Propagation: From Them to Us

Transcription

1 Propagation: From Them to Us (c) 2016 Nils Schiffhauer, DK8OK Utility DXing is like observing other people at work. Their work is to establish communications between two stations or within a network, or to provide a service for an area like weather broadcasts for a region. Planning of time and frequency is the basis to provide a high reliability in this service. As propagation changes during the time of the and also day by day, season by season, most stations do have a set of frequencies, mostly different frequencies for day and night. Keep in mind that they must fulfill their work and are not transmitting for the fun of shortwave listeners. He is just the third and often unwanted man in the middle, but never a partner in the service of utility stations. He has to cope with the schedule (time/frequency) of the transmitters. Knowing about propagation is like studying a weather report to set sails to the wind. Statistics and Probability: Software Ionosphere propagation can be largely described by statistics and probability. These methods are delivering by far better results than to conclude from the climate to the actual weather of a day. There are mainly two different approaches to do a prediction: VoACAP and ASAPS Both are available online (free) and offline, with only VoA- CAP being free of charge offline. I work with both of them. Let me show some examples with ASAPS. The most important question surely is: What combination of time and frequency will provide best reception? You have to answer this question twice: one time for the purpose of the circuit (e.g. Cairo <-> Khartoum), and a second time for the man in the middle (Cairo/Khartoum to your location). Obviously, the answer to the second question is the vital one regarding successful utility DXing. But you often need to answer the first question as to understand the schedule of a circuit. Let s first take the view of the professional, we want to observe. How do they do their planning? Generally, they have to plan for a reliable communications in one out of these five cases: Pointo to Point between two fixed stations (e.g. MFA Helsinki and their Embassy in Riyadh Between a fixed and a largely mobile station, like an aircraft Within a network of more than two fixed stations, e.g. a Ministry of Foreign Affairs with their embassies) Serve a large area, as for weather broadcasts Serve a small area of km around the transmitter. As ASAPS perfectly matches all these challenges reliably and with a GUI easy to handle, I mostly will opt for this solution. Point to Point Take a circuit between Helsinki and Riyadh. Set the path, and do a GRAFEX- and an URSL calculation. The first one returns rather detailed information, whereas the URSL calculation gives concise recommendations for the upper, the recommended the secondary and the lower frequency (hence URSL) out of a given frequency set. See Figures on the right. Figure 1: First, set the path from Helsinki to Riyadh. Figure 2: Then do a GRAFEX calculation for detailed information.. Figure 3: An URSL calculation gives some concise information. 2

2 Fixed and Aircraft ASAPS Air Route Chart shows the best frequencies for contact an aircraft en route (start/arrival) by a third station. The example shows flight AF 128 on April 23, 2016 from Paris to Beijing, holding contact to Stockholm Radio, a private Airline Operational Control Center on a choice of seven HF channels. By this chart, Stockholm Radio and the pilot exactly know the best frequency to call each other successfully at each point of the route. Don t forget to convert local time of start and arrival as given in the airline s schedule to UTC. Figure 4: Set the flight route, start time, possible stop-overs, the time of arrival and the corresponding communications station and their frequencies available (above). Figure 5: The result (right) will be shown as diagram and as a table. Network: Let s work together ASAPS let you build up a network of up to ten stations. It then calculates a set of two to six frequencies for communications 24 hours a day, 365 days a year, and this through a whole sunspot cycle. The example shows the French net with Paris as net control and Tahiti, Martinique, Senegal, Djibouti, Reunion and New Caledonia as outposts (map on the right). The result almost perfectly matches the actual STANAG 4285 net (see below). Figure? 6 The French worldwide network (right). Figure 7 The result (left) shows here four valid sets of two (skip that), three, four, five and six frequencies, respectively. They should assure a service during each 24 hours in each year over a full solar cycle at the given reliablitiy. 3

3 Wider Area Here ASAP s Hourly Area Prediction Chart will help. Example: Charleville Radio/Australia provides FAX weather broadcasts on up to five frequencies to be used in the shaded region according to a published schedule (see map below. From this schedule you see that Charleville provides three frequencies during 24 hours plus a night and a day channel each. How this does allows for reception in Hamburg, Germany? Just do a GRAFEX prediction for the circuit Charleville-Hamburg and concentrate on the optimum working frequencies (OWF) in the result table on the right. Match it with the frequency/hours combination of the schedule. ASAPS also delivers hourly charts that show the recommended frequencies at any location within the actual service areal (see Figures at the bottom of the page). Figure 8: Map with service area (shaded) of Charleville Meteo.. Figure 9: Here GRAFEX recommends some combinations of time and frequency for best reception for a DXer in Hamburg. Figures 10 and 11: Twelve maps with results of the Hourly Area Prediction Chart for the actual service region of Charleville Meteo. 4

4 1.000 km around a transmitter To serve a smaller region, ASAPS provides the Local Area Prediction Mode (LAMP) with a circle of km around a transmitter. Due to the projection of the map this area will look like a circle just on the equator line, and distorting towards the poles. In nature, however, it always represents a circle of km in diameter. Example: A nationwide ALE-network in Uzbekistan. Default mode of ASAPS LAMP predictions shows a set of four channels between 4 and 10 MHz to provide a near-24 h service throughout the country. Actually, there had been monitored six channels from khz to khz. With these you get Figure 12 as LAMP output. From a DXer s point of view these nets are the most difficult as they usually use frequencies under 10 MHz and low-hanging inefficient antennas. This combines high-angle radiation with a low radiated power. Such an installation works for the intended purpose but poses a real challenge many thousand kilometers aborad. Multipath the hidden Enemy ASAPS has some other unique feature, called Digital HF Prediction. Here you get not only information on the power of a received signal, but also on its quality. This is vital for all digital wireless communications, so it needs a closer look. You already know about the ionosphere is providing over-the-horizon HF communications. Don t imagine HF communications as a signal like a laser beam reflected by a plane and stable mirror, bouncing back the same signal. Rather think of it as a highly dynamic piece of wrinkled kitchen foil waving by the wind, thus reflecting a somewhat distorted and ever changing picture. For HF signals it works like an extra modulation, added to the signal. The degradation of the signal may vary from mild under so-called quiet conditions to severe distortion caused by a high degree of geomagnetic activity. Figure12: LAMP calculates HF propagation within a circle of km. Figure 13: It recommends these frequencies for April 23rd, Figure 15: An example for selective fading, see text. The nature of HF communications leads to a phenomenon called multipath, i.e. a signal arriving by different paths at slightly different times and phases. Surely, you already have seen the effect: specific patterns of trenches and ridges over a broad signal (see Figure??). These are caused by a constructive (ridges) and destructive (trenches) addition of at least two paths. As this effect occurs mostly within small bands of the signal, it is dub- Figure 14: If you apply LAMP for the frequencies already observed in this Uzbek net, you get this diagram. 5

5 bed selective fading. Trenches and ridges may differ by 30 db and more. Moreover, the scene is dynamically changing with ever new patterns occurring. You have seen these patterns also in the real world where two identical patterns do have a distance from each other with leads to a third pattern, or moiré (Figure 16). How does multipath degrade HF reception? See Figure 17 for a FAX picture from a 10 kw strong station, just 180 km away from may location, and producing a strong signal. But as you see, the picture is somewhat distorted. A closer look (Figure 18) reveals two stronger, vertical lines in parallel. Obviously, the signal is received at least twice, with some difference of time. As a FAX signal is written horizontal line by horizontal line, it s easy to calculate the difference: One horizontal line is written in 500 milliseconds (120 rpm/rotations per minute) which in this case leads to a chart of pixels of width. So, each pixel represents a time of 0,25 milliseconds. Figure 18 takes an even more detailed look down to pixel level. Figure 16: Selective Fading, demonstrated at a waste paper bin. Figure 17: Ten kilowatts, 183 km distance, excellent signal. But are you suffering from diplopia? Forget about that, plus the slant, so far! You can clearly distinguish both vertical lines, see their width of about four to five pixel and measure the distance between the start of the line. Remember this is a detail from a FAX transmission, writing line by line from left to right. Hop1 is the signal received first, followed by Hop2 after around 1,5 to 1,75 milliseconds. The reason for this effect is multipath : the first signal is followed by an echo. See overleaf how propagation causes this quite common effect on HF. Figure 18: Two vertical lines from the pciture above, at 300 %. They clearly show a doppelganger, or echo. Figure 19: Two hops are causing an echo, see text. 6

6 Figure 20: This diagram shows the solution. The signal reaches my location on at least two different ways one hop from a vertical angle of about 70 and the two hops from an vertical angle of about 80. The second signal has to travel a slightly longer distanace and will arrive a bit later. Simulation has been done with PropLab 3.0. Figure 21: A much more detailed analysis with PropLab 3.0 reveals the time of flight of each path, ordinary and extraordinary. The difference between the first signal and the strongest echo is about 1,75 milliseconds. This ray tracing simulation perfectly matches the real world, shown in Figures 18 and 19. The example above gives clear results in praxi. Alas, ASAPS doesn t to handle such a short range in its online Digital HF Prediction mode. So I took Tokyo Weather Fax transmitter JMH as example for multipath propagation, see diagram on the next page. Its a real stroke of luck that the Space Weather Services of the Australian Bureau of Meteorology offers these pages. There ou also find up-to-date space weather and much more. The scientist and engineers there a doing a fine job,, as is the Australian tax payer doing as these services are for free. Thanks! 7

7 Figure 22: This is part of some results of ASAPS Digital HF Prediction. It is centred around Tokyo, showing some multipath in the window bottom right ( Strongest Interfering Mode.) Such a simulation explains why even a strong signal may suffer from severe impairments. 8