FM Radio stations make excellent beacons. They are plentiful and widespread. Several new tropospheric ducting paths were first identified and conformed by listening for FM radio stations, including the paths from St. Helena Island to Southern Africa, and Ascension Island to Brazil. Most FM stations stay on the air around the clock, and do not shut down late at night and the early morning hours when transoceanic ducting is at its best. A log periodic antenna is essential to cover the whole band. Quite a few low power FM stations transmit with vertical polarization, so an antenna that can be rotated in polarity, or two separate antennas for horizontal and vertical polarization, are required. If two antennas cannot be used, then a single yagi at a 45 degree slant may be a good compromise as it is equally down -3dB from both horizontal and vertical polarities. A good receiver is also essential, requiring excellent sensitivity and selectivity. High performance wideband receivers such as the Icom R7000, R7100, R-8500, R-9000, and R-9500 are all good receivers. Professional grade receivers like the Rohde & Schwarz ESM-500A or similar are also very good. Other surprisingly low cost receiver with stunning performance are the Sony HD radio series. The XDR-S10Hdip is a full receiver, while the XDRF1HD is a tuner only without an amplifier. A good source of information on FM stations is the www.fmscan.com web page.
Analog (non-digital) TV Stations are also plentiful and widespread. VHF High Band stations in the 174-216 MHz range are god indicators of 144 MHz propagation, while UHF stations are excellent indicators of 430 MHz openings. Many TV stations go off the air late at night. They often run higher power than FM stations. The video carrier is the strongest component of the TV signal, but is not easily identified. The only practical way to identify Video Carriers is by knowing their frequency VERY precisely. However, they are useful for showing that the band is at least open in a general direction. Log periodic yagis are the best antennas. Wideband receivers such as those used for monitoring FM stations are good choices for listening to TV stations. The analog TV signal has to be very strong before a picture good enough to serve as identification is visible, but the FM audio channels a can be well received well before the video comes in well.
Aviation Band signals from 108-136 MHz have also been long used to indicate the presence of ducting. Some years before amateurs first worked the duct between California and Hawaii, pilots taking off from San Francisco would report contacts with the control tower in Honolulu as they climbed above the Golden Gate Bridge. As they climbed higher, above the duct trapping layer, the communications was lost until the aircraft came within visible sight of the station in Hawaii. Airport stations are also on well publicized lists, and the air traffic controllers usually identify on every transmission.
Marine Band stations in the 156-163 MHz range useful to monitor for band openings. The duct from Ascension Island to Cape Town was first identified by the reception of Cape Town Harbor Radio on Ascension. However, transmissions can be fairly sporadic, and ships usually only communicate with the port facilities when they are fairly close to the harbor. With commercial vessels ranging the full extent of the oceans, it would be very helpful if they would be on transmitting frequently as they crossed the oceans, our area of interst. Essentially, that would amount to a beacon network with thousands of stations spread across the oceans. Fortunately, that beacon network is exactly what is available!
The maritime Automatic Identification System (AIS) is a systems used by ships for identifying and locating vessels. AIS provides identification, position, course, and speed, with other nearby ships and VTS stations. This information can be displayed on a PC using simple receivers and PC software, or dedicated receivers with internal modems can also be used. While intended for exchanging data between vessels directly, and between vessels and shore-based Vessel Tracking Services, it is also monitored by many private individuals, operating on two channels around 162 MHz. Several of the software packages also link the data onto Internet position servers
Public Service Band signals are not as universally helpful as the other services listed above. As with television video carriers, they are typically more useful for showng the prescense of an opening rather than the specific source. Most operators do identify with a location. Many public service radio systems are switching to digital radio, further complicating the issue.