A New Model of VHF Sporadic E Propagation

[VK2ZRH]

This is the first publication, anywhere in the world, of this proposed propagation model.

I am posting it here because, without the facility of the VKlogger, the research behind it would have been much more difficult and lengthy, if not impossible. For that, we have Adam VK4CP to thank. Articles for print journals are in production. One, for the European audience, has already been submitted to DUBUS. Another, for the Australasian audience, will be with the WIA's Amateur Radio magazine shortly. (c) Roger Harrison 2011.

A NEW MODEL OF VHF SPORADIC E PROPAGATION - PART 1

Roger Harrison VK2ZRH

Es propagation on 50 MHz is generally considered to be via conventional ionospheric propagation modes. The simple geometry you learned about when studying for your licence exam. But many amateurs are skeptical of or don’t believe this could hold up at 144 MHz (or even at 100 MHz in the FM broadcast band). Or if it did, such events would be extremely rare. But reports of widespread 144 MHz Es DX over decades are now so numerous as to confound that [Kraft, J. DL8HCZ/CT1HZE 2008, "Midlatitude Sporadic E on VHF in Correlation to the 22-year Magnetic Cycle of the Sun", DUBUS Technik 8], while the observations of Pocock and Dyer on the 88-108 MHz FM broadcast band are legendary [Pocock, E. and Dyer, P.J. 1992, "Eleven Years of Sporadic E”, QST, March, pp23-28.]. So what’s happening ?

With the advent of the VKlogger for reporting VHF propagation, and the availability of IPS ionograms online [www.ips.gov.au/HF_Systems/1/3], I have been able to scrutinise VHF propagation paths in Australia where the mid-points are located within ‘view’ of an ionosonde as this enables direct modelling of the propagation geometry and its relation to ionospheric conditions. The results have been both ‘as expected’ and delightfully surprising !

I have found that VHF propagation by sporadic E occurs by at least two principal modes:
(a) conventional ionospheric reflection (“classical”) by a thin, ‘plane’ Es layer, and
(b) by successive reflections via the crests of ripples or other structures in an Es layer that subsequently returns the raypath to Earth – which I call ‘petit chordal hop’.

In each case, I can demonstrate with case studies that the well established propagation geometry and ionospheric science can be applied to analyse and model the propagation and the maximum usable frequency (MUF) for a path. Mode (b), petit chordal hop, nearly doubles the MUF for a path, yielding MUFs to at least 230 MHz with intense Es.

VHF Propagation Via a Thin, ‘Plane’ Es Layer

Stay with me. You're going to need this stuff.

The geometry of a propagation path via plane Es is illustrated in Fig. 1, below. A plane Es layer lies parallel to the Earth’s surface. The scale is exagerated to make things clear.

Fig.1

RF travels from A to B. The common convention refers to this as reflection. Here, (i) is the angle between the incident raypath and the vertical line through P, while (r) is the angle between the vertical and the emerging raypath. Angle (e) is the raypath elevation angle, while angle (b) is that between the incident raypath and a tangent to the reflection point at P, which is a horizontal line. R is the radius of the Earth (6371 km, mean). D is the distance over the Earth’s surface between A and B. The line from C to P is at right angles to the Earth’s surface and has a length of R + h. Angle (b) = 90 – (i).

For a given path, the usable operating frequency and the Es vertical incidence penetration frequency (foEs) are related by the secant of the angle of incidence, as per equation 1.0 below. This is the well known "secant law" relationship from which the "classical MUF" can be evaluated. Angle (i) reaches a maximum when the elevation angle is tangent to the Earth, ie. angle (e) = 0. Triangle CAP is now a right angle triangle. The length of CA is R, while CP is R+h, so we can find the maximum of angle (i) from equation 1.1. Call this the "limiting case". These conditions set the maximum distance and the maximum frequency for the path when (e) is zero - equations 1.2 and 1.3.

All the critical parameters of Es propagation on a given path are determined by foEs and h’Es. For a given value of foEs, the maximum path distance and maximum possible frequency vary directly with the Es layer height, as shown in Table 1 below for the limiting case. For four Es heights, I've calculated maximum distances, the angle of incidence and MUF for an foEs of 9 MHz (commonly encountered).

Table 1.

Table 1_lo-gif.gif (10.9 KiB) Viewed 17657 times

Achieving a raypath elevation of zero is generally impractical, but many Es propagation paths occur at remarkably low angles, often in the range 2-3 degrees, or occasionally below. Remember that aircraft enhanced propagation on long paths (700+ km) occurs at angles below one degree, for example.

On to Part 2.

[Ref: "Ionospheric Radio" by Kenneth Davies,ISBN 10 086341186X; chapter 6, Oblique propagation].

[VK2ZRH]

A NEW MODEL OF VHF SPORADIC E PROPAGATION - PART 2

Roger Harrison VK2ZRH

For path geometries other than the limiting case (ie. "real life"), a little trigonometry provides the following equations for evaluating (i), D and the MUF:

The value of sec(i) is called the "M factor", for obvious reasons. To make life easier, it's more convenient to deal with sine and cosine trig functions. The secant of an angle is the inverse of its cosine, to the equation 2.3 can be rewritten as in 2.4. As angle (b) in Fig.1 is the complement of (i), and sine is the complement of cosine, 2.4 can be rewritten as in equation 2.5. Thus, the M factor can be evaluated using either cosine or sine, as in equation 2.6.

The relationship between the raypath elevation angle (e) and the M factor is non-linear, with a different curve for different Es layer heights, as illustrated in Fig.2. Es layers at the lower heights yield a higher M factor and thus higher MUFs. A lower raypath elevation angle, with longer paths, rapidly improves the M factor, but angles below 2 degrees experience a flattening of the M factor increase in all cases.

Fig.2

Fig_2_Elevation vs M factor vs h'Es_lowgif.gif (9.88 KiB) Viewed 17653 times

On to Part 3.

[VK2ZRH]

A NEW MODEL OF VHF SPORADIC E PROPAGATION - PART 3

Roger Harrison VK2ZRH

Table 2 below illustrates the MUFs achievable for a variety of ionospheric and path parameters. The range of h’Es values here are commonly observed on ionograms and the path lengths are generally typical. Note how relatively small changes in h'Es and the path elevation angle (e) affects the MUF.

Table 2.

For Es propagation at 144.5 MHz, note that foEs needs to be above 24 MHz for elevation angles up to 4 or 6 degrees. I have personally observed such values of foEs on ionograms when ‘sondes swept 1-30 MHz (1950s-70s era). Indeed, I have seen ionograms with off-scale Es (at 30 MHz) from that era. However, while memorable, they were not common. Instances of off-scale Es (at 20 MHz) on present era ionograms are readily found among the online displays of the IPS network stations

Figure 3 sums up the case for the geometry of VHF propagation via plane Es. As Es is very thin compared to its altitude, the trigonometry is much simpler than that employed for F-layer propagation and parallels optical reflection from a mirror.

Fig.3. Close up of the geometry for plane Es propagation.

All the above is the reality of what happens now. Stick with me. The good bits are coming up.

On to Part 4.

[VK2ZRH]

A NEW MODEL OF VHF SPORADIC E PROPAGATION - PART 4

Roger Harrison VK2ZRH

A Case Study of Plane Es VHF Propagation

Fig.4 shows a path between VK4 and VK7 where the path mid-point passes within the view of the Canberra ionosonde at Es heights. The mid-point, and likely point of reflection, is marked PoR. Scott VK4CZ frequently spots the VK7RAE 50.057 MHz beacon on the VKlogger with RST reports ranging from 419 through 599. Path length is 1648.7 km.

Fig.5 is the Canberra ionogram nearest to the time of one such spot – 2304 UT on 2/01/09. Here, the top frequency of the Es trace (ftEs) is 10.2 MHz. So, foEs is ftEs minus half the gyrofrequency (the natural spin rate of electrons in the ionosphere - 1.6 MHz here). Hence, foEs is 10.2 – 0.8 = 9.4 MHz. h'Es is 92 km.

Fig.4. VK7RAE-VK4CZ path.

Fig.5. Canberra ionogram relating to the VK7RAE spot by VK4CZ on 2/01/09 at 2304 UT.

The circles around the 'sonde locations show the 'view' of their antennas at Es heights; the outer circles are 200 km diameter.

For the ~1650 km path, the elevation angle (e) is calculated to be 2.6 deg., and angle (i) to be 79.98 deg. Hence, angle (b) is 10.02 deg. Thus, MUF = 9.4/sin(b) = 9.4/0.17399 = 54.026 MHz. We can be confident that it was Es within the Canberra ‘sonde’s view that supported the propagation on this occasion.

So. This is how the "classical" propagation model works.

What if we have an ionogram like that in Fig.6? It looks puny, and a litttle messy. You'd never expect Es like this to support 6m propagation, as foEs here is 4.7 MHz.

Fig.6. Canberra ionogram showing "spread Es".

This is an example of "spread Es", where the Es echo is spread or smeared in virtual height, although the bottom of the echo is fairly well defined. The spreading likely arises from crinkles, ripples or other structures in the Es layer, which reflect the ionosonde transmitter pulses from varying ranges at oblique angles, as well as from directly overhead, perhaps at different heights. Group retardation also contributes to the spreading. The structure of Es layers has been the subject of considerable scientific research and discussion over decades (see Refs 1-6).

Now to Part 5.

Refs:

[1] Smith, L.G. and K.L. Miller 1980, “Sporadic-E layers and unstable wind shears”, Journal of Atmospheric and Terrestrial Physics, Vol. 42, January, pp 45-50.

[2] From, W.R., and J.D. Whitehead 1986, “Es structure using an HF radar”, Radio Science, Vol. 21, No. 3, pp 309-312.

[3] Barnes, R.I. 1992, “An investigation into the horizontal structure of spread-Es”, Journal of Atmospheric and Terrestrial Physics, Vol. 54, Issues 3-4, pp 391-399.

[4] Barnes, R.I. 1993, “HF observations of the vertical structure of mid-latitude spread-Es”, Journal of Atmospheric and Terrestrial Physics, Vol. 55, Issue 6, pp 863-871.

[5] Bernhardt, P.A. 2002, “The modulation of sporadic-E layers by Kelvin-Helmholtz billows in the neutral atmosphere”, Journal of Atmospheric and Solar-Terrestrial Physics, Vol. 64, August-September, pp 1487-1504.

[6] Bernhardt, P.A., Werne, J. and M.F. Larsen 2006, “Modeling of Sporadic-E Structures from Wind-Driven Kelvin-Helmholtz Turbulence”, Characterising the Ionosphere, Meeting Proceedings RTO-MP-IST, Paper 34, pp 34-1 – 34-14.

[VK2ZRH]

A NEW MODEL OF VHF SPORADIC E PROPAGATION - PART 5

Roger Harrison VK2ZRH

It seems that wind shear turbulence in the neutral atmosphere modulates the ionisation in complex ways. While ‘structured’ Es is likely to take a number of forms, From and Whitehead [2] and Bernhardt [5,6] describe layers having “crinkles” or being “rippled”, or having “clumps” of greater electron density within the cloud. Likely models of Es structures are illustrated in Fig.7.

Fig.7. Turbulent wind shear modulates the Es ionosation, creating structures like these and spread Es on ionograms.

It appears that Es ripples, as in example (1), are of small scale, perhaps 1-5 km crest-to-crest, with vertical amplitudes very much less than that. Other likely periodic structures include lobes on the underside of Es clouds [as in example (3)] of up to 10 km crest-to-crest and around 1 km deep, or elliptical structures some 5-10 km long by about 1 km deep.

The proposed principle of petit chordal hop Es VHF propagation is illustrated in Fig.8. The raypath is refracted to the horizontal at P1 via a suitable tilt in the rippled or structured Es layer, then refracted back to ground at P2 via a reciprocal tilt. The distance from P1 to P2 may range from 1 km to perhaps 10 km. The upward tilt of the ripple improves the raypath’s obliquity to the Es layer and thus the path MUF, which is very nearly doubled!

Fig.8. The general geometry of petit chordal hop Es propagation.

A close look at the geometry is in Fig.9. The incident signal will reach the Es layer over a range of raypath elevation angles. If one raypath strikes a ripple at a tangent such that the tilt angle (t) equals the raypath-to-tangent angle (c), the refracted raypath will be horizontal (ie. angle (d) = 0). A higher incident angle will refract the raypath below horizontal, a lower incident angle will refract the raypath above the horizontal. The M factor (now MR factor) is determined by a smaller angle and is thus greater than for the plane Es case, as is the MUF (now MUFR). The ripples do not need to be orthogonal to the propagation path. On an ionogram, h’Es will be the lower height of the crests and the Es trace will be spread, as in Fig.5.

Fig.9. Close up of the geometry of petit chordal hop.

Now for Part 6. Stick with it !

[VK2ZRH]

A NEW MODEL OF VHF SPORADIC E PROPAGATION - PART 6

Roger Harrison VK2ZRH

For petit chordal hop via spread Es, the variation of the MR factor with elevation angle and h’Es is illustrated in Fig.10. Compare this to Fig.2.

Fig.10. The M factor improves with petit chordal hop propagation.

Fig_10_Elevation vs M(r) factor vs h'Es_lowgif.gif (11.18 KiB) Viewed 17641 times

If the ripples are sinusoidal in shape (or roughly so), with a depth of at least 5% of the crest-crest distance, the tilt angle will range from 0 degrees at the lower crest to 5.7 degrees maximum at half depth, which is sufficient for paths having elevation angles up to 6, 5, 4, and 3 degrees when h’Es is, respectively, at 90, 100, 110 and 120 km. Ripples with a crest-to-crest scale of 1 km may be less than 100 m deep; at 3 km crest-to-crest, only 150 m deep. Greater depth/crest-crest ratios provide a greater range of angles. The range of tilt angles required for petit chordal hop extend from about 4.8 degrees up to about 7.4 degrees for h’Es ranging from 90 km to 120 km.

If the spread Es consists of structures as in Fig.7 (3) and (4), their cross-sections may range from roughly circular to elliptical and thus present a suitable range of tilts facing the ground.

Table 3 (like Table 2) illustrates the MUFs achievable for a variety of ionospheric and path parameters under spread Es conditions. Values of h’Es and path lengths (D) are generally typical. For Es propagation at 144.5 MHz, note that the required foEs ranges between only 12.2 MHz and 16.7 MHz! It is clear from the foregoing that the MUF can potentially reach 350 MHz when foEs reaches 30 MHz, but we already know that such occasions are rare.

Table 3.

A Case Study of 6m Petit Chordal Hop Es Propagation

Scott VK4CZ spotted the VK7RAE 6m beacon at 0255 UT on 29/12/10, giving a 569 report (Fig.4 relates). The Fig.5 ionogram relates to this spot. foEs was 4.7 MHz. h'Es was 101 km, giving a raypath elevation angle of 3.2 degrees, and angle (b) of 10.62 degrees. The classical M factor would be 5.42 (eq. 2.6) and, with an foEs of 4.7 MHz, the MUF (eq. 2.5) would be about 25.4 MHz. With spread Es, the VK7RAE raypath would only need to find an Es tilt angle (t) of half 10.62 deg. = 5.31 deg. Now, the MR factor is 10.81 (eq. 2.7) and thus the MUF (eq. 2. was 50.8 MHz!

More revelations in Part 7!

[VK2ZRH]

A NEW MODEL OF VHF SPORADIC E PROPAGATION - PART 7

Roger Harrison VK2ZRH

A Case Study of 2m Petit Chordal Hop Es Propagation

Doug VK4OE (Brisbane) spotted a contact on 144.13 MHz SSB with Karl VK7HDX in Launceston on 10/01/08 at 0533 UT, giving a 52 report. Fig.11 shows the path, which reveals the mid-point (PoR) within the Sydney ionosonde’s view. Path distance is 1592 km.

Fig.11. The VK4OE-VK7HDX path.

Fig.12 is the Sydney ionogram related to this 2m contact. h'Es is 98 km. foEs is 13.7 MHz. The E, F1 and F2 layers are fully blanketed and the Es shows spreading. This ionogram could be interpreted in several different ways, but the fact that it shows spread Es is sufficient for the purpose in this case.

Fig.12. The Sydney ionogram related to the VK4OE-VK7HDX 2m contact.

The raypath elevation angle (e) is 3.4 degrees, (i) is 79.46 degrees and (b) is 10.54 deg. In this case, the classical M factor would be 5.47 and the related MUF almost 75 MHz. With spread Es, the raypath would only need to find an Es tilt angle (t) of half 10.54 deg. = 5.27 deg. For this, the MR factor is 10.89 and thus the MUF was 149.16 MHz!

Concluding Remarks

In relating reported contacts to ionograms at or near paths’ mid-points, I have found that spread Es is more the norm than the exception. I can conclude that, for a given path, the sporadic E MUF depends on at least three things:
(a) the height of the Es layer (h’Es),
(b) the peak electron density (foEs) and
(c) the presence or absence of spread Es at the path mid-point.

However, as spread Es can arise from a variety of structural morphologies in an Es layer, for petit chordal hop VHF propagation, spread Es is a necessary but not sufficient condition of itself. The spread Es needs to arise from ripples or other favourable periodic structures that present a series of small tilts in the vicinity of the propagation path mid-point.

The case studies presented here are not "singular" examples. I trawled the VKlogger History database from January 2008 to January 2011, extracting 6m and 2m spots where path midpoints are within the view of the Sydney or Canberra ionosondes. There are many, many spots that are clearly supported by this propagation model as the ionograms show spread Es, but foEs values lower than the classical propagation model requires. I limited my search to the Jan08-Jan11 period as there is currently online access to the Sydney and Canberra ionograms. Anyone can repeat my observations for themselves.

Consider this: on the admittedly rare occasions that foEs on ionograms reaches the 25-30 MHz region, spread Es of this order would yield MUFs of 300-350+ MHz !

Further research is under way. This was just a beginning.

73, Roger Harrison VK2ZRH

[VK6SIX]

There have been reports of 220mhz Es [usa only ham band] occuring in the USA.

Very interesting thank you.

vk6six

[VK5ACY]

Great work Roger, in both your theories and your use of the VKLOGGER resources.

The VKLOGGER is so much more than mere chat !

[VK4GHZ]

This is the first publication, anywhere in the world, of this proposed propagation model.

I am posting it here because, without the facility of the VKlogger, the research behind it would have been much more difficult and lengthy, if not impossible. For that, we have Adam VK4CP to thank. Articles for print journals are in production. One, for the European audience, has already been submitted to DUBUS. Another, for the Australasian audience, will be with the WIA's Amateur Radio magazine shortly. (c) Roger Harrison 2011.

Roger, this is a lot to absorb, but a quick reply to this.

The value of spotting things, no matter how trivial they may seem, is lost on many people.

Thank you for your on-going work, and a thank you to those VK Logger users who do make the effort and report what they are doing on air.

I think we will all ultimately benefit from research like this.
Let's see if the non-spotters will ever wake up to this!

[VK3AIF]

Truly well done Roger.

I thought you had been a little quiet of late, not stimulating discussion on this medium and now I know why.

Have you come up with a plausible hypothesis on the LDEs as yet?

Cheers

[VK3BCZ]

At last I am via the forum.....VERY well done, Roger, (and a question at the end for you)

As I said on vk-vhf, you have certainly put in the hard yards and come up with great numbers that describe the limits of what the 'spread E' blobs can do. I am also wondering whether 350 MHz could be exceeded for very shallow ripples.

As you might recall, we have both been aware of this spread E effect for several years, now. My personal interest has always been as to the feasibility of being able to predict the formation of both the basic E layer (being able to predict the occurrence of the vertical/horizontal shears that are thought to give rise to the E layer), and the ripples or clouds or even multi-levels of E layer ionisation.

My friend Dr Bob Roper in Atlanta, Georgia has pointed out that in many places around the world, the E layer wind data can be obtained (in addition to the satellite wind data that has been available since the early 1990s) from the Dynasondes with their scans between regular ionosonde vertical scans. However, accessing these winds in a useful format and covering the geographical area of interest seems to be a problem.

Question:
Do you know of any work being done in this area of neutral winds, particularly regarding their relevance and predictability?

Brian Tideman, VK3BCZ

[VK4WDM]

Roger, are you trying to tell us (and the wider world) that hams do REALLY USEFUL experimental research and that we are not all a bunch of black box operating, spectrum-hogging, old chatterers after all?

Outrageous!

73

Wayne VK4WDM

[VK2ZRH]

RESPONSES:

Taking Wayne VK4DWM's comment first:

I have long had a reputation in the amateur fraternity for doing outrageous things [ winning the 1965 JMMNFD VK3 section operating VHF only - as a zed-call! ~ VK3ZRY; editing/publishing 6UP (1972-75, 1984-85)for VK-VHF/uwave enthusiasts, which campaigned for VHF beacons in VK2 and for better band-planning for beacons and repeaters; editing CB Australia magazine 1976-79; co-authoring the "Linton-Harrison papers" Amateur Radio and the Challenge of Change published 1985 and 2003; co-authoring Kiss Your Last Big Sunspot Maximum Goodbye with Dr Leo McNamara, Australian Electronics Monthly, Jan 1987; leading The A-Team, successfully elected to VK2 Div. WIA Council in the 1990s, which 'cleaned up' the Division's governance and operations and saved it from penury when threatened with court action over the 78m Dural tower ; offering a lengthy criticism of the (new) National WIA's draft recruitment brochure at a recent AGM - and then rising to the challenge to "do it proper" - which I did, with others; including art and music (clips) in technical presentations to GippsTech and club meetings - enough! ]

Why should I stop now

Graham VK6SIX:

Damn, your callsign's hard to type Yep. I'm aware of the 220 MHz Es reports. Emil Pocock W3EP, who edited QST's World Above 50 MHz column for many years, reported on it in QST, back in the 1980s, IIRC.

However, at GippTech 2007, during a discussion following my presentation on sporadic E, an audience member related an instance of one-way 432 MHz signals during an especially intense Es 6m+2m opening. The propagation mode detailed above won't go this far . . . but I believe there's another high-MUF Es mode (heh, heh . . . more in a later posting).

Brian VK3BCZ:

I am also wondering whether 350 MHz could be exceeded for very shallow ripples.

The 'shallowness' of the ripples doesn't matter so much, it only limits the range of incident raypath-to-tilt angles [angle (c) ]. However, an MUF over 350 MHz can happen. What limits the MUF is the height of the Es (h'Es) which of course determines angles (b) and (c), and the layer's electron density, as measured by foEs. Lower heights mean lesser (b) and (c) angles and thus higher MUFs.

Take "the limiting case", where the raypath elevation angle [angle (e)] is zero, and for Es at 90 km the MR factor is 11.99 (do the arithmetic !). If foEs hits 30 MHz, the MUF is 11.99 times 30 = 359.7 MHz over a 2141.8 km path. You could only do better by transmitting aeronautical mobile while flying at 10 km altitude

. . . feasability of predicting wind shears and turbulent wind wind shear.

Do you know of any work being done in this area of neutral winds, particularly regarding their relevance and predictability?

In Europe, researchers derive wind speeds and wind shears in the E-region from the characteristics of MF transmissions. However, I don't know if this can be done in real time or over wide geographic areas.

What goes on "up there" is indeed a rich field for research.

73, Roger Harrison VK2ZRH

[VK2ZRH]

An article on this topic is now safely with AR magazine; 7,468 words, plus 32 image files [20 figures, 12 equations].

That's a bit over twice the 'size' of what's published here.

I got replies to my emails.

I'll post some updates here on evidence supporting the model in due course.

73, Roger Harrison VK2ZRH

[VK2ZRH]

Further support for Petit Chordal Hop Es propagation Part 1

On 30 December 2009, Richard VK2SWL2, of Numeralla NSW, posted a report on a Forum here about the reception of TNQ7 Townsville. You can read it here: ttp://www.vklogger.com/forum/viewtopic.php?f=2&t=8940. Scroll down the page to where Richard posted a Brisbane ionogram.

Richard recently confirmed by email to me that the vision carrier of TNQ7 Townsville, on 182.25 MHz, was received briefly by him at Numeralla - first at 0041 UT on 30 December 2009, at S6, then peaking again at 0050 UT at S2. Reception was made on a TV Band II dipole antenna.

In response to Richard's posting, Peter VK3QI reported:

FWIW, at the time you mentioned, there were very strong signals on 6 mx between VK3 (Melbourne) and VK4 (Brisbane).

I have analysed these reports and the associated propagation paths. A map of the situation is below (Fig. A), and below it (Fig. B), the 0040 UT Brisbane ionogram Richard put in his Forum posting:

Fig. A. Propagation paths around 0041 UT 30/12/09.

Fig. B. Brisbane ionogram 30/12/09 0040 UT, showing spread Es; h'Es 91 km, ftEs 14.0 MHz.

The Townsville-Numeralla path is 1982 km long, while the Brisbane-Melbourne path is around 1365 km.

The path mid-point for Townsville-Numeralla is about 520 km WNW of the Brisbane ionosonde. That's too far from the Brisbane ionosonde for the 0040 UT ionogram to be relevant. However, sporadic E cloud systems in this region are known to often travel in a generally westward to WNW direction, driven by the background wind at the relevant heights in the E-region at speeds ranging from 40 metres/second up to 100 metres/second [1,2].

To get a better idea of what the ionosphere was doing at the Townsville-Numeralla mid-point at 0040 UT, we could look at what was happening at Brisbane earlier because what happened there then may have blown across the path mid-point around the time TNQ7 was received in Numeralla. I decided on four likely background wind speeds because they're in the range likely to cause turbulence to Es layers, creating spread Es on ionograms.

1/ At 85 m/s - that's 306 km/h - it would take 1 hr 42 min for an Es cloud to travel from Brisbane to the path mid-point.

2/ At 80 m/s (288 km/h), it would take 1 hr 48 min.

3/ At 75 m/s (270 km/h), it would take 1 hr 56 min.

4/ At 70 m/s (252 km/h), it would take 2 hr 5 min.

So. I looked up the nearest-time ionograms for (1/) 0040 UT minus 1 hr 42 min = 2300 UT 29/12/09; (2/) for 0040 minus 1 hr 50 min = 2250 UT 29/12/09; (3/) for 0040 minus 1 hr 55 min = 2245 UT 29/12/09; and (4/) for 2 hr 5 min = 2235 UT 29/12/09.

Case 1/ The 2300 UT 29/12/09 ionogram shows spread Es with the following characteristics: h'Es 96 km. ftEs 16.1 MHz, foEs = 16.1 - 0.7 = 15.4 MHz. (Refer to Part 4, earlier. The gyrofrequency, fH, at Brisbane is 1.4 MHz).

As the path distance (D) is 1982 km, angle of elevation (e) is 1 deg. and angle of incidence (i) at the Es is 80.0654 deg., making angle (b) 9.9346 deg. The raypath would need to find an Es tilt angle (t) at the path midpoint of half that, making angle (c) 4.9673 deg. (See Fig.9 in Part 5).

The M factor, 1/sin(c), is 11.549. Thus, the MUF in this case would have been at least 177.8 MHz. A tad too low, unfortunately.

Case 2/ The 2250 UT 29/12/09 ionogram shows spread Es with these characteristics: h'Es 96 km. ftEs 17 MHz, and thus foEs = 16.3 MHz.

Angles (e), (i), (b) and (c) are as for Case 1. With an M factor of 11.549, the MUF would have been at least 188.2 MHz.

Clearly, this could have been the scenario at the time Richard VK2SWL2 heard TNQ7.

However, the two other cases could also have been equally likely.

Case 3/ The 2245 UT ionogram shows spread Es at 96 km and a derived foEs of 18.8 MHz.

With all the angles as before, the MUF would have been 217.1 MHz.

Case 4/ The 2235 UT ionogram shows spread Es, still at 96 km, with ftEs off-scale at 20 MHz ! The MUF would have been at least equal to, or greater than, 222.8 MHz !!

These results are summarised in Table A, below.

Table A.

On to Part 2 of this update.

73, Roger Harrison VK2ZRH


[1] From, W.R., and J.D. Whitehead 1986, “Es structure using an HF radar”, Radio Science, Vol. 21, No. 3, pp 309-312.

[2] Whitehead, J.D. 1997, “Sporadic E – A Mystery Solved?”, QST, October and November.

[VK2ZRH]

Further support for Petit Chordal Hop Es propagation Part 2

As quoted above, Peter VK3QI reported 6m propagation between Melbourne and Sydney around the time of TNQ7 reception in Numeralla in southern NSW. The 6m paths are identified in the Figure A map, in Part 1 of this update.

The rough path midpoint (PoR) is about 130 km WNW of the Sydney ionosonde.

In this instance, if the background wind speed is travelling roughly W or WNW at 75 m/s (270 km/h), an Es cloud system would take about 29 minutes to travel the 130 km. Below is the Sydney ionogram (Figure C) near to 29 minutes earlier than the 0041 UT reception report (0012 UT).

Fig. C. Sydney ionogram relevant to Melbourne-Brisbane path midpoint at about 0041 UT on 30/12/09. Note the spread Es.

For this ionogram, h'Es is 92 km and, with ftEs of 6.3 MHz, foEs is 5.5 MHz (fH is 1.6 MHz at Sydney).

Raypath angle of elevation (e) is 4.4 deg., giving (i) of 79.3765 deg and angle (b) of 10.624 deg. Thus, angle (c) would be 5.312 deg. and the M factor is calculated to be 10.8. Hence, the MUF would have been 59.4 MHz. QED

If the background wind speed were a little faster, the Es cloud system would be at the PoR a little earlier. The next Sydney ionogram, at 0018 UT, shows spread Es at 96 km, with a calculated foEs of 9.6 MHz. In this instance, the M factor would be 10.4 and the MUF 99.8 MHz.

So it appears that the propagation reports are supported by the ionospheric conditions around 0041 UT on 30/12/09, with spread Es creating the necessary conditions for petit chordal hop propagation.

The ionogram relating to Case 4 in Part 1 (above) is shown below (Fig. D).

Fig. D. The spectacular Brisbane ionogram for 2235 UT on 29/12/09. Note the spread Es. These ionospheric conditions would support an MUF greater than 230 MHz over a path of ~2000 km.

73, Roger Harrison VK2ZRH

[VK2SWL2]

On 30 December 2009, Richard VK2SWL2, of Numeralla NSW, posted a report on a Forum here about the reception of TNQ7 Townsville. You can read it here: ttp://www.vklogger.com/forum/viewtopic.php?f=2&t=8940. Scroll down the page to where Richard posted a Brisbane ionogram.

Richard recently confirmed by email to me that the vision carrier of TNQ7 Townsville, on 182.25 MHz, was received briefly by him at Numeralla - first at 0041 UT on 30 December 2009, at S6, then peaking again at 0050 UT at S2. Reception was made on a TV Band II dipole antenna.

Just to clarify, while I made the initial post I wasn't one of those at Numeralla at the time but was in contact with the group down that there consisted of Todd VK2SWL1, Will VK2SWL3 and Geoff VK2SWL7 at Geoff's QTH.

Back on the technical side of things, I guess these irregularities or bumps in the Es layer could also go a long way to explaining short distance backscatter or cases where the strongest signal isn't necessarily coming from the direction you would expect.

[VK2ZRH]

The full paper (international version) is published in DUBUS Issue 2, 2011 - out now. [ In English and German ]

73, Roger Harrison VK2ZRH