VHF Communication

 

Electromagnetic waves ranging from 30 to 300 MHz frequencies are classified as "Very High Frequency" or VHF. Their wavelengths range from 10 meters to 1 meter.

These radio waves propagate via line of sight and are blocked by large masses such as hills and mountains, but refraction causes them to travel a distance of about 100 miles. VHF is commonly used in television broadcasting, two-way walkie-talkies, Digital Audio Broadcasting (DAB), FM radio, long-range data communication via radio modems, Air traffic control, Air navigation systems, and marine communications.

Characteristics

VHF propagates via ground pounce paths or by the line of sight; there is some form of reflection at the lower end of the band from the skywave propagation (ionosphere). Hills and mountains block the VHF because they do not follow the earth's contour as ground waves. They can, however, travel beyond the visual horizon about to 100 miles (160km) because they are weakly refracted by the atmosphere. Still, sometimes they can be reflected by said buildings causing multipath propagation, which is the usual reason for bad reception. In urban areas, VHF can penetrate building hence being received indoors.

All electrical equipment emits radio interference and radio noise, but they don't affect VHF much due to its higher frequency band. If the conditions are ideal, with the help of tropospheric ducting, VHF can travel long distances because of refraction by atmospheric temperature gradients. They are also one of the only bands where transmission is efficient enough to use small-sized antennas, making them viable for portable devices like marine radio, walkie-talkies (two-way land mobile radio systems), and two-way radio communication with aircraft.

Transmission Of VHF And Their Antennas

As written above, VHF is one of the only bands with wavelengths small enough that efficient transmission can be achieved by very small antennas that can be installed in handheld devices. A quarter-wave whip antenna with frequencies from 10 inches to 8 feet (25 cm to 2.5 meters) long can be used.

Large base stations use big fiberglass whips (collinear arrays) of vertical dipoles, while portable radios usually use whips or rubber ducky antennas. The Yagi antenna is the most commonly used beam antenna (high gain) for directional antennas. Yagi antennas are also used for television reception alongside log periodic antenna because of wider bandwidth. The turnstile and helical antennas are used for satellite communication since they employ circular polarization. If there is a need for even higher gain, we can make an array of antennas using multiple Yagi and helical antennas together. To make high gain omnidirectional antennas, vertical collinear arrays of dipoles are used, the power is radiated in the horizontal direction. Television stations use the batwing antennas as collinear arrays of a specialized dipole.

Universal Use

Certain parts of the VFH band have standard uses across the world

·         Certain parts of the VFH band have standard uses across the world

·         50–54 MHz: Amateur Radio 6-meter band.

·         108–118 MHz: Air navigation beacons VOR and Instrument Landing System localizer.

·         118–137 MHz: Airband for air traffic control, AM, 121.5 MHz is emergency frequency

·         144–146 MHz: Amateur Radio 2-meter band (Extends up to 148 MHz in some Regions).

·         156–174 MHz: VHF maritime mobile band for two-way maritime radio on ships.