Transequatorial Propagation (TEP): VHF DX Across the Equator
Most VHF propagation stories are about weather, meteors or clouds of ionisation in the E layer. Transequatorial propagation, usually shortened to TEP, is different. It is a long-distance mode that connects stations on opposite sides of the geomagnetic equator, over distances of several thousand kilometres, and it can bring 6 metre and even 2 metre signals through when nothing else is open. It depends on the structure of the ionosphere near the equator, it follows a rather predictable daily and seasonal pattern, and it is strongest in years of high solar activity. If you live in the right part of the world and watch the right band at the right hour, TEP can deliver some of the most exciting VHF contacts you will ever make. This guide explains how it works, the two types of opening you will meet, the seasons and conditions that favour it, the paths that are known to work and how to detect an opening before it is over.
What TEP Is
TEP is a form of ionospheric propagation that crosses the region around the geomagnetic equator, roughly in a north-south direction. A signal from a station on one side of the equator reaches a station on the other side at a distance that is typically in the range of several thousand kilometres. It is not related to the sporadic-E clouds that our guide on sporadic E and gray line techniques describes, and it is not the same as the low-altitude bending covered in the article on tropospheric ducting. TEP happens high in the ionosphere, in the F region, and it relies on the particular behaviour of the ionosphere above the equator.
For a general background, the ionosphere is a region of the upper atmosphere that contains free electrons and ions produced mainly by solar radiation. The overview of the ionosphere explains its layers and how they vary with time of day, season and the solar cycle. Radio waves are refracted or reflected by these layers, and the details decide which frequencies and distances work at a given time. The equatorial region has its own characteristics, which is where TEP comes from.
Why the Equator Is Special
Near the geomagnetic equator, the Earth’s magnetic field is roughly horizontal, and the interaction between the field, the electric fields in the ionosphere and the daily solar heating produces an unusual pattern of ionisation. After sunset the ionisation on either side of the equator remains strong, while ionisation directly over the equator can become structured and irregular. These structures can act as a path for signals travelling across the equator, bending them in a way that allows frequencies far above the normal limit of HF propagation to propagate.
It is worth being modest about the detailed physics. The equatorial ionosphere is an active area of research, and several mechanisms contribute. For an amateur the practical summary is sufficient: the ionosphere around the equator, in the afternoon and evening, can support very-high-frequency signals along north-south paths that cross it at close to right angles. Paths that run east-west along the equator do not benefit from the effect in the same way, and paths that do not cross the equator rely on other propagation modes.
The Two Types of TEP Opening
Operators generally distinguish between an afternoon type and an evening type. They differ in time, in the highest frequency they support and in the way the signal sounds.
| Feature | Afternoon TEP | Evening TEP |
|---|---|---|
| Local time | Mid to late afternoon, into early evening | After dusk, into the late evening |
| Typical bands | 6 m, sometimes slightly above | 6 m and 2 m, in the best years |
| Signal character | Relatively clean, steady and strong | Fast fluttery fading and distortion, often called rough or hashy |
| Typical distance | Several thousand kilometres, often shorter than evening TEP | Very long, often the longest TEP paths |
| Best conditions | High solar flux, quiet geomagnetic field | High solar flux, equinox seasons, quiet geomagnetic field |
Afternoon TEP signals sound much like ordinary skip. Evening TEP generates signals with a rapid flutter, because the signal is scattered by structures that are moving and changing constantly. SSB signals sound distorted and CW signals acquire a rough, broken tone. That roughness is a strong clue that you are listening to TEP, and experienced operators learn to recognise it immediately.
Seasons, Solar Activity and Geomagnetic Conditions
Three factors dominate the likelihood of an opening.
- Season. TEP is most often observed around the equinoxes, in spring and autumn, and tends to be weaker around the solstices. In October the Northern Hemisphere is in the autumn TEP season, which makes this a good moment to watch.
- The solar cycle. Higher solar activity produces a more heavily ionised F region, and TEP on 6 metres and 2 metres is most reliable when the solar flux is high. You can follow the numbers using the explanation in our article on solar flux and the A and K indices.
- Geomagnetic quiet. A disturbed geomagnetic field tends to disrupt the equatorial structures that TEP depends on. Check the current K index and forecasts at the NOAA Space Weather Prediction Center before assuming a closed band means nothing is happening.
None of these factors guarantee an opening and none of them is absolutely necessary. TEP has been recorded in less favourable conditions, which is why long-term beacon monitoring is such a valuable tool.
Paths and Distances
TEP paths cross the equator in a north-south direction. Well-known examples are Europe, particularly the Mediterranean region, to southern Africa; Japan and the western Pacific to Australia; and parts of the Americas on both sides of the equator. Distances are generally in the range of several thousand kilometres, much more than a single sporadic-E hop provides on 6 metres. The best distances tend to be greatest in the evening type of TEP, when signals can sometimes extend further than the ordinary afternoon opening.
The important point for any station is the geometry. If a straight line from your location to a distant station crosses the geomagnetic equator at a large angle, TEP is a real possibility. If your path stays on one side of the equator, look for other modes instead. Operators who live at moderate latitudes and want to use TEP should look at a map with the geomagnetic equator drawn on it, to see which regions are plausible partners.
Equipment: What You Need
TEP does not demand extreme stations, but it rewards sensible ones. On 6 metres a modest Yagi or even a good dipole can work TEP signals when the band is open, and a receiver with good sensitivity is more important than a large amplifier. On 2 metres, evening TEP is rarer and needs a more capable station with a directional antenna, a low-noise preamplifier and the ability to work SSB or CW with weak signals. Our article on the 6 metre band covers antennas and operating habits for the magic band in detail.
Narrow modes are your friend. Weak-signal digital modes such as FT8 are popular on 6 metres, and the standard calling frequency for FT8 on 6 metres, 50.313 MHz, is where many TEP contacts are logged by stations who are monitoring the band. The fluttery character of evening TEP degrades SSB, but FT8 and CW often survive the distortion, so a station that can run several modes has a clear advantage.
How to Detect an Opening
- Beacons. A beacon on 6 metres in the target region is the earliest indicator. If you hear it, or if it appears on a spot map, the band is open towards that area.
- Reporting networks. A reception-report map for digital modes shows where your signal is being heard. If stations on the far side of the equator are decoding you, the opening is in progress.
- DX clusters and alerts. Cluster spots reveal what other operators are hearing. A notification tool that raises an alarm when a callsign you want appears is helpful when you cannot watch the band continuously; LY4A DX Reminder, for example, can watch 6 m and 2 m among its bands.
- Listening for flutter. If the band sounds quiet in the afternoon and then starts to carry rough, fast-fading signals from the south as the evening begins, that is the characteristic sound of evening TEP.
Planning a Watch Schedule
You do not need to sit at the radio all evening to benefit from TEP. A simple schedule is enough. During the autumn and spring seasons, check the solar and geomagnetic numbers once a day. If the flux is healthy and the field is quiet, plan to monitor 6 metres in the late afternoon, and again for an hour or two after dusk. Leave a receiver tuned to a beacon or to the FT8 frequency with a reception-report program running, so that the computer keeps a record of who hears you while you do something else.
It is also worth keeping notes. Write down the date, the time, the direction, the band and the solar conditions of each opening, even a weak one. After a season you will have a personal picture of when TEP works from your location, and that is more useful than any general rule. Share reports with local clubs and with the reporting networks; the more stations that log openings, the better everyone’s understanding of the path becomes.
One more practical point: the quality of your own noise floor decides how many weak TEP signals you will hear. A local noise source such as a switching power supply, a charger or a neighbour’s lighting can mask exactly the flutter-affected signals you are trying to catch, so it is worth spending an hour to find and reduce such noise before the season starts.
Operating Tips
When an opening begins, speed matters: TEP can last a short time, sometimes only an hour or two, and fade as local conditions change. Be ready with your log and your antenna pointed in the likely direction, make short exchanges and give the other station time to copy the callsign. Use CW or a digital mode if SSB is difficult because of the flutter. Do not assume a station you can hear is hearing you; the path may be reciprocal, but local noise and power differences matter, so work stations by their actual reports. And always record the time and the grid squares, because TEP contacts are often among the longest you will ever log on the band, and they tend to count for awards and personal records.
Common Misunderstandings
- Thinking TEP is sporadic E. It is a different mechanism, in a different layer, with a different daily pattern.
- Looking in the wrong direction. TEP is a north-south mode; paths along the equator are not its home.
- Ignoring the evening. Many of the best TEP openings occur after dark, when many operators have already stopped listening.
- Judging by a quiet band. A band that sounds dead at your end may still be open for a path the other side of the equator; check beacons and reports.
The Bottom Line
Transequatorial propagation is one of the great long-distance VHF modes: predictable enough to plan around, rare enough to be exciting. It needs a north-south path across the geomagnetic equator, is most likely around the equinoxes in years of high solar activity and splits into a cleaner afternoon type and a rougher, longer evening type. Watch beacons and reporting maps, keep an eye on geomagnetic conditions, be ready with a mode that tolerates flutter and listen after dark. If conditions line up, a modest 6 metre station can make a contact across the equator that nothing else on the band would allow.
Frequently Asked Questions
What does transequatorial propagation mean?
It is a mode of ionospheric propagation that connects stations on opposite sides of the geomagnetic equator, mainly on 6 metres and sometimes on 2 metres, over distances of several thousand kilometres.
When is TEP most likely?
Mostly around the spring and autumn equinoxes, in years of high solar activity and when the geomagnetic field is quiet. Afternoon and evening hours are the usual windows.
Which direction should I point my antenna?
Along a path that crosses the geomagnetic equator, roughly north-south. Paths that stay on one side of the equator rely on other modes.
Why do TEP signals sound distorted?
Evening TEP signals are scattered by rapidly changing structures in the ionosphere, which produces fast flutter and a rough tone on SSB and CW.
Can I work TEP on 2 metres?
It happens, mainly in the evening type of opening and in good solar conditions. It is much rarer than on 6 metres and usually needs a stronger station.
How do I know that an opening is happening?
Monitor 6 metre beacons, a reception-report map and DX cluster spots, and listen for the characteristic flutter. Notification tools can alert you when a wanted callsign appears.