To understand propagation on the Tropical Bands it is useful to know something of the properties of the D-layer of the ionosphere. This layer only exists during daylight and has different effects on three parts of the radio spectrum.


1.Frequencies below about 3 MHz (including the LF and MF bands) are completely absorbed by the D-layer. Thus skywave propagation on these frequencies normally only takes place during darkness (when there is no D-layer) or in the period around dawn and dusk (when the D-layer is weak). The only circumstances in which skywave propagation can take place during the middle of the day is during winter in the high latitudes, when the sunís rays are weak and hitting the ionosphere at a shallow angle. This explains why mediumwave DXing is possible throughout the day in northern Scandinavia during the winter.


2.Frequencies above about 9 MHz (the exact frequency varies with latitude and season) pass through the D-layer unaffected (except during Sudden Ionospheric Disturbances - SIDís) and are reflected back to earth by the F-layer, which lies above the D-layer.


3.Signals between about 3 and 9 MHz (including Tropical Bands) are absorbed by the D-layer if they strike it at a shallow angle, such as is necessary for long distance propagation. However, if they strike it at an acute angle, close to the vertical, they will pass through it and be reflected by the F-layer. It is this principle which allows stations to use these frequencies during the day to cover areas up to a few hundred miles from the transmitter (i.e. beyond the groundwave range of mediumwave transmissions).


In Europe many stations use the 6 and 7 MHz bands during the daytime on this basis. Radio Luxembourg on 6090 kHz and relays of West German domestic services on 6005, 6030, 6085, 6190 and 7265 kHz are good examples.


On the lower frequencies, below 6 MHz, the angle the signals hit the D-layer, and hence the distance they cover, becomes very critical. In southern England, for example, daytime reception of the Austrian scouting station on 5035 kHz (about 800 miles distant) is normally possible. However, reception of 5057 kHz Tirana (about 1,200 miles away) is not possible until dusk approaches.


Here in Nairobi, this propagational characteristic means that for most of the daylight hours the only stations audible on the tropical bands are those in neighbouring countries of Burundi, Rwanda, Tanzania, Uganda, eastern Zaire, as well as Kenya itself.


In a future report I hope to talk further about listening in East Africa on the tropical bands as well as reception on other bands. In the meantime, it may be interesting for members in Europe to see what stations are audible here on the 90 metre band during the evening.


3200†††††††† Fuzhou, China (audible before TWR sign-on)

3200†††††††† TWR Swaziland (not 3205 as per WRTH)

3204.5††††† RRI Bandung ( Indonesians fade-in shortly before networked news at 1500)

3205†††††††† AIR Lucknow

3210†††††††† Maputo, Mozambique (// 4866)

3215†††††††† SABC Radio Oranje/Radio Orion

3220†††††††† CPBS Beijing (1st Programme // 9064)

3230†††††††† Radio RSA (Portuguese 1900-2000)

3230†††††††† Radio Nepal (// 5005)

3231.5††††† Antananarivo, Madagascar (French programme with short English news bulletin

††††††††††††††††† ††††††††††††††††††††††††††††††††††††††††††††††††††† just before 1800, sign-off at 2000)

3235†††††††† AIR Gauhati

3249.8††††† RRI Banjarmasin

3264.8††††† RRI Bengkulu

3270†††††††† SWABC (1st Programme)

3270†††††††† Radio Beijing

3276.7††††† RRI Jakarta

3277†††††††† Radio Kashmir (relays AIR English news at 1530)

3279†††††††† Beira, Mozambique

3287.5††††† Antananarivo, Madagascar (Malagasy programme, sign-on at 1515 to sign-off at

††††††††††††††††† ††††††††††††††††††††††††††††††††††††††††††††††††††† 1900 - sometimes later)

3290†††††††† CPBS Beijing (2nd Programme // 6890)

3295†††††††† SWABC (2nd Programme)

3295†††††††† AIR Delhi

3300†††††††† Bujumbura, Burundi (// 6140 also audible in daytime)

3305.7††††† Zimbabwe BC (Radio 2)

3315†††††††† AIR Bhopal

3320†††††††† SABC Radio Suid Afrika/Radio Orion

3330†††††††† Kigali, Rwanda (// 6055 also audible in daytime)

3330.8††††† Moroni, Comoros (normally swamped by Kigali)

3347.4††††† Radio Zambia (General Service)

3355†††††††† AIR Kurseong

3356†††††††† Radio Botswana (// 4820)

3365†††††††† AIR Delhi

3366†††††††† GBC-2, Accra (this is the only West African audible on this band)

3370†††††††† Beira, Mozambique

3375†††††††† AIR Gauhati

3380.8††††† MBC, Blantyre

3390.1††††† Radio Candip, Bunia, Eastern Zaire (also audible in daytime on // 5066)

3395.7††††† Zimbabwe BC (Radio 1)




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Updated: 6 Nov 2001