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Rain scatter – Where, When, How ?

By PA5DD, Uffe Lindhardt

This paper forms the basis of a talk on rain scatter given at the Adastral Park Microwave Roundtable on 14 November 1999. It deals with the practical aspects of using rain clouds as a mean of reflection on frequencies around 10 GHz. It also includes a look back on the rain scatter season 1999 as seen from the Netherlands.

Frequencies

For radio amateurs the main frequency band of interest for rain scatter contacts is 10 GHz. Rain clouds offer very good reflectivity at this frequency, and atmospheric losses are low. At the same time the availability of high performance components and circuits for this band has increased the last few years. On lower bands the reflectivity decreases drasticly due to the size of raindrops compared to the wavelength. According to WA1MBA (see Internet ressources below) this amounts to –12dB at 5.7 GHz and –19dB at 3.4 GHz in relation to 10GHz. These figures seem to match the practical experience of many amateurs. DX-contacts (over 400 km) are quite difficult on 5.7 GHz. Also the number of stations QRV on this band is much smaller. Rain scatter on higher bands than 10 GHz is an area where very little experience has been gathered. Due to the still very merger activity and the small output powers on 24 GHz, very few contacts has been made. Certainly atmospheric losses start to play a role on this band.

It seems though, that shorter contacts of up to 200 km can be made when strong forward scatter is present on 10 GHz. For these contacts elevation is essential.

Equipment

Which kind of equipment is needed to make rain scatter contacts ? Basically any equipment used to make troposheric contacts will do, but to make full advantage of this propagation mode a narrowband mast mounted home station is essential. The weather patterns that rain scatter are linked to do not invite for hill top portable operation. It is also very difficult to predict when good rain scatter openings will occur, and it is therefore essential, to monitor the conditions over longer periods.

QTH

It is not essential to have a high-elevated QTH for working rain scatter, as it is often the case with troposheric contacts. High altitude will give only little enhancement in the achievable ODXs. What is important is a clear horizon because even a few degrees of horizon elevation are going to block for DX contacts. The rain clouds resides only up to a maximum height of 10 – 12 km, and it is essential to have a clear view of the reflection areas on the horizon if you want to achieve contacts of 600 – 800 km.

TX

A reasonable ERP output power is important. Output powers of up to 1W are available at low cost (e.g. QUALCOMM surplus modules), and with that power and a parabolic dish of 50cm, you have a good rain scatter station. In fact most of then QSOs reported at the end of this paper was worked with 1 W and a 45cm dish, including several QSOs of 600 – 700 km. Since the station should preferably be mast mounted because of the high feed losses at 10 GHz, solid-state amplifiers are easier to install. On the other hand the cost of solid-state transistors at a power outputs above 1 W are still very expensive. Installing a TWT amplifier in your mast can be quite cumbersome. Nevertheless many stations using 10 W or more are QRV via rain scatter, so be prepared for some frustration if you go on the air with 200 mW.

RX

Since low cost HEMTs like the NE325 are now available, the receiver should have a noise figure of 1 2 dB. On the other hand since many high power stations are QRV any receiver will do. 

Antenna

For maximum size/gain performance a parabolic dish of at least 40cm should be used. Using a relative small dish gives some advantages in finding the optimum reflection point and also removes the need to elevate the antenna at medium distance contacts. A larger antenna gives the advantage of a larger ERP output power, which can prove essential for DX contacts. Exact pointing gets difficult from a dish size of 70cms onwards using the normal commercially available azimuth rotators. My advice is to use a 40 - 50cm dish if you are looking for many QSOs (like in a contest), and a larger (70 90cm) if you are a DXer. Elevation can be very useful in rain scatter openings. For medium distances (300-500 km) it can mean a difference of 10 20 dB in signals. It is however my experience, that you will work these stations also without elevation, but probably AFTER that the stations with elevation are done rag chewing. For 24 GHz elevation is essential for rain scatter.

Modes

All though rain scatter is possible using broadband equipment real DX requires the better system performance of narrowband modes like CW , SSB & narrowband FM. CW  (telegraphy) is by far the most efficient mode. Due to the fast random orientation wind speeds in rain clouds, reflected signals are subject to Doppler distortion, which makes the signal sound like white noise. The size of the Doppler effect is on 10 GHz very similar to the one known from 144 MHz Aurora reflection. This distortion makes it difficult to understand SSB signals, especially when using side scatter (i.e. the reflection point being offset from the direct line between the two stations). On the other hand SSB is normally quite useful for making DX QSOs, which are always forward scatter. For local or medium distance contacts - where signals can be very strong – narrowband FM is also an option. In FM the Doppler distortion disappears all together, due to the low deviation of the Doppler effect as compared with the modulation. For making fast comfortable QSOs or for rag chewing FM is the perfect mode, it is however recommended to QSY from the narrowband part of the band to prevent disturbance to other stations.

Finding rain scatter

Finding rain scatter openings are quite a challenge because of the sparse distribution of stations and beacons, and because of the narrow  beam angles associated with 10 GHz antennas. The rain scatter season – at our latitudes - extends from approximately begin of May to the end of September, with a peak in the month of June. Certainly rain scatter contacts can be made outside this season, but they rarely produce any DX contacts. At present most of the rain scatter QSOs made are made along the 50 latitude. From my experience rain scatter is less frequent at higher latitudes like 55. It is however difficult to separate the effect of the generally low 10 GHz activity level there and possibilities for rain scatter. There seems however to exist a belt of high thunder activity going from the Biscay bay and North- East towards central Europe. In any case there will certainly be some variation in rain scatter activity from region to region. At present the main warning channel for rain scatter openings is the Packet Cluster network. This network links most of Europe together for fast real-time spotting of rain scatter observation. The ideal rain scatter spot contains information of the QTH of the two stations, and the azimuth angle QTF of the reporting station. Unfortunately only the newer DX cluster software has a good support for reporting 10 GHz contacts, and this software is not yet installed throughout Europe (e.g. not in the UK). For example CLX provides excellent features like dedicated 10 GHz spotting and reflection point calculation. WW-convers is also available via the Packet network. It is a "chat mode", and allows real-time communications between rain scatter stations on ch. 10368. Unfortunately WW-convers is also not available everywhere, and normally less than 10 stations van be found here during openings.

The Packet cluster & WW-convers can also be reached via various "back doors" on the Internet. So can some of the professional weather radar’s. These radar’s typically work at around 9 GHz, and they can therefore provide very useful information on current rain scatter conditions. The main problems in using these sources are the cost of real-time information, which is sold at commercial terms. The information that is publicly available is normally some hours old, and does for example not include elevation scans, which are essential for the evaluation of DX possibilities. A few exemptions are mentioned under the Internet ressources below. Beacons are another essential tool in finding good rain scatter reflection points. It is however essential with high power beacons (1W or more), since the beacons are required to have an omni-directional antenna pattern, and hence have relative low ERP. It is also important that beacons are placed between places, where actual activity can be expected. A good example of such a beacon is DB0JK in JO30LX, which serves the three main areas of activity at present, namely the Randstad NL (JO22), Ruhr D (JO31) & Rhein/Main D (JN49). Unfortunately the lesser-activated areas (like the UK at present) suffer from the mutual concentration of activity between these centres. One answer to solve this problem is the deployment of beacons to attract attention.

A final means of locating rain scatter reflection points is the use of another local station as "sounder". In my experience the backscatter signal of a station close to you gives the best indication of the beam angle towards the DX stations. Beacons which are often received via side scatter will give a beam angle slightly offset to the forward scatter beam angle. A very good teamwork can evolve between two close-by stations alternately giving CQ, while the other station are optimising his beam angle. Combined with some kind of backbone communication, this can be a powerful tool to find the reflection in directions, where no beacons are present. It can however be difficult to determine the distance to the scatter point using this technique. Maybe in the future amateurs will be able to do their own "sounding", using fast RX/TX switching and precise time mapping of the return signal. Finally it cannot be stressed enough, that activity brings on more activity in quite a surprising way. It is my estimate that at present, there are close to a doubling of rain scatter activity every year on the continent, at least if measured by the number of contacts made.

For an indication of the present activity, you can check the list of stations compiled by DG1VL and referenced below under Internet ressources. This list is only the top of the iceberg !

Operating

The rain scatter activity is concentrated around 10368.100 MHz. During openings the activity can extend well beyond the band 10368.080 – 10368.150 MHz. It is apparent, that more spectrum will be needed if activity continue to increase at the present level. It is not unheard of to hear 10 stations giving CQ at the same time in this band segment during an opening. Most contacts are made randomly following a CQ without any prior arrangement. This makes rain scatter contacts an extra treat, since most other contacts on 10 GHz are made after contacts made on lower bands. The normal operation style is to have an automatic keyer make a CQ in the direction, where the rain scatter reflection point is assumed to be. These CQ calls can be quite long, but in order to find the rain scatter it is important, that the on-air time is kept high. Unfortunately this leads some operators to call CQ for more than 10 min without checking for stations coming back to their call. If this happen to be the rare DX station you are looking for, be sure to have some tranquillisers at hand. Calls in SSB are also made, but usually even stations that cannot read CW  uses an automatic keyer to make the CQ.

A main activity during a rain scatter opening is to keep track of the reflection area. The reflection point is often moving (though not fast), and DX stations will have slight variations in the optimal azimuth angle, depending on the angle they have towards the reflection area. In some big openings there are 2 or 3 different reflection areas, which can be challenging to the operator. A good working Packet cluster can help to focus on the best direction. On the IARU conference in Lillehammer 1999 it was decided to replace the last character of the RST report with an S (e.g. 59S ) in rain scatter contacts. This reporting is used on CW  as well as phone.

Log analysis 1999

To round off this paper I have made some simple analysis on the rain scatter contacts that I have made during 1999. All of the contacts were made from my home QTH that is situated at –2m ASL, but with 360 of free horizon take-off. Most of the contacts were made with 1 W output and a 45cm parabolic dish (actually a lampshade). Towards the end of the period I have upgraded to 10 W output and a 70cm dish. These improvements were made as an effect of the good results made with the original set-up. The first figure in appendix 1 shows the distribution of contacts and distances over the season. Of course the number of contacts are highly dependent on when I was actually QRV, but still the figure are rather interesting. It should be noted that many stations have been worked more than once. For a detailed analysis I refer to the complete log in appendix 2. The first figure shows 5 – 6 real DX openings concentrated in the period May, June and July. The DX openings are characterised in that contacts over 450 km are possible. These contacts are much rarer than contacts of 200 – 400 km. This is shown more clearly in the second figure, where the same contacts has been sorted after distance. DX contacts are more difficult to achieve due to the fact that openings are shorter, and DX openings can rarely be detected using beacons. The signal strengths of DX stations can be quite impressive though.

Conclusion

It is my hope that this paper shows that rain scatter reflection on 10 GHz is a mode that everybody can enjoy, and that we can continue the rising trend of rain scatter activity in Europe in the coming years. Today the achievable DX results on 10 GHz far suceed those of the lower microwave bands like 2.3 GHz. This is a surprising development, from which we have only seen the beginning yet.

Internet ressources

 WA1MBA, on the basics of rain scatter: http://www.wa1mba.org/10grain.htm

 DG1VLs list of rain scatter stations: http://www.qsl.net/dg1vl/RS_05_02_99.txt

Weather radar for the Netherlands: http://weerkamer.nl/radar/

Weather radar for Bonn, Germany with elevation scans: http://www.meteo.uni-bonn.de/Deutsch/Forschung/Gruppen/radar/radar_en.html

PA5DD, Authors homepage: http://home.worldonline.nl/~nouchavw/

G4DDK, Activity reports: http://www.btinternet.com/~jewell/

DF6NA, Sound recordings of rain scatter contacts: http://www.df6na.de/df6na/audio.htm

 

Appendix 1

 

 

Appendix 2


Rain scatter LOG for PA5DD  (JO22IC)  1999


Date     UTC  Call      Loc    QRG   2*  RSTs  RSTr  QRB(km)

19990505 1154 PA3DYS    JO21JP 10368 CW  59RS  57RS  50
19990507 1657 F6DWG/P   JN19AJ 10368 CW  59RS  55RS  353
19990507 1702 F5HRY     JN18EQ 10368 CW  57RS  55RS  413
19990507 1722 F6DKW     JN18CS 10368 SSB 58RS  54RS  409
19990507 1726 F6DWG/P   JN19AJ 5760  CW  55RS  57RS  353
19990507 1746 F1PYR/P   JN19BC 10368 SSB 55RS  55RS  378
19990507 1754 DG1KJG    JO30NT 10368 SSB 57RS  55RS  221
19990507 1902 DL3NQ     JN49IN 10368 CW  57RS  42RS  398
19990507 1912 DC9YC     JO31PJ 10368 SSB 57RS  52RS  195
19990507 1921 DC6RW     JN49HL 10368 SSB 56RS  41RS  401
19990507 1931 DF6NA     JN49XS 10368 CW  55RS  52RS  450
19990507 1945 DJ1KP     JO40JJ 10368 CW  55RS  55RS  342
19990510 1541 DG1KJG    JO30NT 10368 SSB 59RS  59RS  221
19990510 1544 DG1KJG    JO30NT 5760  SSB 42RS  52RS  221
19990510 1549 DJ6JJ     JO31LG 10368 SSB 59RS  59RS  181
19990510 1559 DH8AG     JO31RL 10368 SSB 59RS  55RS  202
19990510 1724 DH9NBB    JN49WS 10368 SSB 56RS  51RS  446
19990510 1731 DF6NA     JN49XS 10368 CW  56RS  53RS  450
19990510 1746 DH6FAE/P  JO40PL 10368 SSB 59RS  59RS  367
19990510 1856 DL3IAS    JN49EJ 10368 CW  55RS  55RS  396
19990519 1539 F6DWG     JN19AJ 10368 CW  54RS  51RS  353
19990529 1758 F6DWG     JN19AJ 10368 CW  57RS  55RS  353
19990529 1812 DG1KJG    JO30NT 10368 SSB 59RS  56RS  221
19990529 1822 G3LQR     JO02QF 10368 CW  59RS  57RS  227
19990529 1850 G4DDK     JO02PA 10368 CW  57RS  55RS  232
19990529 2004 DG1KJG    JO30NT 5760  SSB 55RS  53RS  221
19990529 2119 DL3NQ     JN49IN 10368 SSB 59RS  58RS  398
19990529 2124 DF6NA     JN49XS 10368 CW  57RS  54RS  450
19990530 1046 ON7WR     JO20EP 10368 CW  59RS  55RS  162
19990530 1153 OK1JKT/P  JO60OK 10368 CW  56RS  57RS  620
19990530 1159 DL2ABO    JO51CR 10368 SSB 59RS  59RS  381
19990530 1205 DH4AE/P   JO51DQ 10368 CW  55RS  55RS  387
19990530 1212 DH6FAE/P  JO40PL 10368 SSB 59RS  59RS  367
19990530 1217 DK3FF     JO30MT 10368 SSB 58RS  58RS  216
19990530 1230 DJ5VW     JO31RJ 10368 CW  59RS  56RS  206
19990530 1243 F6DPH/P   JN28QJ 10368 SSB 57RS  55RS  414
19990530 1328 DL4EAU/P  JO51DR 10368 CW  58RS  58RS  386
19990530 1342 DK1PZ     JO41TH 10368 CW  54RS  55RS  351
19990530 1404 DL3IAS    JN49EJ 10368 CW  55RS  55RS  396
19990530 1528 LX1DU     JN29XM 10368 CW  59RS  55RS  300
19990530 1534 F6DPH/P   JN28QJ 5760  SSB 59RS  59RS  414
19990530 1701 DK9MN     JN58TC 10368 CW  55RS  41RS  664
19990602 1451 DF6NA     JN49XS 10368 CW  59RS  55RS  450
19990602 1500 DK2GR     JN59IE 10368 CW  52RS  52RS  533
19990602 1508 OK1JKT/P  JO60OK 10368 CW  55RS  55RS  620
19990602 1520 DF3CK/B   JN57UV 10368 CW  55RS  HRD   685
19990602 1520 DK9MN     JN58TC 10368 CW  58RS  52RS  664
19990602 1658 DB6NT     JO50TI 10368 SSB 59RS  54RS  520
19990602 1741 DL2ABO    JO51CR 10368 CW  55RS  53RS  381
19990602 1754 DG1VL/P   JO61XE 10368 SSB 52RS  52RS  647
19990603 1546 F6DWG     JN19AJ 10368 CW  54RS  41RS  353
19990627 1808 G3LQR     JO02QF 10368 CW  57RS  55RS  227
19990703 0745 G3LQR     JO02QF 10368 CW  59RS  59RS  227
19990703 1620 OK1JKT/P  JO60OK 10368 CW  59RS  59RS  620
19990703 1635 DK0FLT    JN59FW 10368 CW  59RS  55RS  469
19990703 1649 DL3YEE    JO42GE 10368 CW  57RS  59RS  263
19990703 1711 DM2AFN    JO61WB 10368 SSB 55RS  55RS  645
19990703 1735 OK1KIM    JO60RN 10368 CW  559   559   632
19990703 1948 SM7ECM/B  JO65NQ 10368 CW  54RS  HRD   681
19990703 1951 DL5CC     JO64AD 10368 CW  55RS  55RS  541
19990707 1453 G3LQR     JO02QF 10368 CW  59RS  55RS  227
19990707 1521 DG1KJG    JO30NT 10368 SSB 59RS  59RS  221
19990707 1627 F6DWG     JN19AJ 10368 CW  59RS  57RS  353
19990712 1459 DJ1KP     JO40JJ 10368 CW  58RS  58RS  342
19990712 1509 DC6RW     JN49HL 10368 SSB 56RS  51RS  401
19990713 1522 DL3YEL    JO41EV 10368 CW  52RS  54RS  253
19990713 1708 DF6NA     JN49XS 10368 CW  56RS  52RS  450
19990713 1855 DM2AFN    JO61WB10368  CW  55RS  57RS  645
19990713 1901 OK1JKT/P  JO60OK 10368 CW  54RS  55RS  620
19990713 1908 DB6NT     JO50TI 10368 SSB 57RS  53RS  520
19990714 1942 G3LQR     JO02QF 10368 CW  58RS  57RS  227
19990718 1801 DH8AG     JO31RL 10368 SSB 53RS  52RS  202
19990718 1814 DF6NA     JN49XS 10368 CW  57RS  55RS  450
19990718 1830 DC9YC     JO31PI 10368 SSB 53RS  53RS  197
19990719 1424 DK4VW     JO40IT 10368 CW  53RS  53RS  313
19990719 1430 DL3IAS    JN49EJ 10368 CW  55RS  55RS  396
19990719 1452 DG1KJG    JO30NT 10368 CW  59RS  59RS  221
19990719 1520 DJ1KP     JO40JJ 10368 CW  54RS  55RS  342
19990719 1528 DK8ZP     JO40JJ 10368 CW  55RS  57RS  342
19990719 1611 DF9QX     JO42HD 10368 CW  59RS  59RS  269
19990719 1631 DK1KR     JO53HW10368  CW  55RS  59RS  447
19990719 1711 DF1OI     JO42TF 10368 SSB 55RS  52RS  337
19990719 1927 DL3ALI    JO50JW 10368 CW  56RS  59RS  441
19990719 1943 DL2DR     JO31TO 10368 CW  53RS  55RS  209
19990808 1524 F6DWG     JN19AJ 10368 CW  53RS  52RS  353
19990808 1526 DL3EAG    JO31DK 10368 CW  56RS  55RS  132
19990808 1531 DF6NA     JN49XS 10368 CW  53RS  54RS  450
19990808 1540 F6DKW     JN18CS 10368 CW  52RS  51RS  409
19990808 1547 F6DWG     JN19AJ 10368 CW  59RS  59RS  353
19990808 1619 F5HRY     JN18EQ 10368 CW  57RS  57RS  413
19990808 1907 OK1JKT/P  JO60OK 10368 CW  56RS  59RS  620
19990814 1926 F6DWG     JN19AJ 10368 CW  59RS  59RS  353
19990814 2006 F6DKW     JN18CS 10368 SSB 59RS  59RS  409
19990816 1850 F6DWG     JN19AJ 10368 CW  57RS  57RS  353
19990816 1853 F5HRY     JN18EQ 10368 CW  55RS  529   413
19990818 1522 G3LQR     JO02QF 10368 CW  57RS  55RS  227
19990818 1719 F5HRY     JN18EQ 10368 CW  58RS  57RS  413
19990818 1800 F1PYR/P   JN19BC 10368 CW  55RS  55RS  378
19990819 1624 PE9GHZ    JO11TL 10368 CW  54RS  52RS  100
19990825 1812 F5HRY     JN18EQ 10368 SSB 59RS  59RS  413
19990825 1815 F6DKW     JN18CS 10368 SSB 59RS  59RS  409
19990825 1824 F6DWG     JN19AJ 10368 CW  59RS  59RS  353
19990825 1906 G4BYV     JO02LQ 10368 SSB 58RS  58RS  262
19990825 1946 F5JTA     JN08VN 10368 CW  57RS  55RS  443
19990826 1627 DH8AG     JO31RL 10368 SSB 59RS  59RS  202
19990826 1631 DG1KJG    JO30NT 10368 SSB 59RS  59RS  221
19990826 1641 DF6NA     JN49XS 10368 CW  58RS  59RS  450
19990905 1714 F5HRY     JN18EQ 10368 CW  55RS  55RS  413
19990905 2028 F6DWG     JN19AJ 10368 CW  56RS  55RS  353
19990906 1729 DF6NA     JN49XS 10368 CW  56RS  55RS  450
19990906 1851 DL2DR     JO31TO 10368 SSB 55RS  57RS  209
19990906 1853 DH8AG     JO31RL 10368 SSB 59RS  59RS  202
19990906 1914 DB7QE     JO32VF 10368 SSB 56RS  57RS  212
19990906 2006 PE1CQQ    JO22VQ 10368 SSB 54RS  57RS  100
19990913 1535 DG1KJG    JO30NT 10368 SSB 57RS  57RS  221
19991002 1446 G8P       JO01QD 10368 CW  55S   53S   252
19991002 1727 DL3NQ     JN49IN 10368 CW  54S   54S   398
19991002 1800 ON7WR     JO20EP 10368 CW  569   569   162
19991002 1807 DH6FAE/P  JO40PL 10368 SSB 55    55    367
19991002 1810 DF0BG     JO30OX 10368 SSB 56    59    214
19991002 1911 PA3DYS    JO21JP 10368 CW  59S   59S   50
19991002 2315 G3LQR     JO02QF 10368 CW  599   599   227
19991003 0725 G0EMG/P   JO02OD 10368 SSB 59    59    238
19991003 1130 M1CRO/P   JO01PU 10368 CW  539   55S   234
19991003 1348 PI4GN     JO33KK 10368 SSB 59    55    210
19991003 1350 PA0WSO    JO22XD 10368 SSB 59    59    87
 

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