Reflections on reflections – Clive VK6CSW

Since moving to Perth at the end of March this year I have not yet been able to re-establish my home-brewing facilities, although I am now back on the air with dipoles tuneable on all HF bands except 160m. The shack is now in a spare bedroom with brand-new wall-to-wall carpet gracing the floor and is in stark contrast to the back of the garage which was my old shack at Miranda. The downside is that I dare not splatter solder droppings and other detritus everywhere. Cutting a couple of holes in the bedroom ceiling for the antenna downleads was traumatic enough; re-decorating the walls and floor with ferric chloride would be a hanging offence!

A consequence of this is that, until I can get a workshop built, I cannot experiment with interesting little circuits and describe the results for your entertainment, experimentation, and (maybe?) edification. How then to brew-up something which might be of interest to the readership of Dragnet, yet which doesn’t involve etchant, solder, fumes and all the other good things calculated to enhance and strengthen marital bliss? One answer might be just to pick out some practical amateur topics and comment upon them, even at the risk of airing my ignorance and boring you silly. So here goes!

Anyone who dabbles with HF knows that in 1994 even 20m is not much good for reliable DX, or even DX of any sort, and the reason is “sunspots”, or rather the lack of them. Towards the end of May this year I had a three-way QSO with Bill VK2AGF and Ellis VK2DDW on 20 metres which began at 05Z on 20m with S9 plus signals between Perth and Sydney, yet, within  minutes, the signals were barely audible. What had happened? Probably we had hit the MUF right on the knocker with a single-hop propagation, and very quickly that optimum condition had changed. Many of you will have had similar experiences and it seemed a good excuse for a little revision on the nature of the ionosphere.

On 12th December 1901, Marconi transmitted a signal across the Atlantic from Cornwall to Newfoundland but, since Hertzian or radio waves travel only in straight lines this was somewhat startling to the scientific cognoscenti of the day. Oliver Heaviside of the UK and Arthur Kennelly of the USA jointly suggested that there must be some sort of charged layer in the sky which had reflected the signal back, but it was not until 1925 that the “Kennelly-Heaviside” layer was actually located by the British physicist Edward Appleton. In principle, Appleton measured the time difference between a wave received directly from the transmitter (the ground wave) and one received via the upper layer (the sky wave). By assuming the speed of propagation of both waves to be the same, it was a fairly simple matter to calculate the additional distance traveled by the sky wave and hence estimate the height of the reflecting layer. His estimate of 65 miles or 104 km was pretty good!

We know now that the “ionosphere” is far more complex, consisting of both layers and regions, and that these are in a state of constant change and motion which, to us as Amateurs, manifests itself as ever-changing propagation conditions (mainly on HF, but occasionally on VHF as well). The highest, or F layer, is named the Appleton Layer after its discoverer, while the mid or E region is also known as the Heaviside or Kennelly-Heaviside layer.The lowest, or D region, seems to be called just that. Interestingly, a Japanese physicist named Nagaoka also theorised about the existence of a reflecting layer at much the same time as Kennelly and Heaviside published their thoughts, but like Uda – co-inventor with Yagi of the beam antenna – he seems to have missed out on his place in history. Maybe that says something about the relative importance of Japan to the scientific community in 1901! Appleton was awarded the Nobel Prize in physics in 1947 for his discoveries about the ionosphere and propagation.

The term “ionosphere” was coined in 1930 by Robert Alexander Watson-Watt, whose investigations into radio-wave propagation subsequently led to the development of radio-detection-and ranging or “radar”.  What causes the ionosphere and why does it so affect HF radio propagation? Only an outline answer can be attempted here. In essence, its existence is directly due to two ingredients, the sun and our atmosphere. The sun is continuously converting hydrogen into helium and emitting incredible quantities of energy in the process. It has been estimated that a million tons of matter is blown out of the sun every second, much of it in charged particle form to cause the solar wind, and also much radiation including ultra violet and X-rays. All life on earth depends upon this solar energy. The earth’s atmosphere is densest at the earth’s surface because of gravity, but because of molecular motion this atmosphere extends upwards to great distances, though at minute densities and pressures. At around 350km, pretty much the height of the F region,the atmospheric density is only one-trillionth of that at the earth’s surface, but in terms of molecules per cubic metre it is still significant.

An ion (the word is derived from the Greek for going or traveling) is an electrically charged atom formed by the loss (or gain) of one or more electrons. Ultraviolet is an ionising radiation, which means that it is capable of chipping electrons away from atoms and so “ionise” them, resulting in free electrons and ions. The natural tendency is for the electrons and ions to collide and re-combine into a neutral atom, but at heights where the pressure is extremely low, free electrons and ions can move about for some time without recombining, and the nett effect of the ionising radiation is a region of free electrons which form the conducting shell so important to HF communications. To try to put this in some perspective, at sea level an estimate of the average distance that an air molecule travels before colliding with another one is around one-hundredth-millionth of a millimetre, and one molecule can expect about five billion collisions per second; the life-expectancy of an ion at sea level is miniscule indeed! At 105 km high – D region – the average free-path distance is ten centimetres (4 inches), at 225 km – F region – this distance has increased to about one thousand two hundred metres, dramatically illustrating the effect of altitude on density.  Significant ionisation starts at around 50 km above the earth and peters out at about 500km. These figures may vary significantly for many reasons, but are representative. The ionising is not homogeneous but occurs in layers or regions which themselves may be ill-defined with rather fuzzy edges. The ionosphere is wholly within the earth’s atmosphere and lies between the mesosphere and the thermosphere. Indeed, the atmosphere extends well beyond the useful ionosphere; auroral displays occur above the ionosphere and are due to particle radiation from the sun ionising the extremely thin atmosphere to produce visible light.

The quantity of energy emitted by the sun varies from moment to moment; sunspots are regions of intense magnetic activity which vary cyclically over an eleven and a half year period approximately, and solar flares are regions of intense radiation which occur unpredictably. The sun rotates roughly once every 27.5 earth days, and as it does so the position and effect of sunspots and flares relative to the earth will vary. The distance between earth and sun varies seasonally, and the earth itself rotates with respect to the sun. Add together all these factors and it becomes very clear that the ionosphere, and thus HF DX communication, varies constantly. Solar flares and sunspots cause disturbances to the earth’s magnetic field as well as to the ionosphere, and if severe enough may wipe out HF DX altogether.

A moment’s thought and you will realise that the ionosphere above a given point will be densest when the sun is highest, which is substantially true. At all levels the free electrons and ions are in motion and  will collide, collision causing the ion to revert to an uncharged atom. At lower heights the density of particles is greater, collision more likely, and the ionising radiation becomes weaker. Because of these factors, the lowest, or D region, cannot form until local sunrise and dissipates once the sun’s ultraviolet rays are no longer available. The D region, which extends from about 50 to 90 km, is highly absorptive of frequencies from roughly 500 to 5000kHz; consequently daytime communications in this band tend to be via ground wave only. The exception is Near Vertical Incidence Propagation where the wave may be able to penetrate the D region and be returned by a higher region, thus giving relatively close-in comms via skywave. When this region disappears at night, long distance comms on the lower HF frequencies are enabled via the E or F regions.

The E (or middle) region, at a height of about 110km, has an ionisation density which closely corresponds with the elevation of the sun and is important for daytime HF propagation to around 1600km or less, and for night-time MF propagation in excess of 160km. Sporadic E, an unusually highly ionised cloud-like formation which forms irregularly by day or night sometimes permits the return of VHF signals which otherwise would be lost to space, and its presence may also inhibit penetration of HF to the higher F region.

The highest, or F region, consists of two layers. The lower, known as F1, exists only during daylight and extends vertically from about 175 to 250 km. Above this is the F2 layer from about 300 to 500+km. Here the wide ion-electron spacing minimises collision frequency, the ionising radiation is strongest, and in addition solar energy is
stored, which, by energy transformation, delays the dissipation of this layer until well after sunset. Hence, although the F2 region weakens and slowly descends to merge with the lower F1 region at night, it does so but gradually and lasts well into the night. It is the F2 region which is the main reflector for HF DX.

In 1994 we are approaching a cyclical sun-spot minimum. The foregoing should offer a reasonable, if simplified, explanation of why the higher HF bands are not much good now, but DX is still to be had on the lower HF bands. This article really only touches on a complex topic but may serve as basic revision as to the formation of the ionosphere. No explanation of HOW waves are bent (reflected or refracted), attenuation, critical frequency, maximum and lowest useable frequencies, Pederson rays, absorption limiting frequency, optimum working frequency, the difference between reflection and refraction, or why ionising radiation is largely absorbed before it reaches ground level, and a host of associated topics has been attempted. You probably know it all anyway, but maybe you have forgotten! If perchance all this has rung a bit of a distant bell and you have been moved to open a text-book on the subject to do a bit of “boning-up”- terrific!

Equally, if there is some further interest, I will endeavour to explain a little more in due course; then it is me that has to do the boning-up!!

Clive – VK6CSW

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