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FM 24-18: Tactical Single-Channel Radio Communications Techniques


B-1. Recognition of Propagation Path Outages

High frequency radio continues to play a critical role in the communications architecture for the near- and far-term needs of the Army. The reliability of the HF nets has been plagued by outages directly related to changes of the characteristics and conditions of the ionosphere which are unpredictable. The number of outages can also be increased by action of enemy radio electronic combat (REC) units. When outages occur, it is first necessary to recognize that one has occurred and then change frequencies to one which is capable of supporting the propagation over the path desired. The AN/TRQ-35(V) meets this need by displaying those frequencies which presently are propagating and also by displaying a summary of the amount of time each channel has been used in either the last 5 or the last 30 minutes, whichever is desired.

B-2. Capabilities

The AN/TRQ-35(V) has the capability of improving the connectivity and reliability of HF communications. The employment of the sounder system enables the frequency manager to determine those frequencies which actually are propagating and the mode or modes by which the propagation is accomplished. The spectrum analyzer portion of the sounder system allows the operator to determine which frequencies are being used and how busy they are. With this information, the operator can ascertain whether or not the transmissions should be received and whether or not multipath interference should occur. If it is determined that communications are possible but are not being achieved, an operator or equipment problem is probably occurring. If the operator finds that propagation paths will not support communications with the frequency assigned, the operator can select frequencies that avoid multipath interference, are presently propagating over the desired link, and are unused or have minimal use. A phenomenon, known as sporadic-E ionization, occurs occasionally and sometimes provides for the propagation of frequencies which would not be possible under ordinary circumstances. It also has the effect of preventing propagation of some of the frequencies which were or should be propagating at that time. The duration of the sporadic-E ionization can be very short or for fairly long periods of time. The AN/TRQ-35(V) has the distinct advantage of being able to display the effects of the sporadic-E ionization as it is occurring and to allow the operator to use its advantages and recognize its detrimental effects to other propagation paths. Frequency managers using this device will be able to select frequencies that will assure consistent and effective communications.

B-3. Propagation

There are two principal paths by which radio waves can travel from a transmitter to a receiver. One is by ground waves, which travel near the ground from the transmitter to the receiver, and the other is by sky waves which travel up to the ionosphere and are refracted (bent downward) back to the Earth. In the HF radio band, 2 to 30 MHz, both modes of propagation are used. The distance over which the signal is able to propagate via ground wave is primarily dependent upon the conductivity and the dielectric constant of the Earth for a given radio system. The ability to propagate by ground waves is best over sea water and poorest over deserts and through jungles. The ability of the ionosphere to refract radio waves is dependable upon the height of the ionized layers in the Earth's upper atmosphere, the density of the electrons in the layers, and the frequency and incidence angle of the radio waves. The electron density and height of the layers vary with the time of day, seasons of the year, and sunspot activity. Present methods for determining usable frequencies use predictions based on historical interpretation which shows cyclic trends in ionospheric conditions. There are sudden ionospheric disturbances in addition to the large excursions in the undisturbed ionosphere. All of these phenomena lead to very conservative frequency selections in order to achieve high predicted reliability probabilities. The result is that many usable frequencies are neither specified nor used and many outages occur which could be predicted.

B-4. Characteristics


The T-1373/TRQ-35(V) transmitter transmits a CW signal which is swept linearly between 2 to 16 MHz or 2 to 30 MHz in 4 minutes and 40 seconds. The maximum power output is 100 watts (a selectable attenuation can reduce the power output to 10 W). If the diplexer is employed to allow the use of a common antenna with the communications transmitter, only 2 percent of the sounder transmitter power is coupled to the antenna and the remaining power is dissipated in an internal dummy load. The communications transmitter power output cannot exceed 2.5 kilowatts if the diplexer is to be used. Switches are provided in the transmitter to select a sweep ON or OFF at each 5-minute interval in the hour. Switches are also provided to enable blanking out of up to 16 frequencies in order to prevent interference to any critical links in either co-sited or adjacent receivers and to prevent interference to authorized frequencies such as distress and international time frequencies. A transmitter may be colocated with other communications equipment or may be completely detached as long as it has the required power source. Since its emission has a distinct electronic signature and the radiated output has a long time duration, it is an easily isolated and identifiable target. From the viewpoint of both physical and intelligence security, the transmitter should be detached from any major node as far as possible. The transmitter operates unattended after it has been initially programmed and time synchronized. The antenna used should be broadband and have a high take-off angle since most tactical HF communications use near-vertical incidence. The colocated transmitter may share the transmitter antenna of an HF communications transmitter but the sounder transmitter emitted power is then reduced to 2 percent of the available power. The sounder also is restricted by the characteristics of the communications transmitter antenna which may be narrowband and directional. This would identify the actual operating characteristics of the link involved but could affect the usefulness of the sounder system elsewhere in the operating area.


The R-2081/TRQ-35(V) receiver sweeps the HF spectrum in synchronism with the sounder transmitter. This allows the receiver to have an extremely narrow bandwidth which offers tremendous immunity to interference from other users. A cathode-ray tube (CRT) display is used to provide a visual representation of the strength of the received signal and the time delay due to the propagation path. The display of time delay can be interpreted to determine the mode by which propagation is achievable. Propagation by more than one mode (known as multipath) normally results in interference which is exhibited as fading. The display of only one mode of propagation is desirable for selection of the optimum usable frequencies. The receiver can store and display data from up to three separate transmitters. A sounder receiver may be colocated with the HF communications receiver or operated completely detached. It may share an antenna with the colocated receiver or use a separate antenna. The antenna should be wideband and nondirectional if more than one transmitter is being used. After time synchronization and initial programming have been completed, no operational requirements exist except for interpretation of the displayed data.

Spectrum Monitor.

The R-2093/TRQ-35(V) spectrum monitor is an HF receiver, processor, and display system which presents a visual representation of the occupancy statistics of 6 kHz wide channels through the HF band. The entire HF spectrum is scanned every 11 seconds and results are compiled and updated in 5-minute and 30-minute time blocks. A width of either 100 kHz or 500 kHz is displayed around a selected center frequency and the resulting "histogram" depicts occupancy of each 6 kHz channel. Selection is provided for any of four different received power thresholds spaced by 10 dB and the resulting analysis shows the frequency of occurrence of the crossing of the selected threshold. The operator can select by pushbutton control, and view, received signal amplitude expressed in dBm and percentage of use for each 9,333 channels scanned. The operator can monitor any selected center frequency using internal speaker or headphones--selecting USB, LSB, AM, or FM. The spectrum monitor may be located wherever a power source and an antenna are available. No synchronization is required, so that the operator only is required to turn the monitor on and interpret the display. Colocation of the sounder receiver and spectrum monitor provides the operator with the necessary information to enable selection of the optimum frequencies from the viewpoint of received signal strength, lack of multipath occurrence, and unused or minimally used frequencies.

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