IPS GRAFEX frequency predictions contain a wealth of radio propagation information for HF communicators. For any (time) period, the predictions provide information concerning transmission conditions and possible propagation modes for a given circuit. Information is presented for frequencies in the range 1-40 MHz using tables and pictorial format of symbols and letters.

The ionosphere is that part of the upper atmosphere which is partially ionised by the sun's radiation and consists of three regions which are denoted by the letters D, E and F. The level of ionization is strongly influenced by the activity cycle of the sun and also by the combined effects of the earth's magnetic field and upper atmosphere winds.

HF communications rely on radio waves being reflected back to earth from the ionosphere. As radio waves pass through the lower ionosphere (the D region) the signal is attenuated; lower frequencies being attenuated more than higher frequencies. Radio waves that are not completely absorbed in the D region will be reflected in either the E region or F region of the ionosphere; normally the higher the frequency, the deeper into the ionosphere the radio wave penetrates before reflection. If the frequency becomes too high, then the radio wave will pass completely through the ionosphere.

b) Propagation Modes

HF radio waves will propagate from a transmitter to a receiver via the ionosphere by whatever path (or mode) is available. If any path is available, no matter how unexpected, the radio wave will use it. However, rather than model all paths, computations are performed for those paths most likely to be useful to radio communicators. For the majority of cases, the most likely path is also the simplest path.

GRAFEX predictions are a compact description combining information of up to four likely paths for a given circuit. Two of these paths are propagated via the E region and two by the F region.

Each path can be ranked in order of likelihood. The most likely path is usually the one that has the least attenuation (or pathloss). This normally corresponds to the path with the least number of reflections, or hops, in the ionosphere.

The communication path or mode that requires the least number of hops (via the F layer) between transmitter and receiver, is called the first mode. The path that requires one extra hop is called the second mode. For more detailed information, see Appendix A

The following list identifies various features of the frequency prediction chart.

The following examples are to be read in conjunction with the GRAFEX prediction shown on each page.

Example 1

A radio operator in Sydney wishes to contact a fellow operator in Wellington at 10:00 UT. What will be the propagation conditions for the frequencies available? The Sydney operator has an antenna which radiates energy at 5 - 50 degrees above the horizontal.

Solution:

1. Determine the possible propagation modes. The fourth line of GRAFEX shows the possible modes and corresponding radio wave elevation angles; first mode 1F
2. (6 - 8 degrees), 1E (0 degrees); - second mode 2F (20 - 23 degrees), 2E (6 degrees). Since the antenna angular range is 5 - 50 degrees above the horizontal, it will transmit via the 1F, 2F and 2E modes. No radio wave will be transmitted via the 1E mode because the circuit length exceeds 1800 km.
3. Locate the time 10 UT in the first column of numbers on the left side of the chart. Values for the first mode OWF, EMUF and ALF are read from successive columns and have values: 6.8, 0.0, 0.0 MHz respectively. Corresponding values for the second mode are read from the columns on the right side of the chart: 5.3, 1.6, 0.0 MHz.

Determine the propagation conditions from the letters and symbols read from left to right at 10 UT.

Analysis:

Conclusions:

1. Frequencies below 5.3 MHz will propagate via both 1F and 2F modes on nearly all days of the month. A fading signal will result from multimode propagation unless the antenna provides more than 3 dB gain to either the 1F or 2F mode.
2. Frequencies in the range 5.3 to 6 MHz will succeed on more than 27 days of the month. Propagation will be via the IF mode and possibly the 2F mode (> 50% chance). A steady signal will result from propagation via the IF mode alone. Again, a fading signal could result from multimode propagation, as described above.
3. Frequencies in the range 7 to 9 MHz will succeed on 15 to 27 days of the month, while frequencies in the range 10 to 11 MHz will succeed on 3 to 15 days of the month. On those days when the signal does propagate, it will do so via the IF mode only. Therefore a steady signal should result.

Example 2

What are the best times for a Brisbane radio operator to contact Noumea on 7 MHz in June 1994? The operator's antenna has maximum gain at an elevation angle of less than 20° above the horizontal.

Solution

1. Determine the possible propagation modes. Since the antenna has maximum gain at less than 20 degrees above the horizontal, the possible propagation modes will be only: First mode 1F (14 - 17 degrees), Second mode 2E (12 degrees). The 1E (3 degrees) and 2F (30 - 36 degrees) modes are not supported as the radio wave vertical elevation angles are beyond the antenna's transmitting limits (the lower limit of the most commonly used antenna is 4 degrees).
2. Determine the propagation conditions from the letters and symbols for 7 MHz. Symbols are read from the top (00 UT) to the bottom (23 UT) of the chart.

Analysis:

1. For 00 UT to 03 UT the symbol "S" applies. This indicates that propagation is possible via the 2F mode only. Since the antenna does not transmit energy at high enough elevation angles for this mode, these hours are not suitable. The IF mode is not propagated because 7.0 MHz is below the first mode ALF (7.3 to 7.6 Mhz).
2. From 04 to 05 UT the symbol "X" applies and indicates that propagation is possible via the 2E mode as well as mixed E and F modes on nearly all days of the month. Although the 1F mode is available, because 7.0 MHz is below the first mode OWF (12.7 MHz at 04 and 05 UT), it may be a weak signal because it is close to the first mode ALF (6.3 to 6.9 MHz). However, since both the E and F modes are available, there is a possibility that interference fading of the signal may occur.
3. From 06 to 07 UT the symbol "M" applies. This symbol usually indicates that propagation via the 1F, 1E and 2F modes is possible but since the antenna can only support the IF mode, the symbol will refer to this mode only. Since 7.0 MHz is below the first mode OWF (12.1 to 10.0 MHz) for 06 to 07 UT and above the first mode ALF (4.9 to 0.0 MHz), the 1F mode at 7 MHz will be available on nearly all days of the month.
4. The symbol "F" applies at 08 and 20 UT and the symbol "%" applies from 09 to 19 UT. Each symbol indicates that propagation is possible via the 1F mode only. The symbol "%" indicates that propagation is possible on 15-27 days of the month, while "F" indicates that propagation is possible on nearly all days of the month.
5. For 21 UT the symbol "M" applies, for 22 UT "X" applies and for 23 UT "S" applies. These conditions have already been discussed.

1. The hours 00 to 03 UT are not suitable for communications on 7 MHz since no propagation is available.
2. Propagation is possible 04 to 05 UT and at 22 UT via the 1F and 2E modes. However there is the chance of fading.
3. From 06 to 07 UT and at 21 UT, propagation is possible on nearly all days of the month via the 1F mode only. Therefore a steady signal should result.
4. Between 09 and 19 UT communications will succeed on 15 to 27 days of the month via the IF mode only, and so a steady signal should result.
5. At 08 and 20 UT communications on 7 MHz will succeed on nearly all days of the month with a steady signal propagated via the 1F mode only.

For any circuit there is a maximum usable frequency (MUF), which is the highest frequency supported by a particular region of the ionosphere. The MUF for the E region is denoted by EMUF and for the F region, FMUF or MUF.

The MUF varies from day-to-day due to prevailing conditions in the ionosphere. Consequently, it is not possible to predict exact values. However, it is possible to predict frequencies with a 10%, 50% or 90% chance of successful propagation. These frequencies are called the upper decile, median and lower decile MUFs. The lower decile MUF is also known as the Optimum Working Frequency (OWF).

Frequencies greater than the upper decile will be propagated on less than 3 (10%) days of the month, while frequencies less than the lower decile will be propagated on at least 27 (90%) days of the month. The median MUF represents the frequency value for which that frequency will be propagated up to 15 (50%) days of the month.

The symbols which make up a GRAFEX represent propagation information on a range of frequencies which lie between the upper decile MUF and the absorption limiting frequency (ALF) for the first and second modes. The ALF indicates the lower limit of the usable frequency band and takes into account signal attenuation due to absorption and the effects of E layer screening.

b) Propagation Modes

GRAFEX predictions provide radio propagation information for only the E and F layers. This information does not include sporadic E (Es) propagation. Sporadic E is a thin layer of unusually high ionization (thus a good reflector of HF signals) located in the E region. Es is particularly prevalent during the summer. The presence of sporadic E can lead to signal fading and screening. However, if conditions are appropriate, Es can also help to provide strong signal reflection. This is why sporadic E is often blamed when predictions are unsuccessful.

The upper panel of Figure A1 shows examples of one-hop modes (propagation via one layer only). Thus 1E, 1Es and 1F means one-hop propagation via the E, Es and F layers respectively. Simple multihop modes are shown in the middle panel which illustrates 2 hops via the E and Es layers (2E, 2Es) and 3 hops via the F layer (3F). Examples of complex modes are shown in the lower panel where propagation can be via the E, Es and F layers. IPS predictions do not contain any modes which include an Es reflection.

For circuit lengths of less than 4000 km, the first mode is usually 1F or 1E, so that the second mode is specified by 2F or 2E. For lengths greater than 4000 km, the first mode can be either 2F or 3F, so that the second mode is specified by 3F, 4F or 3E (propagation via the E layer is not considered for distances greater than 6000 km).

If the antenna beamwidth is too wide, no discrimination between modes can be achieved and multipath or interference fading may result. Some conditions where multipath fading can occur are shown in Figure A2. These occur when two or more modes of comparable signal strength are received simultaneously. This condition is described by the symbols "M" and "X" in a GRAFEX prediction.