V.H.F / U.H.F RADIO SYSTEMS
Propagation in the VHF and UHF bands between 30MHz and 3GHz takes place in the tropospheric .node. The major use of two-way radio communications in the VHF and UHF bands is communications between a fixed base station and several mobile units, located on vehicles, ships, or aircraft in the frequency band 30-470 MHz. Typical applications are control-tower-to-aircraft (CTTA) communication at airport, fire departments. Ship control within harbors, police departments, armed – forces field operations, pipeline and transmission line maintenance, highway maintenance, taxicab and delivery vehicle dispatch, and personal paging systems. Since these systems operate in frequencies above 30MHz, their range of operation is limited to within the line-of-sight horizon of the base station. Distortion systems for automobiles are usually required to cover as much area as possible, and omni directional vertically polarized antennas are usually used to accomplish this , both at the base station and in the mobile units. In some applications , such as pipeline and highway maintenance systems, the field of operations is strung out in a liner over many miles, and for these systems vertically polarized multi-element yagi antennas aimed along the path are frequently used. This provides little coverage off the sides, but does provide better coverage alone the line up to the horizon. The antennas used on the vehicles are nearly always short ground- whip antennas mounted on top of the vehicle. The longer whips used for the 50- MHZ VHF band are not as popular.
Transmission power in both the mobile units and the base station units is usually limited to about 150W, mainly because of the limited power available from the vehicle system. Voltage supplies for mobile equipment range from 12V nominal for automobiles , 28V and 48V for aircraft , and 48V for railway locomotives. The transmitters are designed to alternately transmit and receive on the same frequency. For air craft and ship control use , and such systems as police and fire operation , the units may be designed to operate on one of several channels, with manual switching between channels provided. Each mobile unit is provided with a control head, which is usually separate from the main chassis, conveniently located near the operator. The control head provides a power on/off switch , muting threshold control , and a handset containing a microphone, a telephone receiver and a push to talk switch. The base station may be self contained and directly connected to an antenna near the operator’s location , but usually the base station transceiver unit is located with the antenna at a convenient high point and the operator is provided with a remote control console. Connection between the base station and the control console is made bye means of a pair of wires if the distance is short. Often it is found that it is impossible to cover the desire area from the single base station location. In this case one or more additional base stations can be established. These may be connected back to the operator’s console over wire lines and operated independently of each other by the operator. Alternatively , a radio receiver link can be established. This requires the use of a second frequency for the link between the main base station and the repeater station. An arrangement that might be used. Under normal operation , the base operator can communicate over the local base station on frequency F1 with any mobile unit within coverage area 1 . When it is necessary to reach a vehicle in coverage area 2 , he may turn off the local area base station and turn on the repeater link on frequency F2 . Now when the base operator transmits, he transmits on F2 and retransmits F1 to the extended coverage area. When a mobile unit in the extended area transmits , it is received at the repeater on F1 and retransmitted on F2 toward the base station.
When a system with several fixed stations is being operated in a repeater mode , the base operator must continuously monitor all the incoming signals from all the repeaters. If a mobile unit should be in an overlap area between two repeater stations , he may “key” both repeaters , causing interference at the control console. The operator must be able to purposely disable all but one repeater during a conversation. This can be accomplished by sending a coded tone signal to turn off the desired repeater stations , and then turn them back on when the conversation is complete. The detection characteristics of FM are such that a receiver located in the presence of two co channel ( same frequency ) transmissions ( such as f1 base and f1 repeater ) will suppress the weaker signal. This is known as capture effect.
The Ionospheric layers
Our planet has a diameter of approximately 7720 mi and is surrounded by a gaseous substance called the atmosphere that extends 250 mi from the surface. Only 2 mi of this atmosphere contains oxygen in sufficient quantity to support life as we know it. From .this level on out , the oxygen content lessens and the composition of ionized molecules increases. The ultraviolet radiation from the sun and the cosmic ray radiation from space cause thinner at distances farther from the earth’s surface. The atmospheric layers and their approximate distance from earth. There is no sharp division between the layers , but rather a smooth measured changes in the ion layers , which have been cataloged and named. The oxygen layer, the troposphere , the stratosphere, and the ozone layer are generally fixed in altitude and composition. The ozone layer is a protective shell that shields us from the ultraviolet and cosmic rays of outer space. The stratosphere is also called isothermal layer because of its constant temperature. The ionosphere, on the other hand, changes in both density and thickness from daytime to nighttime and also undergoes seasonal changes.
The D layer is the lowest layer of the ionosphere and is located between 30 mi and 55 mi above Earth’s surface. Because it is the layer farthest from the sun , there is very little ionization in this layer. Therefore , the D layer has very little effect on the direction of propagation of radio waves. However, the ions in the D layer can absorb appreciable amounts of electromagnetic energy. The amount of ionization in the D layer depends on the altitude of the sun above the horizon. Therefore , it disappears at night. The D layer reflects VLF and LF waves and absorbs ME and HE waves.
The E layer is located between 55 mi and 90 mi above Earth’s surface. The E layer is sometimes called the kennelly heaviside layer after the two scientists who discovered it. The E layer has its maximum density at approximately 70 mi at noon , when the sun is at its highest point. As with the D layer , the F layer almost totally disappears at night. The E layer aids MF surface-wave propagation and reflect HF waves somewhat during the daytime. The upper portion of the E layer is sometimes considered separately and is called the sporadic E layer because it seems to come and go rather unpredictably. The sporadic E layer is caused by solar flares and sunpot activity . the sporadic E layer is a thin layer with a very high ionization density. When it appears, there generally , is an unexpected improvement in long-distance radio transmission.
0.003 MHz Very Low Frequency (VLF)
0.03 MHz Low Frequency (LF)
0.3 MHz Medium Frequency (MF)
3 MHz High Frequency (HF)
30 MHz Very High Frequency (VHF)
300 MHz Ultra High Frequency (UHF)
3000 MHz Super High Frequency (SHF)
30000 MHz Extra High Frequency (EHF) 300000 MHz
The F layer is actually made up of two layers , the F1 and F2 layers. During the daytime , the F1 layer is located between 90 mi and 200 mi above earth’s surface , and the F2 layer is located 200 mi and 250 mi above earth’s surface. During the night, the F1 layer combines with the F2 to form a single layer. The F1 layer absorbs and attenuates some HF waves, although most of the waves pass through to the F2 layer, where they are refracted back to Earth.