USE DIRECTIONAL ANTENNAS

Reception is defined as: when an electromagnetic wave passes through a receiver antenna and induces a voltage in that antenna. Further detailed information on antennas, antenna use, wave propagation and wave generation can be found in NEETS MODULES 9, 10, and 17.

Rotate For Optimum Reception

This is accomplished by both physical and mechanical means of moving the antenna(s) to properly align and tune the antenna.

Align For Optimum Reception

Using the correct antenna location (by rotation) and the correct equipment for the system, you will bring the antenna into alignment and be ready for the final step, which is tuning.

Tune For Optimum Reception

There are two objectives of antenna tuning: (1) to tune out the various impedances and (2) to match the length of the antenna to the frequency radiated at its characteristic impedance.

Impedance: everything exhibits some impedance, Even a straight piece of copper wire 3 inches long will offer some resistance to current flow, however small. The characteristic impedance of this same piece of copper wire is its overall resistance to a signal.

The transmission line between an antenna and a transmitter has a certain amount of characteristic impedance. The antenna also has a certain amount of characteristic impedance. This basic mismatch in impedance between the transmitter and the antenna makes antenna tuning necessary. Naturally, as transmitters, transmission lines, and antennas become more complex, antenna tuning becomes more critical.

Antenna length adjustment: When we tune an antenna, we electrically (not physically) lengthen and shorten it. The radiation resistance varies as we vary the frequency of the transmitter and tune the antenna. The radiation resistance is never perfectly proportional to antenna length become of the effects of the antenna height above the ground and its location to nearby objects.

You will find that the better the ability of the receiver to reject unwanted signals, the better its selectivity, The degree of selection is determinedly the sharpness of resonance to which the frequency-determining circuits have been engineered and tuned. You usually measure selectivity by taking a series of sensitivity readings. As you take the readings, you step the input signal along a band of frequencies above and below the circuit resonance of the receiver; for example, 100 kilohertz below to 100 kilohertz above the tuned frequency. As you approach the tuned frequency, the input level required to maintain a given output level will fall. As you pass the tuned frequency, the required input level will rise. Input voltage levels are then compared with frequency. They can be plotted on paper, or you may can view them on an oscilloscope. They appear in the form of a response curve. The steepness of the response curve at the tuned frequency indicates the selectivity of the receiver, thus allowing for the optimum reception.

RF SAFETY PRECAUTIONS

Although electromagnetic radiation from transmission lines and antennas is usually of insufficient strength to electrocute personnel, it can lead to other accidents and compound injuries. Voltages may be inducted in ungrounded metal objects, such as wire guys, wire cable (hawser), hand rails, or ladders, If you should come in contact with these objects, you could receive a shock or RF burn. This shock can cause you to jump or fall into nearby mechanical equipment or, when working aloft, to fall from an elevated work area. Take care to ensure that all transmission lines or antennas are deenergized before working near or on them.

Guys, cables, rails and ladders should be checked for RF shock dangers. Working aloft "chits" and safety harnesses should be used for your safety. Signing a "working aloft chit" signifies that all equipment is in a nonradiating status (the equipment is not moving). The person who signs the chit should ensure that no RF danger exists in areas where personnel are working.

Nearby ships or parked aircraft are another source of RF energy that must be considered when checking work areas for safety. Combustible materials can be ignited and cause severe fires from arcs or heat generated by RF energy. RF radiation can detonate ordnance devices by inducing currents in the internal wiring of the device or in the external test equipment, or leads connected to the device.

You should always obey RF radiation warning signs and keep a safe distance from radiating antennas. The six types of warning signs for RF radiation hazard are shown in figure 2-40.