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


E-1. Use and Application

The present capability to counter the jamming threat is limited to operator techniques including manual frequency switching, use of terrain features to shield from jammer signals, and the employment of a limited number of directional antennas. These capabilities to counter the EW threat are inadequate to assure essential communications when intense jamming is encountered. Hence, the Steerable Null Processing Group OL-275()/VRC-- commonly called the SNAP-1--is needed to provide an ECCM capability for the AN/VRC-12 family of radios. The SNAP-1 is employed on essential command and control and fire support single-channel radio nets from platoon to division echelon in forward areas. Basic application includes the division command/operations net, brigade/group command/operations net, as well as those FM radios used by separate brigades in the command/operations nets. The SNAP-1 may be used with the SINCGARS equipment in the single-channel mode. A similar device, the SNAP-2, is being developed to be used with the SINCGARS equipment in the frequency-hopping mode.

The OL-275()/VRC consists of a Steerable Null Antenna Processor CP-1380/VRC (hence the name SNAP), Interconnecting Box J-3792/VRC, and Special Purpose Electrical Cable Assembly CX-13179/U (fig E-1). The SNAP-1 combines signals received by two standard VHF-FM antennas to create a radio reception pattern containing a null. The SNAP-1 installation kit includes the additional antenna. When employed as a part of a radio system (for example, with vehicular mount, radio, antennas, processing unit), the SNAP-1 becomes the antenna control system for the radio and must, therefore, interface and operate with the radio. Additionally, the SNAP-1 --

  • Effectively eliminates an unwanted signal from the radio receiver automatically without interfering with the desired signal arriving from a different direction. A 35-dB reduction in the unwanted signal power is accomplished under most operating conditions. The SNAP-1 is capable of operating efficiently when the operator has no prior knowledge of the direction of either the unwanted or the desired signal.

  • Is capable of operation on the move and while stationary. It is capable of effectively eliminating signals automatically from a stationary or moving jammer.

  • Operates with current vehicular mounted VHF-FM radios and COMSEC equipment without requiring modification to the radios or the COMSEC equipment. The SNAP-1 permits normal cipher and plain text voice and data communications over the associated radio and COMSEC equipment.

  • Is capable of passing the desired signal to the receiver from other VHF-FM transmitters whether or not the sending transmitter is equipped with SNAP-1.

  • Has a bypass or override feature for use in a jam-free environment or when a failure occurs in the SNAP-1.

  • Uses standard Army VHF-FM antennas (for example, the AS-1729 or the AS-2731).

Figure E-1. Components of OL-275()/VRC.

E-2. Features

Modes of Operation.

Automatic. Continuous closed-loop tracking of jammer. This enables it to be useful against a moving jammer, or with a moving receiver.

Manual. Permits cancellation of a jammer only slightly stronger than the desired signal. Coarse and fine adjustments are provided to eliminate co-channel interference and unintentional interference by a strong friendly signal.

Bypass. Permits bypassing SNAP applique while retaining channel light function.

Frequency Selector.

Permits direct setting of channel frequency.

Sum-Normal-Null Switch.

Permits manual selection of pattern nulling (jammed condition) or peaking (unjammed condition). The NORM position provides automatic selection based on the 150-Hz squelch tone on desired signals.

Channel Light.

Indicates presence of signal power in the channel, thus providing an indication of a jamming or interfering signal.


Provides an indication of the null depth by monitoring energy of the in-phase component of the canceled signal.

Mounting Case.

The SNAP-1 is contained in a case similar to an R-442 case so as to make use of the existing vehicular mount (MT-1898).

E-3. Associated Equipment

The SNAP-1 operates with the following equipment:

  • Two AS-1729 or two AS-2731 antennas.

  • Interconnecting Box J-3792/VRC. (Included in Group.)

  • Two antenna cables CG-1773.

  • Two control cables CX-4722.

  • Mount MT-1898.

  • Special Purpose Electrical Cable Assembly CX-13179/U. (Included in Group.)

  • COMSEC equipment, TSEC/KY-8, TSEC/KY-38, or TSEC/KY-57.

  • AN/VRC-12 family of radios and AN/GRC-160 and AN/VRC-164.

Figure E-2. Installation of the OL-275()/VRC.

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