![]() | For additional information, refer to the application note 1GP74: Test of DME/TACAN Transponders, available at the download area of the Rohde & Schwarz internet site. |


High output power of the DME transponder (up to 1 kW) might destroy the power sensor.
Therefore, the attenuator at the power sensor input must be selected in such a way, that the power applied to the power sensor is in the allowed range of the R&S NRP-Z81 before connecting the power sensor to the coupler.
The R&S SMA generates interrogation pulse pairs and starts an internal counter for the time measurement.
The DME transponder receives an interrogation pulse pair, and replies after a 50 µs to 56 µs delay with a reply pulse pair, depending on the channel.
The transmitted power is coupled to the power sensor input.
The R&S NRP-Z81 receives the transmitted reply pulse pair, and triggers the R&S SMA to stop the time measurement. It also measures the peak pulse power and transmits the readings to the R&S SMA.
The R&S SMA displays the peak pulse power of the reflected reply pulse pair, and the reply delay of the DME transponder (ideally 50 µs or 56 µs). In addition, it shows the DME transponder efficiency, i.e. the number of response pulse pairs as a percentage of the interrogation pulse pairs.
The reply delay value, caused by the cable between the coupler and the antenna, must be corrected by adding twice the signal delay. This correction compensates both, the delay of the received signal and the delay of the transmitted signal. The following equation provides a typical value for the twofold signal delay due to cable length:
l = cable length in metersεr = relative dielectric (e.g. 2.1 for teflon, 2.25 for polyethylene)c = 3 x 108 m/s
Optional normalization of the test setup removes additional delays that are due to the test setup.

The R&S SMA generates a test pulse for normalization and starts the counter for time measurement.
The dual directional coupler transmits the test pulse to the R&S NRP-Z81 power sensor. The connections to the DME transponder and the antenna are interrupted.
The R&S NRP-Z81 receives the test pulse, and triggers the R&S SMA to stop the time measurement.
The R&S SMA evaluates the test setup delay and considers the value in a subsequent DME analysis measurement. The R&S SMA stores the correction value on the internal flash disk, to provide it also after a restart of the signal generator.
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