Inductive devices for organizing hidden radio communications.

Inductive devices for organizing covert radio communications..

Ovchinnikov Andrey Mikhailovich,
Archagov Oleg Igorevich

INDUCTIVE DEVICES FOR ORGANIZING COVERT RADIO COMMUNICATION

In the practice of security services and various security structures, it is often necessary to use radio communications in an unnoticeable manner for others (ensuring visual and acoustic secrecy of communication). This is achieved by using small-sized radio stations, covert placement of radio stations and microphones on the operator's body, and the use of miniature remote control panels for the radio station.

The main problem in this case is to ensure that the information is received unnoticed by others, since the use of headphones in such situations is unacceptable. The solution to the problem is to use complexes of inductive communication devices consisting of a transmitting antenna and miniature wireless receiver-phones.

The transmitting antenna (inductor coil) is connected to the output of the low-frequency amplifier (LFA) of any sound-reproducing equipment, in this case a radio station, and creates an alternating electromagnetic field of audio frequency around itself. An EMF of audio frequency is induced in the antenna coil of the receiver, which is amplified by the LFA of the receiver and converted into sound vibrations using a miniature electromagnetic telephone.

The transmitting antenna (inductor) can be placed in a small rectangular case, attached to the inside of the jacket or made in the form of a multi-turn loop antenna and placed around the neck. There are designs of inductor in the form of a shoulder strap, which is placed under a shirt or jacket. The receiver-phone is placed in the operator's auricle so that the sound-reproducing phone enters the external auditory canal of the ear. In addition to solving the problem of hidden reception of information from the surrounding, such placement of the receiver-phone improves the quality of speech signal reproduction in conditions of increased acoustic noise, since the acoustic output of the phone is as close as possible to the eardrum, and the body of the product is a sound insulator (soundproofing properties are especially clearly manifested in the frequency range from 1000 to 4000 Hz). At the same time, placing the receiver in the ear has almost no effect on the intelligibility of the speech information received from others.

As a rule, receiver-phones are powered by miniature power elements with a voltage of 1.5 V. There are a large number of companies engaged in the production of equipment of this class, but the following models of such products are best known on the domestic market:

— a set of equipment consisting of a WR4 receiver and a CA19 inductor from SONIC Communications (USA),

— a similar set manufactured by Dynatech Tactical Communication — DTC (Canada): inductor — Z8, Z8L: receivers — R5. R8:

— Phonak Inductor from PHONAK Communication AG (Switzerland):

— domestically produced PA-1 acoustic receiver:

— domestically produced AMF-U-S receiver-telephone.

Comparative characteristics of receiver-telephones, compiled on the basis of advertising materials and passport data for the products, are presented in the table.It should be noted that the receivers from SONIC and DTC have almost identical characteristics and therefore they are listed in one column in the table.

Note: For SONIC and DTC equipment, there is no information on the current consumption and capacity of the battery used, and the cost is indicated for the delivery option with a hidden headset.

As can be seen from the table, the best acoustic properties (higher sound pressure level and wider range of operating frequencies) are possessed by the phones of foreign companies SONIC, DTC and PHONAK. At the same time, the difference in these technical characteristics in comparison with two domestic products is so insignificant that it can be noticed only during special articulation tests, and is practically not felt by the average user. As a rule, all receiver-phones are designed to ensure the intelligibility of reproduced speech information corresponding to the class of GOST 16600-72 in conditions of increased external acoustic noise (in technical characteristics, foreign companies indicate not average, but the best values).

In preparing this article, we conducted comparative tests of the above products. All receiver-telephones were tested using a standard unified method by measuring the sound pressure level developed by the product on an artificial ear device of type 415: with a DB 0138 communication chamber from Bruel & Kjaer. The receiver under test, powered by a standard power source, was connected to the artificial ear with a communication chamber. The signal from a low-frequency generator of type GZ-117 was fed to a transmitting loop antenna located above the product at a distance of 150 mm. The signal level for different receivers was selected based on the conditions of ensuring a fixed (no more than 20%) coefficient of nonlinear distortion in the operating frequency range.

The tests showed that the speech information intelligibility of all products corresponds to class 1 of GOST 16600-72 with minor differences in acoustic parameters. The values ​​of the maximum sound pressure level and nonlinear distortion coefficient obtained during the tests are indicated in the corresponding columns of the table in brackets. As can be seen from the table, the best parameters here are possessed by domestic products. Despite the fact that no unambiguous conclusion can be made based on this sample (one sample of each type of receiver-phones was tested), the results provided definitely provide additional information when choosing specific technical means.

As a rule, the user is interested in such characteristics of the equipment as the operating time of the receiver from one power source, the degree of concealment of placement, the stability of the sound signal, the convenience of wearing the antenna and the receiver itself, the reliability of the product, the completeness of the delivery (the possibility of delivery by the company together with the product of other accessories for organizing hidden radio communication), as well as its cost. In terms of the operating time of the power source, domestic samples are superior to foreign products (at least 100 hours for PA-1 and at least 120 hours for AMF-U-S with 20 hours for Phonak Inductor and 72 hours for the SONIC and DTC receiver). At the same time, in PA-1 this is ensured by the use of a high-capacity air-zinc battery, and in the AMF-U-S product — an economical operating mode (the current consumption is 6 times less than in PA-1, and 4 times less than in Phonak Inductor).The degree of concealment of placement is determined by the dimensions and design of the receiver-phones, as well as their color scheme. The dimensions of receivers installed in the auricle are similar for different companies, but the smallest dimensions of the specified products are those of AMF-U-S. It should also be noted that the design of the WR4 receivers from SONIC and AMF-U-S is made with maximum (in comparison with other products) consideration of the anatomical features of the external auditory canal of the human ear, therefore, with the same dimensions, these receivers are installed deeper and are less noticeable.

The difference in color design is that the SONIC, DTC, Phonak Inductor and PA-1 receivers are flesh-colored and match the color of the human ear, while the AMF-U-S product is darker, its color is selected in such a way that it matches the color of the ear canal. Therefore, the AMF-U-S receiver, installed in the auricle, looks more natural.

In general, we can talk about a greater degree of concealment of the placement of the AMF-U-S receiver-telephone compared to other products. An important requirement for the inductive communication device is the stability of the receiver's audio signal during movement and other active actions of the operator. Stability is characterized by the dependence of the sound level on the position of the operator's head and is determined mainly by the type of the transmitting antenna used. The least dependence is manifested when using a loop antenna, which forms a uniformly distributed electromagnetic field of audio frequencies. the greatest — with an inductor placed in a rectangular housing and having a narrow directional pattern. When using an antenna in the form of a shoulder strap and placing the receiver in the ear on the same side as the shoulder strap, stability of the audio signal is achieved, close to a loop antenna.In the PA-1 product, the transmitting antenna (inductor) is placed in a small rectangular housing, which is usually attached to the back of the operator's jacket or coat. With this design of the antenna and its placement, the volume of the acoustic signal depends significantly on the position of the operator's head. In the AMF-U-S product, the transmitting antenna is made in the form of a loop and is placed on the operator's neck, which allows to minimize the change in the volume of the signal reproduced by the receiver-telephone when the position of the head changes.

Products of foreign companies (DTC, SONIC and PHONAC) are supplied complete with any type of antenna. Convenience of wearing is mainly determined by the reliability of the product fastening (the absence of the receiver falling out when the head position changes) and the comfort of the person's condition with the receiver-phone placed in his auricle and the antenna on the neck or the inside of the jacket. Asymmetrical designs of receivers of SONIC and DTC and the AMF-U-S product, as close as possible to the shape of the external auditory canal of the human ear, in this case provide maximum comfort when wearing compared to other receiver-phones.

In terms of the design of the inductors, foreign companies have a certain advantage. The design of the transmitting antennas provides greater convenience for the operator, for example, the inductor for products by DTC and SONIC is supplied together with a special case for placing it on the operator's side (fastened to the shoulder under clothing). Unfortunately, there is no precise data on the reliability of products by SONIC, DTC and PHONAK, as well as PA-1 (the mean time between failures of the AMF-U-S product is 40,000 hours), which does not allow comparing products by this criterion. Advertising materials of foreign companies indicate that their products comply with military standards.

In terms of the comprehensiveness of the supply, products of foreign companies stand out favorably, offering a wide range of accessories for visually hidden radio communication (covertly portable radio receivers, remote control panels, microphones, accessories for wearing, etc.), working in combination with an inductive communication receiver-telephone. In addition, it is possible to match the receiver with radio stations that have different output resistance (8, 15, 30, 60, 300, 600 0m, 2 kOhm) by selecting the appropriate modification of the inductor.

The PA-1 product is available with only one modification of the output resistance (8 0m), the AMF-U-S equipment kit is supplied in 3 modifications (12, 50, 200 and 1000 0m). The receivers of the Phonak Inductor, PA-1 and AMF-U-S products are supplied in a package that also contains a mirror and a set of devices for installing and removing the receiver from the auricle. Products from DTC and SONIC are supplied without a set of devices.

Products of foreign companies have a significantly higher cost compared to domestic sets of equipment. At the same time, the high cost of American and Canadian products is explained by the declared high reliability, wide range of operating temperatures and compliance with military standards. However, only a very wealthy client can afford to purchase accessories for covert radio communication at a price of $ 800 — 900 (although the price includes a completely covert headset with a microphone and a radio control panel), 2-3 times higher than the average cost of imported analog UKB radio stations. At the same time, the advantage in technical and operational characteristics compared to other products seems ambiguous. A set of inductive communication devices from PHONAK has a slightly lower cost.

The total price of the transmitting antenna, connecting cable and receiver-telephone, at which the set is sold by the company itself, is about $ 400. However, supplier companies in Russia offer this product at a price of $ 500 to 600. In terms of technical characteristics, Phonak Inductor is not inferior to products from DTC and SONIC. The company has the ability to supply a wide range of technical means for covert radio communication, working together with an inductive communication device. Domestic products can be purchased at lower prices: $ 100 — 200 for PA-1 and $ 200-250 for AMF-U-S. (The range of prices for PA-1 is explained by the fact that the product is sold both by manufacturers (minimum price) and by intermediary companies.)

At the same time, in a number of important operational characteristics, the AMF-U-S set is superior to the PA-1 product and is quite reliable. It should also be noted that the operating costs associated with the purchase of replacement batteries are significantly lower than others for the AMF-U-S. In general, the analysis shows the presence of a wide selection of equipment in the class of inductive communication devices designed to organize covert radio communications. Today, the user can purchase not only expensive foreign equipment, giving preference to it due to the comprehensiveness of the supply, better design, compliance with US military standards and the presence of various accessories for wearing. But also cheaper equipment of domestic production, in no way inferior to imported in technical characteristics, while saving money on the purchase of other equipment for organizing covert radio communications.