06-12)
USCG HF voice guard	4125·6215·8291·12290	7 Feb 2022 (MSIB 10-21)
USCG MF voice/DSC	2182·2187.5·2670	2013

Remaining VOLMET services: Shannon VOLMET (Ireland), 3413/5505/8957/13264 USB, remains in operation and is received in CONUS at night. Honolulu VOLMET (2863/6679/8828/13282) covers the Pacific.

09 · Monitoring Sequence

Monitoring Sequence

Three stages, ordered from the most reliable receptions to the least. On a long leg, the stages are worked in order.

Baseline Receptions

Available at any hour from either coast

WWV — 10 MHz day, 5 MHz night
HFGCS — 11175 day, 8992 night
MMSN — 14300 amateur net
Broadcaster — WWCR 15825 or WRMI 9955 for a strength reference

Favorable Propagation

Requires open bands

COTHEN — 8912 / 10242
Oceanic ATC — 8906 / 8918 / 11336
WWVH — 15 MHz, female voice, received alongside WWV's male voice
Amateur FT8 — 14.074, propagation confirmed on the waterfall display

Rare Receptions

Low probability of reception

HM01 — off air since Aug 2024; current status at priyom.org
Shannon VOLMET — transatlantic reception, at night
UVB-76 — 4625 kHz, the Buzzer

10 · Amateur Licensing

Amateur Radio Licensing

Every signal in this reference is legal to receive without a license. Transmission on the amateur frequencies requires an FCC amateur license. The classes, examinations, and operating rules are summarized below.

License Classes

Class	HF SSB phone privileges
Technician	Minimal. The only HF phone allocation is a narrow segment of 10 m (28.300–28.500 MHz, 200 W maximum). Technician privileges on 80/40/15 m are CW and data only. SSB voice on 80/40/20/17/15 m, the entire amateur portion of this reference, is not authorized at this class.
General	The class required for the amateur frequencies in this reference. Authorizes SSB voice on the principal phone segment of every HF band: 80 m (3.800–4.000), 40 m (7.175–7.300), 20 m (14.225–14.350), 17 m (18.110–18.168, the full band), 15 m (21.275–21.450), 10 m (28.300–29.700). Full 1500 W PEP limit.
Amateur Extra	Adds narrow segments at the low end of four bands (80 m down to 3.600, 20 m down to 14.150, and corresponding segments on 40/15) and eligibility for the shortest call sign formats. HF phone privileges are otherwise the same as General.

Examinations

Technician (Element 2): 35 questions from a published pool, 26 required to pass (approximately 74%). No prerequisite.
General (Element 3): 35 questions, 26 required to pass. Requires Technician credit. Most VE sessions permit both elements to be taken consecutively in one session.
Extra (Element 4): 50 questions, 37 required to pass. Requires General credit. The mathematics and theory content is substantially greater than at General.
Administered by volunteer examiners (ARRL VEC and others), in person or by remote video proctoring. The session fee is approximately $15 and covers every element attempted in that session. A one-time $35 FCC fee applies to the resulting license; the fee is waived for a same-session upgrade that retains the existing call sign.

Part 97 Operating Rules

Station identification by call sign, in English, at the end of every contact and at least every 10 minutes during a contact.
1500 W PEP is the maximum output. The governing rule is the minimum power necessary to carry out the communication.
Encrypted or obscured-meaning traffic, business communications, and paid communications are prohibited.
Third-party traffic (a message relayed on behalf of a non-licensee, as MMSN does on 14300) is unrestricted domestically and, internationally, limited to countries holding a third-party agreement with the United States.

RF exposure: before transmitting from a given location, the licensee is required to confirm that the station will not expose any person to RF above FCC limits. The evaluation is self-administered; VEs have no role in it. A typical 100 W HF station on 15 m and below generally falls under the threshold. Higher power more frequently requires a written evaluation. Mobile stations are exempt.

Call signs: new Technician and General licensees are generally assigned a 2x3 call sign (for example KI5ABC); Extra licensees are eligible for shorter formats. Any class may apply for a specific available “vanity” call sign within its format group for a $35 FCC fee.

Study material: hamstudy.org and the current ARRL license manual. Examination sessions are listed at hamstudy.org/sessions.

11 · Regional Reception

Montana and New York

Montana

WWV — short path, approximately 700 miles; received on at least one frequency at nearly all times.
HFGCS — the Pacific Northwest is a consistently strong reception zone.
San Francisco Radio — the Pacific oceanic families (CEP/CWP/NP) are the nearest ARINC stations.
Honolulu VOLMET — the Pacific aviation weather broadcast, within reception range.

New York

WWV — S6–S9+ on a wire antenna. CHU, formerly the stronger time signal in the Northeast, ceased transmission June 22, 2026.
HFGCS — Cape Cod / Northeast is a documented strong zone.
New York Radio — the transmitter for NAT oceanic traffic is nearby.
Caribbean ARINC — the CAR-A/B families are consistently strong from the East Coast.