TECHNOLOGY AREA(S): Air Platform, Electronics, Ground/Sea Vehicles

ACQUISITION PROGRAM: NAE Chief Technology Office

The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with section 3.5 of the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.

OBJECTIVE: Develop an antenna/sensor package that provides high frequency/very high frequency (HF/VHF) detection and direction finding (DF) capabilities in a 7-inch diameter, flight vehicle cavity.

DESCRIPTION: Achieving high-bandwidth antennas in HF/VHF for transmitters and receivers is a difficult radio frequency (RF) design. References 3 and 4 illustrate that as antennas become significantly smaller relative to the wavelength of the signal, the instantaneous bandwidth of the antenna sharply decreases. Traditional antennas are often at least a quarter of the wavelength of the intended signal, and in HF/VHF applications, this forces antenna to be > 1 meter in size. Therefore, traditional design approaches for high-gain and high-bandwidth antennas onboard tactical and small-unmanned aircraft are not suitable due to the antenna�s physical size.

Specifically, as antennas are miniaturized relative to the signal wavelength, their impedance bandwidth sharply decreases. For transmitters, the antenna rejects and reflects high-bandwidth signals because any frequency outside of its impedance bandwidth is mismatched with the antenna, preventing efficient signal acceptance in the antenna. For receivers, the electrical size of the antenna is so small compared to wavelength, that the gain of the antenna is small, reducing signal to noise ratio (SNR) and sensitivity of the receiver. This is fundamentally due to the conductive and material losses overwhelming the radiation power of the receiver. This prevents the signal from being distinguishable above the noise floor.

References 1 and 2 illustrate a method for overcoming bandwidth-limited electrically small antenna utilizing a transistor switch that directly modulates the signal in order to �time-vary� the impedance boundary conditions of the antenna. If synchronized well, the signal at the input of the antenna is matched exactly at the same moment the impedance boundary of the antenna, due to the transistor, is changed. Yet, both of these references 1 and 2 are methods for electrically small transmitters, and not for electrically small receivers.

For receivers, achieving high-gain, high-bandwidth antennas are difficult as stated above. References 5 and 6 propose a method for using cryogenic systems that significantly reduce the antenna temperature so as the incoming SNR of the signals have significantly lower noise figure at the input of the RF front end. Still, such proposals require additional physical volume to house said-cryogenic systems, significantly increasing the physical area needed. Specifically, this topic seeks a HF/VHF antenna/sensor package capable of direction finding (DF).

Traditionally, high-gain sensor packages are comprised of arrays capable of electronic scanning. The physical size of the package directly increases with demands for higher gain. In rapidly evolving aerodynamic environments, physically large antennas are not practical for tactical aircrafts, unmanned vehicles, and weapons applications.

The proposed antenna sensor system must handle up to 10 Watts, physically sized in all three physical dimensions less than a tenth of the wavelength. The ratio of the radiated power to the total power (i.e., the sum of the radiated power, power lost to ohmic losses, and power lost to material losses) must be as high as possible but greater than 50% or must achieve an antenna gain of at least -6 dBi. The antenna radiation pattern should have a beam width of 3-5 degrees, but an omnidirectional pattern along a vertical axis is acceptable. Clearly state the necessary electronics to achieve direction finding. A 360-degree scan within 2 seconds or a 10 ms dwell time per beam (if antenna is directive) is desired.

An innovative approach to achieving these results would include:
1) Significantly reduce material losses and conduction losses so as the antenna radiation efficiency is almost 100% (0 dB).
2) Reduce the noise figure and antenna temperature so as the SNR of the signal at the input of the receiver RF front end is at least 6 dB.
3) Provide information (i.e., direction finding) on where the signal came from while handling up to 10W of power within an angular resolution of 3-5 degrees.

PHASE I: Design and determine the best low-frequency sensing approaches that are packable into a physically 7-inch diameter volume and used to sense HF/VHF signals, and provide direction-finding capability. The Phase I effort will include prototype plans to be developed under Phase II.

PHASE II: Develop and prototype a solution that can be �flown� in an anechoic RF chamber setting whereas HF/VHF performance can be characterized within proposed electrically small (length, width, height less than tenth wavelength) of volume. Identify and propose solutions to areas that will be difficult to transition to high speed flight.

PHASE III DUAL USE APPLICATIONS: Finalize design and perform testing to ensure HF/VHF performance in a flight operational manner where RF performance from chamber setting is maintained in-flight. Transition final solution to appropriate platforms and end users. Successful technology development would benefit space communications, general aviation, wireless infrastructure, and the internet of things (IoT).

REFERENCES:

1. Yao, W. and Wang, Y.E. "Direct antenna modulation - a promise for ultra-wideband (UWB) transmitting." Microwave Symposium, Dig. 2004 IEEE MTT-S Intl, vol. 2, pp. 1273-1276. https://doi.org/10.1109/MWSYM.2004.1339221

2. Santos, J.P., Fereidoony, F., Huang, Y., and Wang, Y.E. "High Bandwidth Electrically Small Antennas through BFSK Direct Antenna Modulation." Military Communications Conference, MILCOM, 2018.� DOI: 10.1109/MILCOM.2018.8599778

3. Chu, L.J. "Physical limitations of omni-directional antennas." J. Applied Physics, vol. 19, no. 12, 1948, pp. 1163-1175.� https://doi.org/10.1063/1.1715038

4. Hansen, R.C. "Fundamental Limitations in Antennas." Proceedings of the IEEE, vol. 69, no.2, 1981, pp. 170-182.� DOI: 10.1109/PROC.1981.11950

5. Clarke, J. "Principles and Applications of SQUIDs." Proceedings of the IEE, vol. 77, no. 8, August 1989, pp. 1208-1223. https://ieeexplore.ieee.org/document/34120

6. Kornev, V.K., et. al. "Linear Bi-SQUID Arrays for Electrically Small Antennas." IEEE Transactions on Applied Superconductivity, vol. 21, no. 3, June 2011, pp. 713-716. https://ieeexplore.ieee.org/document/5672560

KEYWORDS: Electrically Small Receiver; HF/VHF Antenna; Direction Finding; DF; Antenna; Low Profile