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PARC's Propagation Pages are primarily intended as a 'starter' for members, but everyone is welcome to view them  . . . The content is under constant review in order to add new information and correct any errors. E&OE.
Are you wondering why you rarely hear UK stations on 40m nowadays?


Answer:

The first reason is - it's summer, when propagation is usually pretty poor. Also, we are approaching the 'solar sunspot minimum' which is arguably the worst one for nearly 50 years.

The state of the ionosphere and solar conditions have been poor for HF communications since March 2016, with only a minor respite over the winter. The summer season usually has weak ionisation in the F2 region. However, this is often accompanied by quite strong ionisation in the E region giving rise to Spread-E conditions extending from 160 to 40m and occasionally into the higher HF bands.

Spread-E conditions can block HF radio waves, especially below 14MHz, from reaching the F/F2 region where they would normally be refracted and return to earth. The blocking effect can make contacts on 80 & 40m very difficult, or give 'dead band' conditions.

However, Spread-E can also give short-range NVIS communications on 40m during the daytime and extending into the night, while often being accompanied by Sporadic-E propagation during daytime, ranging from middle HF frequencies up to 10m, 6m and occasionally 2m. That said, Sporadic-E conditions may occur without any signs of spreading of the E-Region!

The key to understanding changing HF radio wave propagation, foF2 (critical frequencies) and prevailing conditions is to look at ionograms. There is a large network of digisonde stations around the world and these can most easily be found at:

http://digisonde.com/stationlist.html

There is more information about ionograms on the main propagation page (link).


Spread-E, Spread-F and foF2


At times, Spread-E propagation is also associated with Spread-F propagation, which may produce NVIS effects on the HF bands up to 17m but rarely any higher.
The MUF  (critical 'foF2') is too low to really register, and all bands are (theoretically) closed or very weak. Nevertheless, weak and unreliable contacts at distances of 400-800km are possible on the 7MHz band. The 20m band will still have incoming signals from DX stations because they may have a very different MUF to the listener in the UK. In other words, propagation can be very asymmetrical, and the epithet"if you can hear them, you can work them" is patently untrue. This 'asymmetric' propagation can give rise to different signal paths for two or more stations on the same frequency (see below).

Time to look at some ionograms . . .

The ionogram below from UKSSDC at Chilton shows conditions in southern England on 27 June at 1520UTC.

This chart shows poor conditions -
Spread-E and Spread-F with a tiny hint of reflections in the F2 region. The E-region shows 'blocking' and there are only diffuse (vertical) reflections in the F/F2 region.



NB: the blue colour indicates X-wave 'scatter or shift' (you'll need to dig deeper into ionograms to get more information about this). Also, the frequency of Spread-E is usually the same as the Es (Sporadic-E) value on the chart; the frequency of Spread-F is the 'fxI' value.

Compare this to a more normal ionogram from February 2017, below. Here the F2 region has fairly good ionisation, although the MUF is only just high enough to sustain reliable contact at 7MHz at distances greater than 200km (bottom two lines on each chart marked D and MUF). Broadly, we are looking for a foF2 of 6.35MHz in order to have reasonably good UK-wide contacts, or an fxI around 7MHz:

 
Changing MUF, foF2 and effects on Skip Distance

As the MUF (foF2) rises or falls the effective skip zone will change: this may result in a signal drop-out which seems to be like QSB but this phenomena is deftly side-stepped in almost all books covering propagation.

A good indication that the critical foF2 frequency is different for two stations is when one gives a "+30" report and the other a S6 or lower report. In such conditions the weaker station may fade away (not really QSB, but very similar) while the stronger one may not.

In the conditions prevailing since April 2017 it has been very rare for the 7MHz skip distance to be less than 400km. Naturally, contacts on 40m have still been possible over longer distances.

Here is an example ionogram, showing a Critical foF2 frequency of 5.325MHz at Fairford in Gloucestershire (click to enlarge).

The calculated Maximum Usable Frequencies for distances of 100 to 3000km are shown at the bottom. In this example, at a working frequency of 7.100MHz, the skip distance will be around 600km, giving poor (or near impossible) two-way communications across the UK (at least, for any station in the middle of the country).

Conditions (critical frequency, MUF and skip distance) will not be exactly the same in another part of the UK at any significant distance from Fairford - such as northern Scotland - but MX0PHX is located close enough to Fairford (and Chilton, both c.(200km) for foF2 conditions and skip distances to be similar.

It is not currently possible to find ionograms from other parts of Britain online, so some extrapolation is needed if you are significantly more than 200km from Chilton or Fairford (see the Propagation page).

The MUF table on this chart also shows that 80m is unlikely to be reliable, while paths of 1500km+ should be workable on 20m/14MHz and higher HF bands.

In Practice

So, when on 40m a falling foF2 will tend to result in an apparent increase in activity from Italy and Spain while UK stations fade somewhat. As the skip distance lengthens stations from Greece and beyond will become stronger. Stations in Germany and France can be strong at almost any time because they are in a 'sweet spot' for 40m when the foF2 is above 5.9MHz; similarly Russia is almost always well beyond the skip zone unless the foF2 falls below c.4MHz and that is why western Russia stations are almost always heard on 40m.

That leaves Belgium & Netherlands: contacts between the UK and ON/PA (for many of us) at a distance of 200-400km  usually require a critical foF2 of 6.9MHz in order for the 2-way contacts to be reliable.

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Rough guide - what you may hear:
  •  if there is increasing activity (or signal strengths) from farther and farther afield - e.g. Italy, Spain, Greece - the foF2 is falling and the skip distance will increase. UK stations will be weak (except for, say, northern Scotland).
  •  if there is an apparent increase in activity from nearer stations (e.g. Belgium, Netherlands & UK south coast) the  foF2 is increasing the skip distance is decreasing. UK-wide contacts may improve significantly.
  • conditions can switch rapidly minute by minute.


Solar Perturbations

Additional complications such as Coronal Mass Ejections and similar 'crud' emanating from the sun, as well as the interplanetary magnetic field, will also affect radio propagation regularly during the sun's (nominal) 28 day rotation cycle. Cyclical 'coronal hole' disturbances recur for several months although their intensity and duration may vary.

For information on current sunspot and coronal hole activity see spaceweather.com




MX0PHX & MX0YHA - in the 'wrong places'

Located in IO92KT, MX0PHX is in the centre of England. If the 'skip distance' at 7MHz is any greater than 200km the likelihood of reliable radio contact between MX0PHX and any station in the more populous parts of England will be very low.  The same applies to all UK stations to a similar degree, not least MX0YHA (see below).

This map (below left) shows various circles (in km) from IO92KT where MX0PHX is located, although 'skip zones' are rarely true circles. However, it is clear that if the observed foF2 critical frequency will only support reliable 40m band contacts at distances much greater than 200km MX0PHX will not normally be heard. Equally, with a 400km skip zone (a regular situation) MX0PHX will normally only be able to hear UK stations in Scotland, Cornwall, Northern Ireland an Europe.  An example ionogram, showing skip distances is given below. The circles on the map are only a guide to distances, in reality 'skip zones' may have a much more irregular and asymmetrical 'shape'.

For comparison, MX0YHA - Youth Hostels ARG - located near Skipton in N. Yorkshire is 150km north of MX0PHX which means that when the skip distance is c.200km on the 40m band much more of England is within range for reasonably reliable contacts.

This map shows the approximate areas which fall within a 200km 'skip zone' centred on the locations of MX0PHX (Nottingham) and MX0YHA (near Skipton). In reality, 'skip zones' are not circular, so the diagram is just a simple guide to distances from each station. It is evident that MX0PHX will have greater difficulty making contacts in southern England than MX0YHA: the same is true for contacts between MX0YHA and stations farther north.


Final word

Nature is fickle . . .  it doesn't always do what you expect: sometimes things can be worse than you hope for and sometimes better. Don't rely on propagation software, tables, websites, social media, books  . . .  try things for yourself. Switch on, tune in, make a call!
 
 
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