HF Bands
|
Short waves (SW) or HF
(high frequency) are electromagnetic waves with frequencies in the range of between 3MHz
to 30MHz. The broadcasting HF bands used by IBB are limited to the International
Broadcasting Bands between 6 and 21 MHz listed below plus 100 kHz bands extending
above and below each of these bands:
Band (MHz)
Frequency Range (MHz)
21 (13 meters)
21.45 - 21.85
17 (16 meters)
17.55 - 17.90
15 (19 meters)
15.10 - 15.60
13 (22 meters)
13.60 - 13.80
11 (25 meters)
11.65 - 12.05
9 (31 meters)
9.50 - 9.90
7 (41 meters)
7.10 - 7.30
6 (49 meters)
5.95 - 6.20
|
SW Stations
Locations of IBB
HF Radio Transmitting Stations |
IBB SW stations receive broadcast programs from production studios via satellites,
and retransmit these programs to designated areas via frequency carriers in the SW
bands. RF power is provided by HF transmitters, the power of each can be as high as
500kW. The RF power generated by the transmitters and
modulated by broadcast signals is then fed to a device called antenna to be transformed
into radio waves. A modern SW antenna can consist of many bays and stacks of
radiators grouped together as a curtain, a network on the ground to allow the
radiated antenna beam to be pointed in certain desired directions, and matching devices to
reduce transmission losses. Click on the
left to see a map of locations of International IBB HF radio transmitting
stations. |
| Propagation
of SW |
 | At SW, ground waves attenuate rapidly with distance and long distance
transmission of short waves is possible only because of reflection and refraction of sky
waves from the ionosphere.
|
| The ionosphere is a region of the upper atmosphere in which the constituent gases
are ionized by extreme ultra-violet and x-rays radiations emitted by the sun.
The ionosphere lies between 50 to 1000km above the surface of the earth.
|
| Several ionized layers in the ionosphere play an important part in the
propagation of radio waves. The lowest layer is the D-layer which lies about 50 to
90 km above the surface of the earth. The highest layer is the F2- layer which lies
at an altitude of about 400km. Between the D and F2 layers are the E-layer, the
E-sporadic layer and the F1-layer.
|
| For radio communications using SW to be reliable, the operating frequency must be
lower than the maximum usable frequency (MUF) and higher than the lowest
useful frequency (LUF). These two frequencies are defined
for each propagation path. The MUF's and LUF's depend on the ionization density of
ionospheric layers. This depends on the amount of radiation each layer received,
hence depends on the latitude at the point of reflection, time of day, season of the year,
and sun spot cycle changes. Burst of solar flares and burst of streams of particles
from the sun causing ionospheric storms also affect HF propagation.
|
| A computer program, titled "Voice of America Coverage Analysis Program"
or VOACAP, was developed by the Institute for Telecommunication Science (ITS) from the
Department of Commerce, and the Naval Research Laboratory (NRL) to assist the IBB to plan
and operate its high frequency broadcast coverage. This computer program is
available to the public on the Internet.
|
|
|
|
| SW Antennas |
|
|