Page:Electronics Technician - Volume 7 - Antennas and Wave Propagation - NAVEDTRA 14092.pdf/44

 Figure 2-38.—Matching network.

the tuner, assembling the connections with the antenna and transmission line, and pressurizing the tuner, if necessary. Access must be provided to the pressure gauge and pressurizing and purging connections.

ANTENNA TUNING

For every frequency in the frequency spectrum, there is an antenna that is perfect for radiating at that frequency. By that we mean that all of the power being transmitted from the transmitter to the antenna will be radiated into space. Unfortunately, this is the ideal and not the rule. Normally, some power is lost between the transmitter and the antenna. This power loss is the result of the antenna not having the perfect dimensions and size to radiate perfectly all of the power delivered to it from the transmitter. Naturally, it would be unrealistic to carry a separate antenna for every frequency that a communications center is capable of radiating; a ship would have to have millions of antennas on board, and that would be impossible.

To overcome this problem, we use ANTENNA TUNING to lengthen and shorten antennas electrically to better match the frequency on which we want to transmit. The rf tuner is connected electrically to the antenna and is used to adjust the apparent physical length of the antenna by electrical means. This simply means that the antenna does not physically change length; instead, it is adapted electrically to the output frequency of the transmitter and "appears" to change its physical length. Antenna tuning is done by using antenna couplers, tuners, and multicouplers.

Antenna couplers and tuners are used to match a single transmitter or receiver to one antenna; whereas antenna multicouplers are used to match more than one transmitter or receiver to % antenna for simultaneous operation. Some of the many antenna couplers that are in present use are addressed in the following paragraphs. For specific information on a particular coupler, refer to the appropriate equipment technical manual.

Antenna Coupler Group AN/URA-38

Antenna Coupler Group AN/URA-38 is an automatic antenna tuning system intended primarily for use with the AN/URT-23(V) operating in the high-frequency range. The equipment also includes provisions for manual and semiautomatic tuning, making the system readily adaptable for use with other radio transmitters. The manual tuning feature is useful when a failure occurs in the automatic tuning circuitry. Tuning can also be done without the use of rf power (silent tuning). This method is useful in installations where radio silence must be maintained except for brief transmission periods.

The antenna coupler matches the impedance of a 15-, 25-, 28-, or 35-foot whip antenna to a 50-ohm transmission line, at any frequency in the 2- to 30-MHz range. When the coupler is used with the AN/URT-23(V), control signals from the associated antenna coupler control unit automatically tune the coupler's matching network in less than 5 seconds. During manual and silent operation, the operator uses the controls mounted on the antenna coupler control unit to tune the antenna. A low-power (less than 250 watts) cw signal is required for tuning. Once tuned, the CU 938A/URA-38 is capable of handling 1000 watts PEP.

Antenna Coupler Groups AN/SRA-56, -57, and -58

Antenna coupler groups AN/SRA-56, -57, and -58 are designed primarily for shipboard use. Each 2-22