**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.

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.

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.

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).

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].