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 B.  The DME antenna provides the means of radiating the RF energy from the DME interrogator's transmitter, and receiving RF reply signals transmitted by the ground station.
 C.  The DME antenna is designed for L-band use. It has a nominal RF impedance of 50 ohms and a maximum VSWR of 2/1 over the frequency range.
 4.  Interrogator Units
__________________
 A.  The interrogator units are standard 1/2 ATR short assemblies which mount on the E2-2 shelf in the electrical/electronics equipment center (Fig. 1).
 B.  DME-1 interrogator receives 115v ac, 400 Hz, power thru DME-1 circuit breaker located on the P18 circuit breaker panel. DME-2 interrogator receives its power thru DME-2 circuit breaker located on the P6 circuit breaker panel.
 C.  Interrogator units provide the capability to transmit interrogation pulse pairs to a ground station, receive and process the returned reply pulses, which provide the slant range distance to the RDDMI's, the EHSI and other airplane systems.
 D.  The slant range capability of the interrogator is from 0 to 389.99 nautical miles.
 5.  DME-1 and DME-2 Indicators
__________________________
 A.  Dual DME indicators are installed in each RDDMI on the captains and first officers instrument panels (Fig. 1).
 B.  The DME indicator is a magnetic wheel type counter which receives a BCD pulse signal from the related DME system. Logic in the indicator decodes this signal and converts it into a digital display showing miles-to-go to the interrogated ground station.
 C.  Warning flags (shutters) cover the distance display when the system is inoperative.
 D.  The RDDMI DME-1 display indicates distance information provided by DME No. 1 transponder. The RDDMI DME-2 display indicates distance information provided by DME No. 2 transponder.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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737-300/400/500MAINTENANCE MANUAL
 E.  The captain's RDDMI DME indications are provided 26-volt ac power from instrument transformer 1 which receives 115-volt ac power through INSTR XFMR 1 circuit breaker from the Standby Bus. The first officer's RDDMI DME Indications are provided 26-volt ac power from instrument transformer 2 which receives 115-volt ac power through INSTR XFMR 2 circuit breaker from Standby Bus No. 2.
 6.  Electronic Horizontal Situation Indicator_________________________________________
 A.  Each of the captain's and first officer's instrument panels has an electronic horizontal situation indicator (EHSI). The EHSI is a multifunction display that gets inputs from and gives displays for several electronic systems on the airplane. Additional description, operation, and maintenance instructions for the EHSI are given in 34-22-00, EFIS.
 B.  The EHSI shows the flight data horizontally. It shows the standard type HSI information and other parameters. It is also used for the WXR output in the MAP, EXP-VOR, and EXP-ILS modes.
 C.  The indicators show the DME slant-range distance to the correct VHF navigation station. The distance is shown when the electronic flight instrument system operates in the VOR or ILS mode. Distance is displayed when the control panel is in the manual mode of operation and the electronic flight instrument system is operating in the VOR or ILS mode.
 7.  Operation_________
 A.  DME operation consists of an operational description and a functional description. The operational description briefly covers the DME system. The functional description provides in-depth coverage of system components and their interface.
 B.  Operational Description
 (1)  
The DME system automatically measures the distance (in nautical miles) between the airplane and a selected ground station. The DME frequency is selected when the corresponding VHF NAV frequency is selected (Fig. 3). Manually, this is accomplished using the VHF NAV control panel. When AUTO is selected on the control panel, tuning of the VHF NAV and DME systems is accomplished by the flight management computer.

 (2)  
When the DME is in an operating mode, a pulsed signal in the 1025-1150 MHz band is transmitted. The DME ground station receives the interrogation and transmits a reply to the interrogator. The reply frequency is either 63 MHz above or below the interrogation frequency. The interrogator receives the reply, measures the time delay between the interrogation and reply pulses and converts the delay time to the equivalent DME distance.
　
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