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Omega-to-VLF Conversion

The Problem
When the worldwide OMEGA navigation system was shut down, the Royal Australian Navy (RAN) obtained a decommissioned OMEGA transmitting facility located in Sale, Australia. The RAN wanted to convert the station for use as a submarine broadcast facility; however, the system architecture to support the OMEGA broadcast is significantly different from what is required to support constant envelope Minimum Shift Keying (MSK) used for VLF communication. Thus, they needed to change its operating frequency from 13.0 kHz to 18.6 kHz, and upgrade the equipment to support the operation of a two channel MSK communications broadcast site. Hi-Q engineers were asked to provide engineering services and installation assistance for the frequency change effort.

Hi-Q's Solution
Hi-Q engineers utilized the network analyzer based measurement system to characterize the antenna tuning and matching components, performed a field strength survey to determine the antenna effective height and radiation resistance, and designed a new architecture for the transmitting system to support the MSK broadcast requirements.

Major modifications to the equipment included the following:

  • Adding a phase shift network by converting some of the existing antenna tuning variometers to Tee-network coils, and adding a capacitor bank
  • Eliminating the helix house matching transformer and converting another antenna tuning variometer to an adjustable coupling coil
  • Reducing the inductance of the remaining antenna tuning variometers
  • Modifying the main helix by tapping it for use with fewer turns
  • Significant changes to the helix house buswork to support the new architecture
  • Addition of digital antenna current metering and an automatic antenna tuner

After the conversion NTS Sale was fully operational at 18.6 kHz, providing 100 baud MSK transmission (2-channel MSK) at its full transmitter output power of 150 kW, radiating 36.5 kW.


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