This is Roger Harrison VK2ZRH with a few words about signal strengths,
\nmirrors in the sky and Foundation operators accused of running more power
\nthan allowed.<\/p>\n
As I was listening around 40 metres the other week, I heard an Advanced
\nlicensee complain about Foundation operators apparently running more power
\nthan allowed because – quote – some of them are SO STRONG that they’ve
\ngotta be running more power than ten watts – end quote.<\/p>\n
<\/p>\n
Well, there ARE other explanations for what’s happening when you hear a very
\nLOUD signal from a Foundation operator located 1000 to 1500 kilometres away
\nin another state, for example.<\/p>\n
After all the years I’ve been on the air, and the knowledge I’ve gained about
\nionospheric propagation, I too, still marvel at how that happens.<\/p>\n
Sometimes, the signal strength defies adequate description, especially when
\nthe S-meter slams against the right hand stop !<\/p>\n
The behaviour of the ionosphere often seems puzzling, often ranging through
\nto perverse !<\/p>\n
As many listeners are aware, during the Summer months, from late October
\nthrough to early March, the phenomena of sporadic E makes its presence felt.
\nAlso through Winter, during a few weeks either side of the Winter solstice.<\/p>\n
But, wait a minute ! Isn’t sporadic E just something that affects the VHF
\nbands – six and two metres ?<\/p>\n
Long story short – no. It also supports propagation across the HF bands.<\/p>\n
Sporadic E ionisation consists of a thin, dense horizontal “clouds” of ions
\nthat are between one kilometre and five kilometres thick. These clouds form
\nat heights between about 90 kilometres and 130 km – that’s the E region.
\nThe ions attract a swarm of free electrons that do the work of reflecting your
\nRF signal.<\/p>\n
These clouds are created by wind shears. That is, winds blowing in opposite
\ndirections. When one wind blows east and the other above it blows west, they
\npush long-lived metallic ions into the “quiet” space between the winds, thus
\nforming those thin clouds.<\/p>\n
The long-lived metallic ions are formed from the ablation of meteors. It is
\nestimated that some 100 to 200 tonnes of tiny meteors, like grains of sand,
\nfall on the Earth’s atmosphere every day. So there’s always a haze of meteor
\ndust and metallic ions drifting around up there.<\/p>\n
So, when sporadic E forms we have a thin, pretty much totally reflecting cloud
\nlayer in the ionosphere. Ionospheric scientists refer to RF propagation via
\nsporadic E as specular reflection. That means – like a mirror. There’s little
\nor no loss.<\/p>\n
The signal attenuation over the path is largely that of it spreading out.
\nOn 40 metres, during daytime, there will be some attenuation by the signal
\npassing through the D region and suffering some absorption. At night,
\nthat absorption disappears. Sporadic E occurs at night as well as through
\nthe day. But that’s another whole story on its own !<\/p>\n
Over a path of 1000 kilometres up to 1500 or 2000 kilometres, if the path was
\ndirect, unobstructed by the Earth’s curvature, a ten watt transmitter will be
\nreceived as a very strong signal.<\/p>\n
But – it can get even stronger ! This comes about through the phenomenon of
\nraypath focusing.<\/p>\n
It’s a curious thing, but the sporadic E cloud can act much like a shaving
\nmirror and focus signal raypaths into a small area where they come to ground
\nat the skip distance. This focusing AMPLIFIES the signal strength.<\/p>\n
Say what ?! Yep – the ionosphere can provide GAIN. Hence, those mighty loud
\nsignals you hear at times.<\/p>\n
The corollary to that is that the area on the ground where the signal can be
\nheard may be quite small, maybe only hundreds of metres to a kilometre or so
\nacross. You can hear someone in your neck of the woods working a DX station,
\nbut can’t hear the DX yourself.<\/p>\n
Of course, the sporadic E cloud can also de-focus the signal raypaths and
\ncause the signal at the skip distance to become quite weak. QSB rules when
\nit comes to sporadic E propagation.<\/p>\n
I know some of you will be wondering if this also happens over shorter
\ndistances. On the HF bands, the answer is yes !<\/p>\n
Specular reflection and raypath focusing can be observed on paths of only
\n200 to 500 km on the HF bands.<\/p>\n
So. Next time you hear a Foundation station whose signal pins your S-meter,
\ndon’t immediately jump to the conclusion that the operator is “cheating” and
\nrunning more power than their licence allows.<\/p>\n
This has been Roger Harrison VK2ZRH for VK1WIA News.<\/p>\n