|Distance Boundaries for Sporadic E Propagation Modes|
This article discusses the practical minimum and maximum range of great circle distances propagated via Es (sporadic E) and variations of this mode. How far one can realistically expect to receive upper HF and VHF signals via single or multi-hop sporadic E is dependent on several variables.
With over 55 years of sporadic E DX logs and observations we now have a very large database which provides a good indication of minimum and maximum great circle distance ranges. This extensive database is largely sourced from VHF ham radio and TV DX enthusiast loggings since the 1950s.
With the advantage of hindsight and 60 years of accumulated Es knowledge, some researchers, notably Pat Dyer, WA5IYX, and Emil Pocock, W3EP, have done a statistical study of Es reports and written articles based on their conclusions. One article called 'The Doughnut Effect' by Emil Pocock, is relevant to this discussion (see below).
Es MUF AND CLOUD IONIZATION
One major contributing factor to the maximum distance propagated by sporadic E is the height of the Es cloud above the Earth. Based on the word-wide ionosondes used to measure electron critical frequency reflectivity of the ionosphere at vertical incidence (foEs), Es mostly occurs around 90-105 km altitude (h'Es). It is rare to find h'Es above 110 km when 50 MHz propagation is supported by either by plane Es or spread-Es. Long 50 MHz paths that are near the geometric path limit are not too often geographically placed where an ionosonde is near or at the midpoint, unfortunately. Hence, performing a direct correlation between the propagation and the relevant ionospheric parameters can be difficult to determine if two single clouds, or one cloud was involved.
Sporadic-E reflections are assumed to be mirror-like and associated with a single E cloud that lies approximately midway along a given radio path at an average altitude of about 105 km. At 105 km altitude the maximum single- reflection (single-hop) distance averages 2350 km (1,433 miles) for most monitoring stations. However, there are variables which can provide reception somewhat below and above the average range.
The highest frequency reflected back to the surface of the earth, the Es MUF, varies from ~ 20 MHz to at least 220 MHz. At the MUF, the angle of reflection is greatest, the single-hop distance is longest and signal strengths are greatest. As the signal frequency decreases from the MUF, the angle of reflection decreases, while the resulting signal path is shorter and signal strength is relatively less. At some critical frequency signals transmitted straight up will be reflected straight down (zero angle of reflection). The minimum MUF of a single sporadic- E reflector can be determined when the frequency and path distance of any observed contact are known.
The main factors that set the minimum and maximum distance limits for Es DX reception are geometry of the earth, Es cloud electron density, the number of Es clouds, Es ionization height above the earth, local horizon take-off, horizon obstructions, antenna height above ground, ground conductivity, and Yagi antenna vertical main lobe response (degrees).
For VHF DX signals are being received from 500 miles, the Es cloud has a high electron density (electron concentration per cubic cm). For signals received from a shorter path distance (e.g. 350 miles), the electron density is higher.
One characteristic of Es is that maximum path distance will occur just below the MUF cutoff. Experience has shown that the maximum distance will be in the 1,430-1,500 mile range. Note that this upper boundary 1,500 mile estimate only holds true for well elevated receiving stations with all other contributing variables approaching optimum (e.g. a zero or even negative angle horizon take-off). Even for 1,450 mile Es signals, the local monitoring station horizon is less than 1 degree.
At frequencies up to 70 MHz, distances in the 1,300-1,450 mile range are relatively common via Es single-hop propagation. This conclusion is based on the relatively frequent reception of trans-Tasman 45-70 MHz New Zealand low-band VHF television signals into eastern Australia.
MONTHLY DISTANCE VARIATION OF SINGLE-HOP Es
Bob Cooper (New Zealand) recently offered the following comments regarding the trend for single-hop Es distances to extend toward the end of the Es season: "Just a "reminder" in the midst of a bumper crop Es summer that right around now the Es distances begin to "go long" for the balance of July and into August. The logical explanation is the Es reflective/refraction layer becomes more elevated, thus producing longer skip distances. If you study the distances now being reported, there is an increase in those 1300-1500 miles which indicates what I am saying (Cuba into New England for example). This "trend" goes way back into the 50s, and what you will notice is that as distances become longer there are TYPICALLY fewer stations coming through in a given opening but the average distances are greater."
The diurnal h'Es characteristics of Es (either plane Es or spread Es) having sufficient electron density to support VHF propagation is not yet fully clarified. The h'Es results reported in scientific papers simply take the ionosonde results, which includes Es of insufficient density. Hence, scientific papers authored by Haldoupis and Vyronides, et al, have not yet provided clear guidance.
The maximum single-hop distance for sporadic-E contacts is approximately 1,450 miles (2300 km), a geometric restraint based on an average height of E-layer ionization of 65 miles (105 km). If the E-layer ionization were higher than 65 miles, greater path distances would be possible. An additional 10 km of h'Es provides ~ 116 km (70 miles) extended path length, which holds for plane Es; but spread Es likely affords marginal additional path length.
It follows that 2Es or multi-hop Es DX will usually be weaker compared to single-hop Es. This is because of extra signal attenuation associated with multiple reflections off E layer ionization, either cloud-to-cloud, or intermediate ground reflections.
DISTANCES PROPAGATED via Es
Here is a general guide to the distances possible with Es, 2Es, and multi-hop Es propagation:
45-70 MHz single-hop Es
Minimum range 250-400 miles.
Optimum range 900-1,300 miles.
Maximum range 1,350-1,500 miles.
45-70 MHz double-hop Es
Minimum range 1,750-1,900 miles.
Optimum range 2,000-2,600 miles.
Maximum range 2,750-3,000 miles.
45-70 MHz triple-hop Es
Optimum range 3,000-4,000 miles.
Maximum range 4,300 miles.
45-70 MHz multi-hop Es
Maximum distance record: ~ 7,750 miles (12,500 km) - 48.2597 chE2 Iran received via multi-hop sporadic E, by N5HV New Mexico N5JHV.
88-108 MHz single-hop Es
Minimum range 350-500 miles.
Optimum range 900-1,300 miles.
Maximum range 1,350-1,500 miles.
88-108 MHz double-hop Es
Optimum range 2,000-2,500 miles.
Maximum range 2,700-3,100 miles.
Maximum Es FM DX distance record: 4302 miles (6925 Km). On 31 May, 2010, 88.7 MHz La Voz de la Luz, a religious station from Salvaléon de Higüey, Dominican Republic received by Mike Fallon, East Sussex, UK. This extreme distance ranges could involve some form of Es-related ducting propagation, i.e. multiple cloud-to-cloud signal transfer. This could explain the relatively minimal signal attenuation for such an extremely long signal raypath.
144 MHz double-hop Es
Maximum distance record: 2,250 miles (3,635 km) WA7GSK (DN13so) -- W4FF (EL96am) 29-May-1998.
Es DISTANCE VARIATION
Aircraft scatter on paths of > 457-mile / 750 km have raypath elevation angles around one degree; the shorter paths have higher angles. Forward scattering from the terrain in front of the transmitter and receiving stations will play a role where raypath elevation angles are low. This scattering effect is more pronounced on the lower VHF frequencies, hence the greater potential distance variation for a given single-hop Es reflection.
Lower frequency Es signals, for example, 45.25 MHz New Zealand channel 1, have more leeway with regard to minimum and maximum distances. For example, 350 mile Es DX from ABSQ1 Warwick is received at times into Sydney. 1,500 mile Es DX from the North Island of New Zealand into Melbourne is also possible every Es season
Higher frequency signals, such as the 88-108 MHz FM band, have more restricted leeway with regard to minimum and maximum distances. 350 miles Es DX is very rare at 88-108 MHz. Also, 1,600 miles Es DX is relatively uncommon at 88-108 MHz.
Lower frequency signals are also much more likely to be received by double-hop Es. Even in a poor Es season, 45-60 MHz double-hop Es is relatively common. One example is reception of 46.17 MHz RTQ channel 0 into Perth, Western Australia (2,200 miles), or 45.25 MHz New Zealand channel 1 into South Australia (2,000 miles).
Double-hop Es at 88-108 MHz FM is certainly not common. A very good Es season ideally during sunspot minima, is needed for double-hop Es FM DX.
Double-hop Es at 88-108 MHz FM may be more common during periods of maximum solar activity, for DXers in the Northern Hemisphere.
Even though there are 'black spots' were Es reception is less likely, there are no areas which can be classed as impossible. This applies to the 1450-1800 mile 'blackspot' or 'doughnut' area.
THE DOUGHNUT EFFECT, W3EP
"Your appeal to explain why it is more difficult to work 1,490-1,740 miles on sporadic E than shorter or longer distances has a relatively straight-forward answer (see page 47, January issue). The effect, by the way, was perfectly described by DL7AV. We have seen that same effect time and time again over here. I call it the doughnut effect."
"Sporadic-E paths between 1490 and 1740 miles are more difficult to complete than longer and shorter paths. The maximum single-hop distance for sporadic-E contacts is about 1430 miles, a geometric restraint based on an average height of E-layer ionization of 65 miles or so. Curiously enough, sporadic-E paths in the 1120-1360-mile range are probably the most common. This is because the single-hop distances near the maximum useable frequency (MUF) are also the longest. As the MUF rises above 50 MHz, the paths shorten up."
"It may be possible that some sporadic-E paths at 1490 miles or even longer are also completed by unusually long single hops, perhaps from patches of E-layer ionization that are somewhat higher than the average 65 miles. Even so, it is more likely that sporadic-E paths longer than 1490 miles are via multiple hops. If that is indeed the case, then a 1490-mile path must involve two hops with an average of 740 miles each (the hops do not have to be of equal length, so long as they total 1490 miles). The problem is that 740-miles paths are unusual at 50 MHz, because the required MUF to create such short hops is high, perhaps in the 100 MHz range. Thus in order to complete a 1490-mile path at 50 MHz, two separate sporadic-E centers with MUFs of 100 MHz and spaced 740 miles apart are needed. That is a big requirement!"
"As the path lengthens from 1490 miles, the required MUF for the two sporadic-E centers drop, thus making it more likely that the required geometry will be achieved. In theory, this suggests that as the distance approaches 2850 miles, there should be a greater incidence of double-hop sporadic-E."
"When the probability of sporadic contacts are graphed in two- dimensional space, a sort of doughnut shape emerges. Sporadic-E contacts are rarely shorter than 250 miles. That is the hole. As the distance lengthens from 250 miles, the occurrence of sporadic-E contacts increases until 1420 miles is reached. That is the main part of the doughnut. There is a sharp drop-off at 1420 miles amounting to a sharp boundary until around 1740 miles or so, then contacts become more and more likely until 2850 miles, when the second, but less sharply defined boundary is reached."
"At 2850 miles and longer, there are many possible configurations of hops that make the 2850 to 3200-mile void less clearly defined. A 3000-mile path could be completed by three 1000-mile hops, for example. The MUF requirements for 1000-mile hops are not as high as for 750-miles, although finding three sporadic-E centers lined up optimally is not common either. You can make your own calculations and discover the various possibilities for difficult distances."
"This line of logic suggest that there may be some prime distances for multi-hop sporadic E. If the most common single-hop contacts near the MUF fall into the 1000 to 1350 mile range, then the most common multi-hop paths might be expected at 2200-2700, 3350-4100 miles, and so forth".
Es DISTANCE BOUNDARIES
One method to identify your single-hop Es target area is to obtain a great-circle map and draw two sets of boundary lines with a compass. For 45-108 MHz TV and FM, draw one at approximately 500 miles, and one at 1,500 miles. This would be your prime target area for single-hop Es. The same method can be applied to double-hop Es, with a boundary line drawn at 1,750 miles, and another one at ~ 2,800 miles.
Great circle Es boundaries also have variations. Assuming that ground reflection points exhibit varying degrees of conductivity (e.g. saltwater vs. vegetation), differences in received Es signal strengths can be expected.
A great-circle distance calculator is also useful for submitting longitude and latitude coordinates (see link below).
A Seven Year Study of 50 mHz Sporadic-E Propagation; PAT DYER, WA5IYX
Mid-Latitude Sporadic-E (Es) - A Review; Michael Hawk
The doughnut effect; Emil Pocock, W3EP
Sporadic-E propagation at VHF - a review of progress and prospects; Emil Pocock, W3EP.
WTFDA members all-time TV distance records
WTFDA members all-time FM distance records
ARRL all-time 144+ MHz VHF DX distance records
Long-haul 88-108 MHz FM DX from Pacific Islands & Asia
Great Circle Distance Calculator
Copyright © 2013 Todd Emslie