Active Antennas for VLF and LF Applications

STAR-H is currently teamed with its electronics materials partners on the development of new Active Antenna Technology (AAT) for the reception of Very-Low Frequency (VLF) and Low Frequency (LF) signals. These signals range from 3 kHz to 300 kHz and are often used in underwater communication and geological surveys. Following is a brief summary of our approach and what sets our team apart from the existing technology. If you are an end-user in need of VLF or LF antennas or a fellow researcher working in this area, we would like to hear from you regarding your needs or experiences and how we can make our technology work for your specific applications.

AAT Antenna Diagram

Background of VLF and LF Antennas

The difficulty with typical VLF and LF antennas is that they are problematically large when using traditional construction techniques. Very large. Some full size VLF antennas are 10's of kilometers in length. On aircraft, Unmanned Aerial Vehicles (UAVs), or even something the size of an aircraft carrier, it is very difficult to install an antenna even a fraction of the size required for efficient transmission. Fortunately, for the bulk of applications, only signal reception, and not broadcast, is required, as the transmitting antenna are on land.

When only reception of VLF or LF signals is required, something called Active Antenna Technology (AAT) can be designed to work reasonably well. The development of AAT dates back over the last ~50 years. The idea is fairly simple: use a high input impedance amplifier tuned for the VLF and LF frequencies mounted right at the antenna terminals to allow reception of very weak signals using only the small aperture afforded by an electrically small dipole or monopole. This works because only a field strength above the amplifier’s noise floor is required. This technology is proven and has been demonstrated many times over the years.

Current Technology and Where STAR-H Comes In

We are confident that, with additional development, AAT can provide a fully-suitable solution for receiving VLF and LF communications, but existing systems are not yet fully-matured for the applications where they are most needed. Active antennas can work well in laboratory settings, but the technology still lacks the packaging, ruggedization, and small-scaling needed even for many towers and building structures, yet alone for extreme military and aerospace applications. Thus far, little effort has been dedicated to ruggedizing the technology for harsh military use or to developing and integrating very short receive antennas suitable for small aircraft and UAVs.

STAR-H, with its extensive experience in successfully designing and deploying electrically small UAV antennas, has begun addressing these issues. The challenges we see generally fall into five main categories:

  • Existing COTS active antennas and amplifiers tend not to be rugged enough for military application
  • Active antennas, while a tiny fraction of a wavelength, are still big compared to a UAV
  • Aircraft or UAV materials tend to detune the input impedence of active antenna amplifiers
  • Large metal or carbon fiber composite aircraft interfere with operation of active antennas
  • Amplifiers and delicate antenna structures are susceptible to high-power RFI and nuclear scintilation

At almost any frequency band, there exists a host of challenges inherently associated with the electrical and mechanical implementation of electrically-small antennas on aircraft and UAVs. All of these challenges, with which STAR-H is very much familiar, apply to active antennas:

  • Limited Placement Locations
  • Restricted Antenna Size and Geometries
  • Weight Restrictions
  • Aerodynamic Parasitic Drag
  • Loading of Carbon Fiber Materials

AAT LayeringIn addition, a number of factors dominate the effectiveness of AAT. For example, great care must be taken to ensure that the coaxial cable provides the only DC return path for the active preamplifier power, as parallel ground currents flowing from the short antenna have been shown to create severe intermodulation distortion effects, which can completely overwhelm the signal the receiver is attempting to detect. This problem is especially likely to manifest when the active antenna is placed next to a large, planar conductive or lossy surface, such as a metallic or carbon fiber UAV airframe. In working with its industry partners, STAR-H has developed high impedance magnetodielectric backings for its conformal applique antennas to mitigate this problem. Construction of an antenna radiator with this magnetodielectric backing is shown to the right.

Challenges can further arise from the FET input stage used in AAT. AAT uses a traditional FET input stage with high impedance input determined by an external input resistor, typically 1 Megaohm or greater. But the high input impedance leads to a high input noise level. Optimization of the input FET and the bias network is required to lower the noise contribution of this amplifier in low-signal level applications.

Bandwidth of AAT is governed by the input filter to the FET, the bandwidth of the FET input stage, and low-pass filters of subsequent active antenna stages. The bandwidth must be specifically tailored to the application to minimize the noise bandwidth contributed by the filter stage.

The Goal of Our Technology Research

STAR-H is fully cognizant of all the above challenges in designing and deploying AAT at LF and VLF and is already working with its partners to meet the active antenna technology challenges on several fronts. Our team plans to develop a complete VLF/LF system that is suitable for military application and can be integrated with both manned aircraft and UAV. The intent is to provide a complete system with a conformal antenna structure and hardened active antenna amplifier, all packaged into a non-intrusive conformal package enhanced with flexible magnetodielectric material for application to metallic and carbon fiber airframes. The system may also include SDR based pre-receiver electronics for noise rejection based on a system calibration once installed on the aircraft.