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:
21
(13 meters)
17 (16 meters)
15 (19 meters)
13 (22 meters)
11 (25 meters)
09 (31 meters)
07 (41 meters)
06 (49 meters)
21.45
- 21.85
17.55 - 17.90
15.10 - 15.60
13.60 - 13.80
11.65 - 12.05
09.50 - 09.90
07.10 - 07.30
05.95 - 06.20
Short wave transmitters in the IBB network typically deliver between 100 and 500 kilowatts into their antennas and operate on all international short wave broadcast bands between 5.9 and 26.1 Megahertz. Newer models being procured utilize solid-state modulators so they are significantly more reliable and cost effective than the older models that employed vacuum tube modulators. Many of the short wave transmitters are fully capable of being automated, or even being controlled remotely, in conjunction with automated antennas and control room equipment.
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 will consist of many bays and stacks of horizontal radiators grouped together as a curtain, matching devices to reduce transmission losses, and a network of slew switches and delay lines to beam the radiated energy in desired directions,
Click on the left to see a map of locations of International IBB HF radio transmitting stations.
- At SW, ground waves attenuate rapidly with distance. 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 radiation 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.
