RADIO COMMUNICATION

Radio Communication

Radio Communication

In this paper, four questions related to radio communication are answered. Each of the four questions is divided into different parts.

Q1. (a). Modes of Wave Propagation

The way in which radio signals travel or propagate it is prime importance to anyone associated with radio communications, be it mobile telecommunications, two way radio communications, maritime or general mobile communications and much more. From radio communications users of the short wave or HF bands through to those using the VHF, UHF and the microwave bands. There are a number of categories into which different types of radio propagation can be placed (ecjones.org). These relate to the effects of the media through which the signals propagate.

Surface modes

Lower frequencies (between 30 and 3,000 kHz) have the property of following the curvature of the earth via groundwave propagation in the majority of occurrences. In this mode the radio wave propagates by interacting with the semi-conductive surface of the earth. The wave "clings" to the surface and thus follows the curvature of the earth. Vertical polarization is used to alleviate short circuiting the electric field through the conductivity of the ground. Since the ground is not a perfect electrical conductor, ground waves are attenuated rapidly as they follow the earth's surface. Attenuation is proportional to the frequency making this mode mainly useful for LF and VLF frequencies (ecjones.org).

Today LF and VLF are mostly used for time signals, and for military communications, especially with ships and submarines. Early commercial and professional radio services relied exclusively on long wave, low frequencies and ground-wave propagation. To prevent interference with these services, amateur and experimental transmitters were restricted to the higher (HF) frequencies, felt to be useless since their ground-wave range was limited. Upon discovery of the other propagation modes possible at medium wave and short wave frequencies, the advantages of HF for commercial and military purposes became apparent. Amateur experimentation was then confined only to authorized frequency segments in the range (ecjones.org).

Direct modes (line-of-sight)

Line-of-sight is the direct propagation of radio waves between antennas that are visible to each other. This is probably the most common of the radio propagation modes at VHF and higher frequencies. Because radio signals can travel through many non-metallic objects, radio can be picked up through walls. This is still line-of-sight propagation. Examples would include propagation between a satellite and a ground antenna or reception of television signals from a local TV transmitter (ecjones.org).

Ground plane reflection effects are an important factor in VHF line of sight propagation. The interference between the direct beam line-of-sight and the ground reflected beam often leads to an effective inverse-fourth-power law for ground-plane limited radiation. [Need reference to inverse-fourth-power law + ground plane.

Radio frequencies and their primary mode of propagation

Band

Frequency

Wavelength

Propagation via

VLF

Very Low Frequency

3-30 kHz

100-10 km

Guided between the earth and the ionosphere.

LF

Low Frequency

30-300 kHz

10-1 km

Guided between the earth and the D layer of the ionosphere.

Surface waves.

MF

Medium Frequency

300-3000 kHz

1000-100 m

Surface waves.

E, F layer ionospheric refraction at night, when D layer absorption weakens.

HF

High Frequency (Short Wave)

3-30 MHz

100-10 m

E layer ionospheric ...