Tekst Daily Minutes 19 feb. over sporadische E (Engels)

This is Roger Harrison VK2ZRH with a few words about signal strengths,
mirrors in the sky and Foundation operators accused of running more power
than allowed.

As I was listening around 40 metres the other week, I heard an Advanced
licensee complain about Foundation operators apparently running more power
than allowed because – quote – some of them are SO STRONG that they’ve
gotta be running more power than ten watts – end quote.

Well, there ARE other explanations for what’s happening when you hear a very
LOUD signal from a Foundation operator located 1000 to 1500 kilometres away
in another state, for example.

After all the years I’ve been on the air, and the knowledge I’ve gained about
ionospheric propagation, I too, still marvel at how that happens.

Sometimes, the signal strength defies adequate description, especially when
the S-meter slams against the right hand stop !

The behaviour of the ionosphere often seems puzzling, often ranging through
to perverse !

As many listeners are aware, during the Summer months, from late October
through to early March, the phenomena of sporadic E makes its presence felt.
Also through Winter, during a few weeks either side of the Winter solstice.

But, wait a minute ! Isn’t sporadic E just something that affects the VHF
bands – six and two metres ?

Long story short – no. It also supports propagation across the HF bands.

Sporadic E ionisation consists of a thin, dense horizontal “clouds” of ions
that are between one kilometre and five kilometres thick. These clouds form
at heights between about 90 kilometres and 130 km – that’s the E region.
The ions attract a swarm of free electrons that do the work of reflecting your
RF signal.

These clouds are created by wind shears. That is, winds blowing in opposite
directions. When one wind blows east and the other above it blows west, they
push long-lived metallic ions into the “quiet” space between the winds, thus
forming those thin clouds.

The long-lived metallic ions are formed from the ablation of meteors. It is
estimated that some 100 to 200 tonnes of tiny meteors, like grains of sand,
fall on the Earth’s atmosphere every day. So there’s always a haze of meteor
dust and metallic ions drifting around up there.

So, when sporadic E forms we have a thin, pretty much totally reflecting cloud
layer in the ionosphere. Ionospheric scientists refer to RF propagation via
sporadic E as specular reflection. That means – like a mirror. There’s little
or no loss.

The signal attenuation over the path is largely that of it spreading out.
On 40 metres, during daytime, there will be some attenuation by the signal
passing through the D region and suffering some absorption. At night,
that absorption disappears. Sporadic E occurs at night as well as through
the day. But that’s another whole story on its own !

Over a path of 1000 kilometres up to 1500 or 2000 kilometres, if the path was
direct, unobstructed by the Earth’s curvature, a ten watt transmitter will be
received as a very strong signal.

But – it can get even stronger ! This comes about through the phenomenon of
raypath focusing.

It’s a curious thing, but the sporadic E cloud can act much like a shaving
mirror and focus signal raypaths into a small area where they come to ground
at the skip distance. This focusing AMPLIFIES the signal strength.

Say what ?! Yep – the ionosphere can provide GAIN. Hence, those mighty loud
signals you hear at times.

The corollary to that is that the area on the ground where the signal can be
heard may be quite small, maybe only hundreds of metres to a kilometre or so
across. You can hear someone in your neck of the woods working a DX station,
but can’t hear the DX yourself.

Of course, the sporadic E cloud can also de-focus the signal raypaths and
cause the signal at the skip distance to become quite weak. QSB rules when
it comes to sporadic E propagation.

I know some of you will be wondering if this also happens over shorter
distances. On the HF bands, the answer is yes !

Specular reflection and raypath focusing can be observed on paths of only
200 to 500 km on the HF bands.

So. Next time you hear a Foundation station whose signal pins your S-meter,
don’t immediately jump to the conclusion that the operator is “cheating” and
running more power than their licence allows.

This has been Roger Harrison VK2ZRH for VK1WIA News.

Source: http://wia.org.au/