This article was originally published in the magazine 'Amateur Radio' in 1984. Although some things have changed, like the break-up of the Soviet Union, and the use of the Internet for propagation alerting, the basic facts in the article remain true today.
It was a hot, sunny afternoon in July, 1974. A newly-licensed radio amateur sat in his shack, drinking a cup of tea and idly tuning the 2-metre band. It was very quiet. Suddenly, he noticed a strong SSB signal.
"HG5AIR calling CQ DX. This is the Radio Club of Budapest Airport calling CQ..."
It must be 10 metre IF breakthrough, he thought, disconnecting the power to the 2-metre converter to check. The Hungarian station disappeared! When he reconnected the power, the HG5 was back again, still calling CQ! So he must be on 2 metres! As if to confirm this, the Hungarian replied to an inaudible G8 (then a VHF only license) mobile on the M6. Then he called CQ again. Hands shaking with excitement, the amateur selected a frequency close to the DX station and called him. But back then, 2 metre SSB transceivers were a rarity. The amateur's transmitter only had four crystal controlled channels of FM. Despite calling several times, the HG5 did not reply to him.
This was how G4ILO (then G8ILO) first experienced sporadic-E propagation. It convinced him that SSB was the mode to use if you wanted to work DX, and some sideband gear was soon acquired. But so elusive are sporadic-E openings, particularly to those who are normally at work during the daytime, that it was four years before G8ILO was once again in the shack, tuning the band, when a strong station with a thick foreign accent was heard calling CQ.
What is sporadic-E?
So what exactly is sporadic-E? Basically, it is long-distance propagation caused when clouds of ionised particles form at an altitude of about 110km up in the E layer of the ionosphere, causing radio waves which hit them to be bent back to earth. The conditions which cause these clouds to form generaly occur between mid-morning and late afternoon during a season that runs from May to August.
Signals are not reflected off these ionised clouds as you might think, but refracted. Some readers may remember from school physics how a beam of white light entering a glass prism is refracted into its different colours. Red light (the lowest frequency of visible light) is bent the most by the prism, while violet light (the highest frequency) is bent the least. Intermediate colours are bent by varying amounts so that the light leaving the prism is split up into all the colours of the spectrum.
The same principle applies to radio waves in a sporadic-E cloud. Low frequencies are bent through a greater angle than high frequencies. The stronger the ionisation, the greater the bending of the waves. So a weakly ionised cloud may cause only 10 metre and Band 2 FM signals to be bent back to earth (shown by a in the diagram below), while a 144MHz signal passes through and out into space as it normally would. A strongly ionised. cloud (shown by b) will bend the signals through a sharper angle so that they reach the ground closer to the transmitter, while even a 144MHz signal is refracted back to earth resulting in unexpected long distance propagation.
Signs of sporadic-E
The distance between the transmitter and the receiver of the refracted signal is called the skip distance. On HF, where refraction of signals by higher layers of the ionosphere cause longer distance propagation, sporadic-E type conditions are often referred to as short skip. Keen sporadic-E watchers will monitor the 10 metre band, taking note of the locations of stations heard, knowing that the shorter the skip, the stronger the E-layer ionization and the greater the chance of propagation extending to the VHF bands, 6, 4 and 2 metres.
Many VHF DXers also monitor Band 2 FM. Propagation must extend to 90MHz before it can reach 2 metres, a1though it is possible for DX to be coming in on Two when nothing is heard on Band 2 if the sporadic-E is from a direction where there are few FM broadcast stations.
Frequency of openings
Because it requires a higher level of ionisation to achieve propagation at higher frequencies, sporadic-E openings are less frequent and of shorter duration on 2 metres than on the lower VHF bands. During the season, there may be openings nearly every day on 10m, while at the other extreme, sporadic-E is non-existent at 70cm. The graph below shows the probability of propagation occurring at 90MHz for days throughout the season. The most likely time for an opening is around the second week in June when there is about a 60% chance of DX being heard at some time during the day.
On 144MHz, openings are about one fifth as likely to occur, which means, taking an average over the whole month, that there are likely to be no more than four good openings on 2 metres during June. Since some 2 metre openings may last for only a few minutes, and since they are most likely to occur between about mid-morning and late afternoon, it is easy to see why most people find then somewhat elusive.
So what is the attraction of this type of propagation? On 2 metres, the shortest skip encountered is about 1200km (anything shorter than this is probably tropospheric propagation) while the maximum distance workable with a single hop is somethinig approaching 2400km. In other words, it is possible to work stations over distances which could normally only be accomplished using meteor-scatter. The map below illustrates the area workable from central England via sporadic-E: it covers eastern Europe, Western Russia, Italy, most of the Mediterranean, southern Spain, Portugal and north Africa.
However, what is characteristic of sporadic-E propagation is the strength of the signals, particularly over the shorter distances. There is no searching for weak signals down in the noise. For many stations, the first warning that an opening is taking place has been when the local station they have been talking to is suddenly blotted out by someone jabbering away in a foreign language at S9 plus! And it is not uncommon for someone using a hand-held radio with a whip antenna to work into the Mediterranean area with good reports both ways.
Making successful contacts
Obviously, a major factor contributing to success in making sporadic-E contacts is luck, particularly luck in being on the band at the right time. However, the level of interest now shown by amateurs in working DX on 2 metres is such that news of an opening soon spreads, and within a few minutes QRM levels can be very high indeed! Sporadic-E signals are subject to sudden, irregular fading, and it is quite common for a 59 station to suddenly vanish for good as if his rig just blew up! So as well as luck, good operating technique aimed at completing the contact in as short a time as possible is very important.
The information exchanged should be simple: callsign, signal report and QTH locator. Keep it short, don't repeat anything unnecessarily, give each item of information no more than twice, once in plain letters and once using standard phonetics. Do not make the common mistake of giving names, equipment details and so on; this simply wastes time, conditions may change at any moment and the DX operator may have limited English and have trouble understanding you anyway. Remember also, in your excitement, not to speak too fast!
Unless you have high power and a big antenna, it is probably not profitable to sit on one frequency calling CQ. Tuning the band will enable you to get a better picture of what DX is coming in, and you may be lucky and beat a pile-up by catching a DX station making his first CQ call: pile-ups can become very heavy, and the DX station may well ask for you to call him a few kHz up or down in frequency, or he may tune a little either side of his transmit frequency anyway in an attempt to resolve something out of the cacophony of calling stations.
Rigs with multiple memories really come into their own during a sporadic-E opening. Frequencies on which a DX station has been heard, or where there is a large pileup, can be memorised and then checked periodically, so that you can tune the band while awaiting your chance to call a station.
Although the majority of sporadic-E contacts are made on SSB or CW, there is no reason why DX cannot be worked via this mode of propagation using FM. As has already been mentioned, low power can still produce good reports at distances of over lOOOkm,and this is equally true of FM, given a clear channel. FM operators tend to be less DX-minded than those on sideband, and it may be difficult to break into a QSO in a foreign language or even to catch the callsigns, as I found one summer when I heard a couple of Spanish-speaking stations on the FM calling frequency. However, it is worth remembering that many of the countries within range of the UK via sporadic-E, particularly north Africa, have few high-power SSB stations on VHF, so if propagation favours that particular direction then it is quite likely that the only activity will be found on FM.
What causes sporadic-E?
In spite of recent advances in knowledge about the ionosphere, little is yet known about how sporadic-E is formed. As a result, it is impossible to predict when openings will occur.
For many years, it has been thought that sporadic-E is caused by a phenomenon called "wind shear". Certain conditions in the upper atmosphere result in winds moving at high speed in different directions. Static electricity is generated where two winds meet, and this produces ionisation, which forms into clouds at a height of about 110km.
Like the high-altitude winds that cause them, these clouds move at a speed of about 100km/hour, generally drifting down towards the Equator. The result is that propagation tends to favour one area only for a short time, and as some stations fade out others fade in.
What puzzles geophysicists is what causes these wind shears to develop? It has been suggested that they are caused by violent thunderstorms, and statistical evidence does seem to show some relationship between storms and occurrences of sporadic-E. However, no convincing explanation as to how storms could cause wind shears to occur has yet been found.
I have often noticed that the bands have a characteristic sound when sporadic-E is about. The normally quiet VHF bands are buzzing with static, which could be the noise of distant storms being reflected back to earth by the abnormally high levels of ionisation.
Whatever the cause, there is no denying that the sporadic-E season is a very exciting time for VHF DX enthusiasts. Good hunting!