Notes on solar flux and Propagation
By Ian G3ROO

What is Propagation ??
Propagation is the word we use to describe the way radio signals are ‘propagated’ round the world.   Depending on the frequency in use there are many types of propagation that can come into play.   On HF we have different layers that the radio signals are reflected from.  They can then be reflected back down to the surface of the earth to bounce back up again to the ionosphere to be reflected again to earth still further round the globe.

The signal may not hit the earth but miss it and be reflected a second time from the ionosphere, this is called a chordal hop.

On VHF when signals are propagated beyond the horizon can be by several methods.  

There is ducting, where the signals are ‘trapped’ in a layer of different density of the atmosphere. 

Sporadic E where small areas of the E layer are intensely ionised and will reflect VHF signals.

Meteor scatter where the signals are reflected from the ionised trails left by meteors entering the earths atmosphere and burning up, these are short livecd, longest only lasting a few seconds and the shortest being accurately described as ’pings’.

An especially interesting method is moon bounce, where signals are reflected from the surface of the moon!   This requires large aerials and high powers but hearing your own signals being reflected from the moon is especially thrilling and the first QSO, unforgettable!

This figure we hear on the RSGB news bulletin … solar flux

Solar flux “A“ is a figure representing the noise radiated from sun at 2,800MBHz…. The higher the better as F1 and F2 will be more intensely ionised…..   When A=300  50Mhz is likely to be open!

In sunspot minima A=<70 and unlikely then  only 10MHz is likely to be open for hops

Don’t be deceived, even with A=200 conditions can be bad….. Ap (Planitary index)… measurement of geomagnetic radiation, can have a dire effect. 

 <10 is quiet,

10-20 unsettled,

20-50 are sub storm,

50-80 storm, and

>80 is a major storm

An Ap index >25 will mean poor conditions on HF bands.

America favours the ‘k’ index, this is Ap index expressed logarithmically, ‘k’ ranges from 1-9.

MUF (Max Useable Frequency)

This is measured by firing a signal vertically at the ionosphere and looking for a vertical reflection.   The frequency where the signal fails to be returned is the MUF.   This does not mean that ‘hop’ communication is not possible above this frequency as rarely we require vertical radiation.  In practice three times the measured MUF is a practical MUF.

Interesting to note that on those days that we manage very short skip QSOs on 20m…. Say working a station only 100 miles away… it means that the MUF is near 14 Mhz…. So it is a time to check to see the conditions on 50MHz… could be excellent!

Another observation I have made is that during short skip conditions worldwide Dx is also audible at low signal strengths….. But workable with QRP!, almost as if increased power levels do nothing with signal strength!

MUF is not constant over the globe but can be displayed on a map as contour lines as barometric pressure on a weather chart.  These are published daily on the internet.

Solar flares

These are sudden releases of energy on the surface of the sun and can last several hours. They are measured by their ray radiation in Mega electron volts (MeV).

A and B flares                 Very low

C flares                         Low

M in a scale of 1-4          Moderate

M in a scale >5              High

X                                Very high

Conclusion…. For working Dx we require A>100, Ap<10 or the k index less than 3, ideally the MUF greater than the band you wish to work, but Dx will still be possible with an MUF of half that frequency.  And, of course, no solar flares!

What is a doublet and a dipole, are they the same?

A doublet is a dipole when used at it’s resonant frequency!  A doublet can be effectively used to a frequency of about ten times its resonant frequency, after that the polar diagram breaks up.

The other difference is that a dipole can be fed either with coax and a balun whereas the double must be fed with twin feeder so as to be able to cope efficiently with the high VSWR and voltages that can be present.

So an aerial does not have to be resonant to work?

Oh no, that is a very common misconception!  The only reason we will cut a dipole to resonance is to ease the feed requirements.

But surely the gain will decrease if the aerial is not resonant?

No, another misconception!   In fact, as the frequency increases the gain in favoured directions will increase.

Favoured directions? What do you mean by that?

Oh heck, I’ve opened a can of worms here because there are so many misconceptions when aerial polar diagrams are concerned. 

Let see if we can clear this up a bit.You’ve no doubt seen polar diagrams of various aerials….. The classic dipole figure of 8 shape.  

Oh yes, I’ve always made sure I’ve made sure that they have been orientated in the best direction!

Ah, just what I thought!