AFTERNOON TRANSEQUATORIAL VHF PROPAGATION
VHF transequatorial propagation (TEP) is so called because it involves the reception of VHF signals, or the making of VHF contacts, over very long paths that cross the geomagnetic equator.
The frequencies are well above those supported by the familiar F-layer propagation at HF, and signal strengths are often much higher than would be expected for the distances involved. In addition, the paths involved are not those commonly experienced with VHF sporadic-E propagation, in length, geography and seasonal characteristics.
There are two types, or modes, of TEP: afternoon-type TEP (aTEP) and evening-type TEP (eTEP), based on the hours they occur during a day.
Afternoon TEP paths cross the geomagnetic equator and range from 4000 to 10,000 km (2400-6000 miles) in length, but longer paths have been recorded (perhaps extended by other propagation modes). The paths most often observed cross the geomagnetic equator at angles within 30 degrees of orthogonal, but can be at quite oblique angles (e.g. Eastern Australia to Mexico).
Path terminals generally lie in a zone between 32 and 62 degrees dip angle (roughly 20 to 40 degrees geomagnetic latitude).
This map of the Pacific sector shows the general terminal zones (hatched areas) and the propagation paths between them (grey arrows). The oblique paths are S-shaped because that's the shape of a great-circle path on a Mercator projection map.
Maximum observed frequencies (MOFs) for afternoon TEP are typically 40-55 MHz, and occasionally extend to the 60-70 MHz region.
On the 5870 km (3600-mile) Townsville-Yamagawa (Japan) path, for example, the MOFs observed are 15-20 MHz greater than the normal two-hop F-layer maximum usable frequency (MUF).
aTEP TIMES AND SEASONS
For paths oriented generally north-south (e.g. VK - JA/HL), afternoon TEP generally occurs between 1400 and 1900 local mean time (LMT), occasionally extending as early as 1200 and as late as 2100 LMT.
On oblique paths where the terminals lie in different time zones, times at the path terminals can be determined from the LMT at the path midpoint. For example, the time difference between eastern Australia and Central America (ignoring the international date line) is eight hours. Hence, from Brisbane you’d be looking for aTEP to Costa Rica or Mexico from 12 noon (1400 LMT in the mid-Pacific), or earlier.
Afternoon TEP is generally experienced during the equinoctial months of March-April and September-October (whereas sporadic-E peaks in the summer months, with a minor winter peak and minimum at the equinoxes).
However, it has been observed as early as two months before and after the equinoctial months (i.e. January and May for the March 20-21 equinox, July and November for the September 22-23 equinox).
Solar maximum years brings more afternoon TEP openings, but openings never disappear during solar minimum.
aTEP SIGNAL STRENGTHS
Signal strengths observed for afternoon TEP range from weak to very strong. Path loss can be significantly less than free-space loss because of signal focussing that occurs in the path through the ionosphere (see the section on propagation mode).
The signal fading rate experienced is generally slow, with shallow fades every few seconds, occasional deep fades of 20-30 dB and often sustained periods of very strong signals. Doppler shift/spread is generally small, generally in the range 1-5 Hz and varying slowly.
Here is a chart recording I made of the JA1IGY beacon on 52.5 MHz received in Townsville on 26 April 1972. It shows some 45 seconds of Doppler shift and signal strength of the received signal. The signal strength, fading and Doppler shift characteristics are typical of afternoon TEP. The Doppler recording is a linear scale, centred on chart line 7, while signal strength is a logarithmic scale from chart line 0.
The propagation mode of afternoon TEP is 'chordal hop', having two F-layer reflections without an intervening ground reflection. The reflection points occur within two zones of enhanced ionisation either side of the geomagnetic equator known as the 'equatorial anomaly'. The diagram shows the general chordal-hop geometry of afternoon TEP, now known as "super-mode" propagation.
Solar radiation causes ionisation over the equatorial region to rise during the morning hours, which then flows north and south along the Earth's magnetic field lines, away from the magnetic dip equator, in a complex process called the 'Fountain Effect'. The ionisation accumulates in the equatorial anomaly zones, becoming more dense as the day progresses.
A VHF signal from a suitable area will encounter the nearest equatorial anomaly at a very shallow angle and be refracted such that its path is above the ground over the equatorial region. When the signal reaches the other anomaly region it is then refracted through a small angle towards the Earth. Over a small range of angles, the signal ray paths from a transmitter in one hemisphere will converge in the opposite hemisphere, as illustrated in the diagram. This signal focussing phenomena can yield very high signal strengths and narrow 'footprints' on the ground – sometimes, signals heard strongly by one station can't be heard by others nearby; after a while, the situation can be reversed!
LINKS AND REFERENCES
Geoscience Australia maps
IPS ionogram viewer
IPS foF2 near real-time ionospheric map
© 2007 Roger Harrison, VK2ZRH