Technical Reports

PTC 220 MHz Radio: BNSF Locomotive GE Dash-9 Locomotive Final Intermodulation Test Report

  • 23
  • Jun
  • 2011
AUTHOR: Meteorcomm LLC
SUBJECT: Positive Train Control, Signal and Train Control, Grants & Financial Assistance
KEYWORDS: Railraod Safety Technology Grants
ABSTRACT: During several testing sessions from May 12 to June 3, 2011, information was gathered on potential sources of intermodulation from on-board and external radio transmitters. No strong evidence of non-locomotive external intermodulation was detected, but it cannot be ruled out at this time. One of the reasons is that very strong local signals from other on-board radios were present and measured on board the locomotive, making it difficult to evaluate external signals that could have been present at a much lower power level. The extremely high signal levels appear to be the result of very close spacing of twenty separate antennas on the locomotive roof, installed within a flat area 57 inches wide by 54 inches front to back. There is very low isolation between the twenty antennas on the locomotive roof, resulting in signals transmitted from any radio reaching each other radio receiver at very high power levels. When high power signals enter a radio front end, they find a non-linearity and beat into intermodulation products, which go back through the antenna and are transmitted again to each other radio in the locomotive. This condition generates a “forest of signals” across the spectrum, affecting the entire 220 MHz band, as well as most of the other bands from 50 MHz to 1,000 MHz. When signals from a transmitting radio reach at high power levels other radios that are also transmitting at the same time, more severe intermodulation products are produced and, in addition, transmitted back through the antenna to other radios. A new testing system and protocol are being developed to measure more precisely the exact amounts of power that arrive at each radio receiver or transmitter when one radio transmits, and how much of that power is reflected back and on what frequencies through the antenna of each radio. This will provide the requisite data to reliably determine the levels of internally generated intermodulation.
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