Commtel Wireless - measured coupling between communication antennas mounted on aircraft scale model

By: Commtel Wireless  09-12-2011
Keywords: Aircraft, Scale Model, Antenna System

Antenna System Evaluation

Antennas Mounted on Large and Complex Structures

Aim

Ordinarily, an evaluation of the coupling between V/UHF and HF communication antenna performance on the aircraft is required. Indication of the coupling between the communication antennas at the respective communication frequencies is determined from measurements on a suitable scale model of the aircraft. Normally, all antennas at VHP and higher frequencies are approximated as simple monopoles whereas, for example, an HF loop antenna would be represented by wires of the correct scaled lengths. It is often desirable to also include in su~.i investigations coupling between the communication antennas and some navigation and other antennas on the aircraft at the communication antenna frequencies. The navigation and other antennas are normally the VHP Homer, VOR/LOC, DME, IFF, TACAN and Marker Beacon antennas. The main aim of such coupling study is to obtain an indication of areas where antenna coupling may result in electromagnetic compatibility (EMC) problems.

Approach

The approach to the coupling evaluation between the communications antennas is outlined as follows:

  • A suitable scale model is constructed and all antennas of interest are incorporated.
  • The coupling between all antennas which exhibited coupling values above the measurement system noise floor is measured using scale frequencies corresponding to the actual frequency range 2-420 MHz.
  • The cable losses associated with the cables leading to the antennas are removed mathematically from the measured results.
  • The matching losses presented by the monopoles representing the V/UHF antennas are removed in the band (30-420 MHz) when the measured couplings between these antennas are above the measurement system sensitivity. The removal of mismatch losses is necessary because broadband antennas, rather than the narrowband simple monopoles (used during measurement), are used for actual communications.
  • The mismatch losses associated with an HF antenna above 30 MHz and the V/UHF antennas below 30 MHz are retained in the coupling results. This is realistic, since similar mismatch losses between actual antennas exist. The actual mismatch losses for the antennas mounted on the aircraft may be different than the mismatch losses presented by the same length monopoles used during these measurements to approximate those antennas. The measurements does, however, provide an indication of the coupling values to be expected. The actual mismatch losses for the antennas mounted on the aircraft can, however, be established by measurements using the actual antenna of interest when mounted either on the aircraft or on the variable fuselage. We are also often required to assess the radiation efficiency and patterns of antennas, using a measurement set-up similar to that shown as follows:

Results

An example of measured coupling (after the removal of cable losses and the appropriate removal of mismatch losses) between the Top-Fin V/UHF antenna and the VHF homer antenna (after removal of mismatch loss) on the C130 aircraft is presented as follows:

Keywords: Aircraft, Antenna System, Loop Antenna, Scale Model, System Evaluation,

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