INTRODUCTION

What are the minimum, maximum, and typical range of distances propagated via Es for various VHF modes? How far can one realistically expect for single and multi-hop sporadic E modes?

With over 55 years of sporadic E DX logs and observations, we now have a very large database of information which gives us a very good idea regarding what distances are possible on the VHF band. This data is mainly obtained from VHF ham radio and TV DX enthusiasts.

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 factor as to the maximum distance propagated by sporadic E is the height of the Es cloud. According to ionosonde (devices used to measure reflectivity of the ionosphere) foEs data, Es usually occurs around 90-105 km altitude.

Sporadic-E reflections are assumed to be (mirror-like) and associated with a single E cloud that lies midway along a given radio path at an altitude of about 105km. At this altitude the maximum possible single- reflection (single-hop) distance is about 2200 km. 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, and Es ionization height.

This means that when TV DX signals are being received from 500 miles distance, the Es cloud has a very high electron density. For signals from 350 miles distance, the electron density would be even 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.

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.

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 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 distances would be possible.

It follows that 2Es or multi-hop Es DX will usually be weaker compared to single-hop Es. This is because of extra signal losses associated with multiple reflections off E layer ionization.

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 300-400 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 2,750-3,100 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 distance record: 3,040 miles - 97.5 MHz WFGY-FM, Watertown, NY, USA received via double-hop sporadic E, by Paul Logan, North Ireland.

144 MHz double-hop Es

Maximum distance record: 2,250 miles (3,635 km) WA7GSK (DN13so) -- W4FF (EL96am) 29-May-1998.

Es DISTANCE VARIATION

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. 1,600 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.

Do the distances for different frequencies vary; are there definite blackspots between single and double hop skip distances where no one has heard DX or propagation is impossible?

Yes, the distances do vary (see above).

Even though there are 'black spots' were Es reception is much 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 would be 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.

A great-circle distance calculator is also useful for submitting longitude and latitude coordinates (see link below).

LINKS AND REFERENCES

A Seven Year Study of 50 mHz Sporadic-E Propagation; PAT DYER, WA5IYX
http://home.swbell.net/pjdyer/cq/cq7208a.htm

Mid-Latitude Sporadic-E (Es) - A Review; Michael Hawk
http://www.amfmdx.net/propagation/Es.html

http://www.anarc.org/wtfda/sporade.pdf

The doughnut effect; Emil Pocock, W3EP
http://www.uksmg.org/doughnut.htm

Sporadic-E propagation at VHF - a review of progress and prospects; Emil Pocock, W3EP.
http://www.uksmg.org/sporade.htm

WTFDA members all-time TV distance records
http://home.twcny.rr.com/nordquistsyr/tvrecdis.htm

WTFDA members all-time FM distance records
http://home.twcny.rr.com/nordquistsyr/FMRECDIS.htm

ARRL all-time 144+ MHz VHF DX distance records
http://www.arrl.org/qst/worldabove/dxrecords.html

Long-haul 88-108 MHz FM DX from Pacific Islands & Asia
http://www.oocities.org/toddemslie/exoticfmdx.html

TV FM DX received in Sydney, Australia via multi-hop sporadic E
http://www.oocities.org/toddemslie//Multi_hop_sporadic_E.htm

Great Circle Distance Calculator
http://www.vwlowen.demon.co.uk/java/circle.htm