Equatorial Propagation Click here to go back to the home page

Ray Cracknell, G2AHU


Measurements of solar flux up to and including October 2000, and the scarcity of long distance F-layer QSOs on 50 MHz, seem to suggest that solar cycle 23 will go down in the records (together with cycle 20) as years when the solar flux maximum was considerably lower than in cycles 17,18,19, and 21,22. In cycles 20 and 23, in the temperate regions of the southern and northern hemispheres in particular, F2-layer ionisations were too low to support regular world-wide radio propagation at 50 MHz.

After the International Geophysical Year in the peak years of cycle 19 (1957 to 1959), nobody was in the least surprised that cycle 20 was an anticlimax. However, after cycles 21 and 22, which were also exceptionally good, very few predictions were made to prepare us for a similar anticlimax in cycle 23.

Nevertheless, most of Britain is only one sporadic-E hop north from the Mediterranean, the northern limit of tropical propagation. In the days of the 50 MHz permit holders, in October 1985, there was a fine opening from Botswana, centred on Yorkshire, during which Botswana was worked from southern Scotland and the northern half of England. Both ends of the QSOs were monitored by an amateur in Malta. This occurred near sunspot minimum, between the maxima of cycles 21 and 22.

The Tropical Ionosphere

It needs an understanding of the tropical F region to appreciate how that Botswana 50 MHz QSO occurred. An alternative to our findings might have been multi-hop sporadic-E (Es) as that mode of propagation is not directly dependent upon solar activity, but strangely enough, in equatorial regions, Es is not usable at 50 MHz as it forms a smooth layer which only reflects lower frequency HF signals. Thus we can eliminate any possibility of multi-hop Es being responsible, although the first hop out of Britain could only have been by sporadic-E.

Fig.1: The effect of the Earth’s magnetic field on free electrons reaching the tropical ionosphere.

All forms of solar radiation, like radiant heat, are at maximum intensity at the equator and at a minimum at the poles. As solar radiation, especially ultra-violet rays, causes ionisation of rarefied air, we expect to find higher electron density in tropical regions and the solar wind with its flow of free electrons adds to the process. Free electrons tend to follow the magnetic lines of force of the Earth's magnetic field, which also acts as a magnetic shield to protect us from bombardment by harmful radiation.

The density of free electrons in the F region of the ionosphere determines its ability to reflect, or more strictly to refract radio signals back to ground at a distant point. The effect of the Earth's magnetic field on the F layers of the ionosphere is illustrated in Fig. 1.

The force exerted on the electrons at any given point is a combination of the vertical component attracting electrons towards the centre of the Earth and the horizontal component attracting them towards the geomagnetic poles. At the magnetic poles there is no horizontal component and they are only pulled down towards the surface. In equatorial regions we find the geomagnetic equator, or more accurately the zero dip equator, where the magnetic lines of force are parallel to the Earth's surface and the vertical and horizontal components cancel themselves out. As one may imagine, the effects on the ionosphere over the zero-dip equator are quite dramatic and of special interest in explaining how VHF and UHF signals encountering the ionosphere near the magnetic equator are influenced.

Like the measurement of latitude, the dip angle changes from 0° at the zero-dip equator to 90° at the magnetic poles (see Fig. 2). Although the measurements are similar, the magnetic poles are situated away from the geographical poles, and the zero-dip equator correspondingly differs considerably from the geographical equator.

At the magnetic north and south poles where there are not any horizontal components, free electrons are attracted downwards and auroras result, especially during magnetic storms.

Elsewhere, free electrons follow the magnetic lines of force as the dip angle varies from the equator to the poles. The vertical component prevents rapid dispersal of the F regions, but after sunset over the dip equator, where the dip angle is zero, night-time dispersal is rapid with no magnetic force to prevent it.

This occurs regularly in a belt which is approximately five degrees either side of the dip equator; but is beyond the apparent five-degree barrier, the vertical component is sufficient to hold the F-region in place, often until the early hours of the morning.

At the line of zero magnetic dip, that is the zero-dip equator, it is important to understand the changes that take place in the morning, afternoon, evening and night, with the rising, setting and disappearance of the sun on the F-region of the equatorial ionosphere. They are illustrated in Fig. 3.

After sunrise, ionisation rapidly builds up, mainly due to ultra-violet radiation and the increasing force of the solar wind, until such time as the sun reaches its zenith and builds up electron density as illustrated in Fig. 3(a).

In Fig. 3(b), after mid-day, the ionising influence decreases but the flow of electrons continues. This gradually reduces the electron density around the zero dip equator, until it reaches the area where the vertical component of the magnetic field exerts sufficient force to arrest it, thereby forming an area of very dense ionisation both to the north and south of the zero dip equator.

Fig. 3(a): Typical F-region ionisations before noon local time.

Fig. 3(b): Changes during the afternoon.

Fig. 3(c): Typical ionisations after 2000 local time.

Figs. 3 a, b & c: The varying effect of the sun during the delay on the tropical ionosphere.

After sunset, in Fig. 3(c), the lower density around the zero-dip equator continues to fall until approximately 20.00 hr local sun time, when it begins to break up and drift away. But the high-density zones remain firmly bound by the vertical attraction of the earth's magnetic field until they finally disappear by recombination into uncharged atoms.

Fig. 4: Dr Aarons’ 50 MHz radar backscatter measurements during the evening from Jicamarca, Peru at 19.00-22.00 local standard time, October 16-17 1976.

Dispersal of charged particles over the dip equator against time is illustrated in Fig. 4, which was published by Dr Jules Aarons in 1977. It is very important to note that the horizontal axis is against time, not distance from the zero-dip line, and illustrates changes from 19.10 to 22.00 hours local time.

In private correspondence with Dr Aarons of Boston University in 1991, he expressed the opinion that the high-density zones tended to split into field-aligned sausage-like structures travelling from west to east. This had been also suggested to us by SV1DH's Doppler shift observations on 144 MHz from ZE2JV in 1980.

When the diagram was published by Basu and Aarons (1977) some amateurs jumped to the conclusion that these irregularities were responsible for transequatorial VHF propagation. But although the regions above 700km are not illustrated, the density is very unlikely to be sufficient, as an altitude of over 1000km is required for one-hop propagation over the distances worked by TEP. Nevertheless, the dispersal of electrons illustrated is undoubtedly responsible for the flutter fading that frequently develops after 20.00 local time.

The final theoretical concept is that reflections from the ionosphere are only symmetrical if a layer is smooth and also of consistent altitude and density, but tilts play a very important part in HF, VHF and UHF DX modes of propagation. If tilts in virtual height or density occur, then the angle of arrival at the layer will be different from the angle back towards the ground, and if the distance is increased thereby a higher frequency is able to be used.

Tilts are operative at the Grey Line and in the use of sporadic-E, which sometimes propagates frequencies up to 144 MHz although the maximum frequency from a vertical sounding may be as low as 5 MHz. Also, transequatorial propagation (TEP) uses the tilts between the high-density zones on both sides of the deep bite-out over the line of zero magnetic dip in the late evenings. The use of tilts in TEP is illustrated in Fig. 5.

The tropical high-density zones are also usable for normal F2 propagation, and with the maximum frequency returned to earth from vertical sounders rising at times to nearly 20 MHz, one-hop 50 MHz propagation can take place throughout the solar cycle.

Fig 5 (a): The geometry of a tilt at the nose of a high quality density region.

Fig 5 (b): The ray path followed by TEP (the billiard ball mode).

Fig. 5: Transequatorial Propagation (TEP) showing the use of apparent tilts from high and low density zones.

East-west contacts along the high-density zones can be multi-hop and the circuit from Greece to South America has often been utilised. North to south TEP circuits are likewise sometimes available at 50 MHz, throughout the solar cycle, between favourably located stations about 6000 km apart. Nevertheless seasonal conditions do vary widely, particularly at solar minimum. In general, it is evident that conditions are best near the equinoxes when the high-density zones are balanced on both sides of the zero-dip equator.

Fig. 6: The TEP zones as visualised from Limmasol, Cyprus by 5B4WR and by ZE2JV from Salisbury, Southern Rhodesia (now Harare, Zimbabwe) in 1958.

Bearing the above considerations in mind, it is possible to appreciate how a sporadic-E transmission from Yorkshire could be bounced off the Mediterranean and carried on with successive F2 reflections to Botswana even at solar minimum.

The reasons for recommending the exercise were based on results during the International Year of the Quiet Sun (IQSY), published by ZE2JV (now G2AHU) and 5B4WR in June 1965 (RSGB Bulletin pp 367-370) aided by reports on the reception of our 70 MHz beacon ZE1AZD in England, also during sunspot minimum conditions.

The persistence of Eric Parvin, G2ADR in 1987 in monitoring the Botswana 50 MHz signal and alerting other operators was most commendable (see page 32 for more on Eric's historic QSO).

The northern and southern TE zones as observed in 1958 from Cyprus in the north and Salisbury (now Harare) in the south are illustrated in Fig 6. It was originally published in "The transequatorial propagation of VHF Signals" by ZE2JV in QST in December 1958, pp 11 to 17, and subsequently in the first edition of the ARRL VHF Manual, p 21 and several subsequent editions.


Edward Tilton, W1HDQ, the long-time VHF editor of QST, described TEP as a unique amateur achievement and a brief description of its development over the Europe to Africa circuit should be a pleasant change from mere theory.

The earliest known use of transequatorial propagation took place in October 1947 almost simultaneously in South America and Africa, when XEIKE worked LU6DO and several other Argentine stations on 50 MHz, and G5KW (operating as MD5KW) worked VQ2PL on several occasions. We all sat up and took notice when PA0UN and PA0UM, G6DH and others worked ZS1P and ZS1T on 50 MHz and Fred Anderson, ZS1LA received audio and video TV from British 40 MHz TV.

The first professional research was published in 1957 by Prof. O G Villard of Stanford University, who was also a well-known W6 amateur. He and his co-authors used ground-backscatter experiments south from the West Indies and proposed that opposing tilts in the virtual height of the F-region on either side of the equator allowed propagation without a ground reflection between them. The transequatorial mode was, as a result, nicknamed 'the billiard ball mode' (see again Fig. 5).

Fig. 7: An extract from Ray Cracknell’s paper from the proceedings of the Science Convention at Salisbury, Rhodesia, May 1960, p 6.

He was right, except that the tilt was not due so much to physical tilts in the virtual height of the layer as to the pronounced variations in electron density between the high density zones approximately five degrees of latitude either side of the zero-dip line and the low density bite-out in between, which produces the apparent tilts required for Prof. Villard's calculations.

It is now difficult to appreciate the pressures and interest in international communications in the days of the Sputniks and propagation research. Transequatorial propagation was no exception.

In 1959, Prof. Obayashi published a paper entitled "Long distance HF propagation along exospheric field-aligned ionisations". He used backscatter soundings south from Tokyo on 28 MHz. This was published soon after my article in QST and a deputation from Boulder flew out to Salisbury to see me. They wanted to set up one end of a TEP experiment using high power sweep frequency sounders and rhombic antennas. The government turned down the project and I told Dr Davis that I was quite certain it was ionospheric propagation and not the extra-spherical mode.

I was in daily contact with the late Chalky Whiting, ZC4WR in Cyprus and we agreed that all we had to do was to measure time delays. I borrowed a CRO and built a pulsing unit to key the TX on 50 MHz, which ZC4WR received and rebroadcast as modulation on 29.5 MHz. I displayed the outgoing and returned pulses together with time pulses on the twin beam CRO and the results appear in Fig. 7, which is an extract from the proceedings of a science congress held in Salisbury in May 1960. After it nothing further was heard of the extra-spherical mode.

Of special interest in the results reported in Fig. 7 is that there were three modes of propagation, namely:

1. Normal two hop F layer (2F2);

2. TEP without flutter-fading in the early evening (F type TE);

3. Pure TE in the later evening with its flutter-fading, diffused characteristics and very high MUFs. In 1980, contacts on 144MHz and 432 MHz were made with Cyprus (5B4WR) and Greece (SVIDH).


The details of work during cycle 21 were published in QST, November and December 1981 by ZE2JV, ZS6PW, and SV1DH but we were mainly concerned with 144 MHz work and confirming our propagation theory. On 144 MHz we did not succeed in working beyond the sharp cut-off line across northern Italy, in spite of a long series of tests from ZE2JV with HB9QQ in Switzerland. Many, including GJ4ICD, listened for the regular test transmissions in the UK without success, but as TEP across Africa on 144 MHz did not commence until nearly 20.00z and we should have been very fortunate to find 144 MHz sporadic-E operative to the Mediterranean at that time of the evening.

The TEP system works best when the high-density zones on both sides of the zero-dip line are balanced, when the vertical sun at noon is over the line of zero dip. It is complicated, as over Africa the zero-dip line is located up to 11 degrees north of the geographical equator (see Fig. 6) and it crosses South America well to the south of it. Nevertheless optimum conditions occur at or near the equinoxes (actually in October over Africa) and is poorest at the solstices. Variations between solar activity maximum and minimum are pronounced, although 50 MHz propagation from the high-density zones is still possible but with reduced reliability during periods of sunspot minimum.

The presence of high electron density areas in the F-layer has other effects as well as boosting transequatorial propagation. These regions are formed approximately 5° to 15° from the zero-dip line and can provide propagation at higher frequencies than are available in temperate zones. They are also interestingly convenient for amateurs working multi-hop QSOs in a line roughly parallel to the zero-dip line. But transequatorial propagation (TEP) is best when working over optimum distances, as near as possible at right angles to the zero-dip line. Seasonal and solar cycle variations are pronounced but, as we saw from our Es plus TEP experiences, TEP from the most favourable locations can also take place during a solar activity minimum.

Another aspect which was a major concern of Dr Aarons was the effect upon communications of the 'plumes of ionisation' illustrated in Fig. 4 when the path to satellites was passing through the ionosphere above tropical areas. Fortunately solar cycle 23 has produced lower solar activity than was predicted; this has resulted in much diminished flutter-fading and the effect on communications with satellites has been minimal. This should be some consolation to those of us hungry for more DX on 50 MHz.


It is very difficult to say who first used the term 'TEP' instead of transequatorial propagation. In the early days it was referred to as 'anonymous propagation', because it was clearly a mode differing from normal multi-hop F layer propagation. It became 'Transequatorial Propagation' in the title and 'TE propagation' in the text of the 1959 QST article and I can only guess that it was Ed Tilton, W1HDQ, the VHF Editor of QST, who initiated the use of 'TEP'. It would be of interest if someone who still has 1959 and 1960 QSTs searched his articles for its first use.

It should always be confined to meaning, 'anonymous' F-layer propagation of VHF radio signals across the equator' and care should be taken to apply it only to the 'billiard ball mode'. It is correct to label the Botswana to Yorkshire contacts as Es + TEP. West African QSOs without crossing the equator should be labelled Es + F and contacts with northern Brazil should be Es + 2F as the path runs parallel to the geomagnetic equator. Contacts further south might be either multi-hop F (nF2) or Es + nF2. Long distance HF contacts with Australia which cross the equator would not be considered as TEP although they may encounter TEP-type flutter-fading at night and may at times miss out an earth reflection due to encountering a Grey Line at dawn or sundown.

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