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Frequencies well below the MUF are affected by the D-layer that shows a strong absorption while the E-layer reflect shortwaves back to earth more often than expected. Above the MUF your chance to make contacts are almost null. Due to their high frequency, sky waves are not more reflected by the F-layers (F2 or F) and escape into space. Using sky waves it is thus impossible to work on frequencies too away below or above the MUF. The Highest Possible Frequency, HPF, is the upper usable limit exceeded 10% of the time, or 3 days per month, or say in other words, in exceptional conditions. During the 90% of time we use the Frequency of Optimal Transmission (after the French words), aka FOT. It is defined as the statistical frequency during which the MUF can be exceeded of 85% (see below). This range of frequencies spreading between the FOT and the HPF is 4 MHz wide or larger, sometimes so wide that it include two ham bands. To know the probability to use such exceptional conditions, there is only parameter to check in propagation prediction programs : the "required reliability"of the signal-to-noise ratio (SNRxx or SN-Rel) for the specified circuit.
At the low-frequency boundary, due to the Sun presence, most radio waves are absorbed by the D-layer before any reflections may occur what drastically impact the lowest frequencies with an increasing of QRM at noon (12:00 UTC). The LUF depends also on the signal-to-noise ratio, the power and the transmission mode. This is for this reason that we also define a LUF, below which radio signals are absorbed by the D-layer and interrupt the propagation on the lower bands. The LUF can also be approximated by 0.25 x MUF with an upper limit of 12 MHz. Hopefully the lower limit usually drop below 2 MHz at night and until sunrise. If you really want to work DX stations in the low bands in these conditions, you can always use power amplifiers and high gain antennas to increase the signal against the noise of even work in CW. Of course, although your signal exceeds by far the noise level in such working conditions, you must absolutely take also in account the geometry of your communication and the radiation pattern of your antenna to expect establish a communication link. At the limits of the MUF or the LUF radio signals can be subject to fading which is a fluctuation of the signal amplitude. The highest frequency will mainly be used at daytime but it will also depend on various parameters like the time of the day, the season, the solar activity or the location of each station (geometry).
At last, a QSO will be possible at the conditions that on both sides of this circuit, the reflecting layer is at same altitude. If on the transmitting side for example the MUF is below the E-layer but if the F-layer only is effective at the receiving station, the F-layer will be too high to reflect shortwaves toward the listening station. The F-layer will attenuate (absorb) most of the signal and this ham will be unable to listen the DX station.
Other ionograms are available on NGDC website, and international ionograms on IPS website (ULCAR is no more avilable). An ionosonde provides a cross-section image of the ionosphere state much more accurate than any simulation or forecast. But a simple test can help understanding how these limits work. If you try to work or listen to a station 1000 km away in the 20 m band, there are many chances that you cannot get a QSO. Why ? Because the MUF is most that probably lower for such a short circuit. You will have a far better result working on the 40 or 80 m band.
At last, to get an accurate status of the bands activity, as we explained in the page dealing with DXing for a SWL it is always interesting to listen to beacons. They are very appreciated emitters to "feel" the propagation towards different continents. As most beacons are emitting continuously between 1.8 and 30 MHz they permit you to estimate the propagation bandwidth, MUF and LUF through all the spectrum for a given heading. Associated to a spectrum analyzer, you can immediately know what are the crowdest bands. Models of the ionosphere Now with all these solar and geomagnetic data in hand we can calculate a propagation estimation to a particular country for a specified date, time and frequency. But far from you the idea to make these calculations manually using differential equations and complex formulae. Propagation prediction programs like DX ToolBox, WinCAP, DXAtlas or PropLab Pro are here to help us in this tedious task. Thanks to these programs able to use online data in near-real-time, we can get a good estimation of propagation conditions for each hour of the day and each HF band. If you wish more information about these applications, I suggest you to read my Review of Propagation analysis and prediction programs where I list most common applications with screens dumps. Good news, most of these applications are freeware or very cheap. In What can we expect from a HF propagation model I explain also what can we expect from these tools in terms of reliability, accuracy and quality of the user interface.
Get online status The propagation being really a complex subject to study, it is not surprising that models have still need to be improved. However thanks to real-time satellite observations and the assistance of supercomputers, astronomers and geophysicists are today able to predict the evolution of the Sun and the upper atmosphere of the earth. They can draw a quite accurate portrait of the behaviour of our star and determine its influence on shortwaves, and this without interruption thanks to a continuous monitoring 24 hour a day all long the year. Most of these data are available online (see end of page). DK0WCY's messages If you understand Morse or want to learn it and in the same time being informed of the last status of the spaceweather, know that DK0WCY transmits for over 20 years in CW information related to the Sun activity and the status of the geomagnetic field. It works on 3.579 MHz between 07-08 UTC and on 10.144 MHz between 15-18 UTC without interruption. A typical message received at medium speed at 15h UTC at 10.144 MHz looks like this, once translated :
For more information Many other very interesting things could be explained about the propagation and how it affects ham activities. In a first time I suggest you to refer to my general article about radio propagation as well as in Pr. Bob Brown, NM7M's HF Propagation tutorial, a cursus given to his student at U.C.Berkeley. You will find additional readings about propagation on RSGB website, in its Propagation Studies pages and at the ARRL's Bookshop. To get a better image of the upper atmosphere status or to get forecasts of the solar and geomagnetic conditions, you can access online to websites, http or ftp, devoted to the study of the space environment physics, that release for public usage tens of warning messages and reports dealing with the occurence of solar flares and CME, geomagnetic storms and associated auroral activity, without to forget solar and geomagnetic indices as well as tens of images of the sun taken at various wavelengths by satellites orbiting earth. SEC/NOAA for example provides various dynamic and near-real-time graphs as the D-Region Absorption Prediction and various solar flux plots, including the solar flares intensity in X-rays. DX.QSL.NET provides also gray line maps and other MUF maps completed with SOHO pictures of the Sun taken at various wavelengths. Occasionally some hams organizations like ARRL, N3KL and personal websites refer to these data too. In that matter, your best sources of information are the next ones :
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