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Modes of Propagation

Frequency Range Band Designation

30-3000 Hz ELF

3-30 kHz VLF

30-300 kHz LF

300-3000 kHz MF

3-30 MHz HF

30-300 MHz VHF

300-3000 MHz UHF

3-30 GHz SHF

30-300 GHz EHF

Ground Wave

The wave which is guided along the surface of the earth,is called the

ground waves. The range of this mode of propagation is upto 2MHz. The waveis

connected at one end to the surface of the earth and to the ionosphere at the

other.The ionosphere is the region above the troposphere (where the air is),

from about 50 to250 miles above the earth.It is a collection of ions, which

are atoms that have some of their electrons stripped off leaving two or more

electrically charged objects.The sun's rays cause the ions to form which

slowly recombine. The propagation of radio

waves in the presence of ions is drastically different than in air, which is

why the ionosphere plays an important role in most modes of propagation.Ground waves travel between two limits, the earth and the ionosphere, which

acts like a duct. Since the duct curves with the earth, the ground wave will

follow. Therefore very long range propagation is possible using ground

waves.

Figure 13



Sky Waves Radio waves in the LF and MF ranges may also propagate as ground

waves, but suffer significant losses, or are attenuated, particularly at

higher frequencies. But as the ground wave mode fades out, a new mode

develops: the sky wave. Sky waves are reflections from the ionosphere. While

the wave is in the ionosphere, it is strongly bent, or refracted, ultimately

back to the ground. From a long distance away this appears as a reflection.

Long ranges are possible in this mode also, up to hundreds of miles. Skywaves in this frequency band are usually only possible at night, when the

concentration of ions is not too great since the ionosphere also tends to

attenuate the signal. However, at night, there are just enough ions to

reflect the wave but not reduce its power too much.

Figure 14

The HF band operates almost exclusively with sky waves. The higher frequencies have lessattenuation and less refraction in the ionosphere as compared to MF. At the high end, the waves

completely penetrate the ionosphere and become space waves. At the low end, they are always

reflected. The HF band operates with both these effects almost all of the time. The characteristicsof the sky wave propagation depend on the conditions in the ionosphere which in turn are

dependent on the activity of the sun. The ionosphere has several well-defined regions in altitude.

. Maximum usuable frequencyThe transmitted frequency was 5 MHz and the critical frequency was 3 MHz in this example.

Maximum Useable Frequency (MUF): defined for two stations. The maximum frequency thatwill reflect back to the receiving station from the transmitter. Beyond the MUF, the wave will

become a space wave. At MUF the skip zone extends to just short of the receiver. In figure xx,the MUF for a receiver at 1400 miles is 5 MHz. Lowest Useable Frequency (LUF): again defined

for two stations. At low frequencies, the signal will be attenuated before it can be reflected. The

LUF increases with sunlight and is a maximum near noon. Optimum Frequency for Traffic(OFT): for two stations, taking into account the exact conditions in the ionosphere, there will be

the perfect frequency that gives the strongest signal. This can be predicted by powerful modeling



programs and is the best guarantee of success in HF. The diurnal variation if HF propagation is

characterized a simple rule-of-thumb: the frequency follows the sun. At noon, the OFT isgenerally higher than at night.

Line of Sight

In the VHF band and up, the propagation tends to straighten out into line-of-

sight(LOS)

waves. However the frequency is still low enough for some significant

effects.

1. Ionospheric scatter. The signal is reflected by the E-region and scattered in all directions.

Some energy makes it back to the earth's surface. This seems to be most effective in the

range of 600-1000 miles.

Fig

What is ISDN?

ISDN is an abbreviation for Integrated Services Digital Network. It is the next-generation, digital

telephone network that integrates circuit-switched voice and data services over a common access

facility. There are two types of ISDN lines. Basic Rate ISDN (BRI) is designed for residential

customers and small businesses. Primary Rate ISDN (PRI) is designed for larger businesses.



The Benefits of ISDN

Reduces the cost of network adminstration

Simplifies wiring

Combines separate voice and data networking requirements

Compatible with BRI/PRI, plus existing analog voice and Switched 56 services

For residential customers, Basic Rate ISDN (BRI) costs about the equivalent of two phone lines.

BRI customers can gain high speed Internet access (64 KBPS to 128 KBPS). BRI improves the

quality of speech in telephone calls. BRI provides an ideal way to keep in touch through personal

videoconferencing. BRI offers improved modem connectivity to non-ISDN systems.

For business customers, ISDN offers cost savings through the integration of voice and dataservices. PRI provides a great backup solution for leased data lines. PRI offers high-quality video

conferencing capabilities. PRI costs about the same as standard "channelized T1" services.

Basic Rate ISDN

Basic Rate ISDN (BRI) consists of three channels. Two channels are used for actual voice or

data traffic with each one operating at a rate of 64 KBPS. These are called "Bearer" channels, orB-channels for short. There is a third channel that is used for call supervision (connecting,

disconnecting, etc.). This channels operates at a rate of 16 KBPS and is called the "Delta"

channel, or D-channel for short

Radio Receiver System



The power supply (not shown) is connected to the audio amplifier block.

Aerial - picks up radio signals from many stations.

Tuner - selects the signal from just one radio station.

Detector - extracts the audio signal carried by the radio signal.

Audio Amplifier - increases the strength (power) of the audio signal.This could be broken down into the blocks like the Audio Amplifier System shown above.

Loudspeaker - a transducer which converts the audio signal to sound

Tropospheric scatter (known as "troposcatter" among practitioners) is a method of transmitting and receiving microwave radio signals over considerable distances – often

up to 300 km. This method of propagation uses the tropospheric scatter phenomenon,

where radio waves at particular frequencies are randomly scattered as they pass throughthe upper layers of the troposphere (hence troposcatter). Radio signals are transmitted in a

tight beam aimed at the tropopause, midway between the transmitter and receiver sites; as

the signals pass through the troposphere they are scattered, allowing the receiver station

to pick up the signal.

Normally, microwave signals, transmitted at various frequencies, usually around12 Gigahertz (GHz) or 19 GHz, are only used for ‘line of sight’ applications, where the

receiver can be ‘seen’ from the transmitter. However, tropospheric scatter signals use afrequency of around 2 GHz

http://en.wikipedia.org/wiki/Microwave_radio



http://en.wikipedia.org/wiki/Troposphere



http://en.wikipedia.org/wiki/Tropopause



http://en.wikipedia.org/wiki/Gigahertz



http://en.wikipedia.org/wiki/Line-of-sight_propagation








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