Frederick H. Raab is well-known and respected for his expertise in the development of high-efficiency power amplifiers and transmitters. He has also been professionally involved with general RF design, antennas, matching networks, communication systems, and signal processing. Graduating from Iowa State University, he received his B.S.(1968), M.S.(1970), and Ph.D. (1972) degrees in electrical engineering. He received the I.S.U. Professional Achievement Citation in Engineering in 1995 and was named an IEEE fellow in 2006. The textbook Solid State Radio Engineering, coauthored by Dr. Raab, is widely used by both academics and practicing engineers. Other professional achievements include publication of over 100 technical papers and award of twelve patents. Professional leadership includes serving as technical program chairman for RF Expo East '90 and founding technical committee MTT-17 that expands the IEEE MTT Society to include HF/VHF/UHF engineers.

Green Mountain Radio Research (GMRR) is owned by Dr. Raab. Since 1980, GMRR has provided his technical expertise to numerous clients in both government and business.

Dr. Raab's ability to translate theory into practice is illustrated by over 100 publications in both IEEE and trade journals. These publications also illustrate the ability to work in a variety of different disciplines, including power amplifiers, transmitters, receivers, signal processing, antennas, and propagation.

Under this SBIR Phase-II contract, GMRR developed a prototype high-efficiency transmitter for L-band radar. This application requires generation of FM-CW pulses at 1.2 GHz and delivery of a variable-amplitude signal into an antenna element with a variable impedance. The transmitter is based upon a combination of a high-efficiency RF-power amplifier, power combiners, high-efficiency class-S modulators, electronic tuning, and digital signal processing. The PA is a class-F design that uses a GaN HEMT. It produces up to 55 W with an overall efficiency of 74 percent, and maintains an overall efficiency of 60 percent or better at power levels of 5 W or more. Drive is provided by 15-W GaN HEMT PA operating in class B. Gysel combiners in a corporate configuration produce an output of 175 W with an efficiency of 60 percent.

The primary purpose of the modulators is to ensure high efficiency at all amplitudes. In addition, however, they allow the PA to generate a wide variety of amplitude-modulated signals with efficiencies two or more times those of conventional linear amplifiers. Signals originate in the digital signal processor and are predistorted to correct for amplitude nonlinearity and amplitude-to-phase conversion. Two-tone signals are produced with third-order IM products at about -26 dBc. Adaptation to load impedances with SWR up to 3:1 is accomplished by a three-stub tuner with stub lengths controlled by pin diodes.

At VHF, class-E provides the highest efficiency with an output network of minimum complexity. The electronically tunable VHF PA is therefore based upon an LDMOS FET operated in class E. A double-T output network allows adjustment of both drain resistance and reactance. Band segments are selected by switching the inductors, after which tuning is accomplished by high-voltage varactor capacitors. The PA operates from 20 to 90 MHz and produces 50 W with an efficiency of about 60 percent. With a different tuner, the PA is capable of operating to frequencies of 300 MHz with good performance and 500 MHz with useable performance.

The electronically tunable UHF power amplifier uses a GaN HEMT operated in class C. The input tuning is accomplished by two arrays of varactors. The output tuning is accomplished by a three-stub tuner. Ten pin diodes on each stub control its electrical length. This PA produces 40 - 45 W from 325 to 800 MHz (2.5:1 tuning ratio) with an overall efficiency of 50 to 55 percent.

GMRR developed a prototype GaN-HEMT class-F power amplifier for use in UHF radar. This project began with an experimental comparison of operation in classes B, E, and F. The prototype PA operates at 500 MHz and produces an output of up to 54 W. An overall efficiency of 70 percent or better is maintained for outputs from 8 to 54 W, and the overall efficiency is 60 percent or better for outputs as low as 2 W. A version of this PA adapted to operation at the 915-MHz ISM frequency has similar performance.

GMRR has developed a high-efficiency transmitter for MRI applications and other applications from 1.8 to 128 MHz. The transmitter is based upon the Kahn EER technique and includes class-E RF power amplifiers, 2-MHz class-S modulators, and a digital signal processor. The RF PA module produces up to 200 W with an efficiency from 70 to 90 percent. The DSP produces the input signals to the RF chain and modulators, and includes correction for amplitude and phase nonlinearities. The system can generate a downloaded signal on command or digitize and amplify a low-level input RF signal. The linearity is excellent, with two-tone IMD products at -35 dBc or lower. [NSF1/NSF2]

The broadband VHF class-D PA uses ferrite-loaded transformers at its input and output. The design was evaluated with a wide variety of transistors, including LDMOS FET, SiC MESFET, and GaN HEMT. The best performance is achieved when using a pair of GaN HEMTs. Thus configured, the PA produces over 100 W from 50 to 300 MHz, with useable power to 500 MHz. The efficiency varies from 55 to 67 percent. No tuning is required.

GMRR has developed electronically variable capacitors and inductors for use in power amplifiers. The electronically variable capacitor is based upon a novel semiconductor structure and provides a 3:1 range of variation. The electronically variable inductor is based upon a novel ferrite configuration and provides a 3:1 variation. A class-D power amplifier was designed that can be tuned electronically from 5 to 20 MHz; it produces 100 W with an efficiency of 70 percent. [SWL7]

This project explored techniques for wideband, high-efficiency power amplification for satellite communication at X and K bands. Included in the investigation were the Kahn Envelope-Elimination-and-Restoration (EER) technique, envelope tracking, and outphasing. Split-band techniques are envisioned to achieve the large bandwidths while maintaining high efficiency. In the split-band technique, a high-efficiency class-S modulator amplifies the low-frequency components of the signal envelope, which typically constitute 80 percent of the envelope power. A combination of the class-S modulator and a class-B linear amplifier offer bandwidths to 100 MHz with a degradation in efficiency of only 10 percent. [AFR1]

Determined power, antenna, receiver, and signal-processing requirements for a low-frequency spread-spectrum navigation system. Prepared plan for development and field-testing. [DOT1]

Designed class-E power amplifier capable of being tuned electronically continuously from 19 to 31 MHz. The amplifier produces a 20-W output with an efficiency 61 to 71 percent across the band. When tuned conventionally, it produces 25 W with an efficiency of 71 to 74 percent, but has a bandwidth of only about 1 MHz. It also has excellent amplitude-modulation linearity, and demonstrated the capability to produce amplitude-modulated signals with efficiency roughly double that of linear amplifiers. [VTE1]

This feasibility study for a high-efficiency amplifier for deep-space missions identified the GaAs pHEMT and InP HEMT as the most promising semiconductor technologies. The former is a mature technology, but the latter offers significantly higher efficiency at Ka band and higher. Basic relationships were discovered that relate the number of controlled harmonics to the maximum attainable efficiency and power-output capability. Class-E and -F power amplifiers offer the same efficiency, but class F offers greater power output for the same ratings. Output tuning networks to provide the required harmonic impedances were determined. A PA based upon GaAs pHEMTs is expected to have an efficiency of about about 40 percent, while one based upon InP HEMTs is exptected to to have an efficiency of about 55 percent. GaAs-pHEMT technology is recommended for short-term development with minimum cost. To obtain the highest possible efficiency, a longer-term approach based upon the InP HEMT is recommended. [JPL1]

Designed and tested experimental broadband class-D and -E power amplifiers using APT ARF449 MOSFETs. The class-D push-pull PA deliver 500-W with 80-percent efficiency from 2 to 4 MHz and 300 W at 21 MHz with 60-percent efficiency. The single-ended class-E PA delivers 250 W from 1.8 to 14 MHz with an efficiency of 80 percent and 125 W at 30 MHz with a 60-percent efficiency. Amplitude-modulation linearity is excellent, allowing the production of AM, SSB, and multitone-data signals through the Kahn EER technique. [IR&D]

A 20-W PEP linear L-band transmitter based upon the Kahn EER technique was developed and tested as part of a consulting contract. The RF power amplifier employs a two-stage MMIC driver and a PA biased for class-AB operation. The class-S modulator uses 0.5-m HFETs and switches at 3.3 MHz. A double envelope-feedback loop assures both high linearity and time-delay equalization for RF bandwidths to 150 kHz. With a two-tone signal the transmitter achieves an efficiency of 56 percent at full power and 35% at 18 dB into back-off. The third-order IMDs vary from -30 to -40 dBc over a 20 dB range of power output. [MOT4]

Components and techniques for high-power electronic tuning were investigated through analysis, simulation, and experimentation under a Phase-I contract. Under an on-going Phase-II contract, two types of voltage-controlled semiconductor capacitors are being fabricated and tested. Several variations on a new type of current-controlled inductor are also being fabricated and tested. Initial results demonstrate the capability to handle 100-W signals with a 2:1 tuning range. [SWL6] [SWL7]

Predicted the SNR for various VLF-communication links for an improved TACAMO system. [RAY1]

A power-combined version of the Adaptive Jammer power amplifier was designed and tested under a quick-reaction contract. [HAC2]

Techniques for implementing wide-bandwidth high-efficiency amplifiers are being investigated for systems such as Iridium, M-Star, and Celestri. Requirements for techniques such as EER, Doherty, and outphasing have been determined through simulation and analysis for a variety of different signals. The investigation includes both RF PAs (e.g., class E and F) and high-level modulators (class S, pulse-step, Meinzer). [MOT3/4/6]

The objective of this project was to integrate the power amplifier, matching network, and antenna to achieve the maximum radiated HF/VHF signal for a given dc-input power. Various whip and loop antennas were compared with the Numerical Electromagnetics Code (NEC). Simulation was also used to evaluate techniques for improving the gain of the whip antenna by both fixed and switched in-antenna inductors. Selected approaches were confirmed by field testing. At most frequencies, the low-angle gain was improved by 3 to 6 dB. Broadband-matching techniques were used to devise a set of filters that provide approximate matching over their respective subbands. Filter and matching networks tuned by pin-diode switches were also evaluated. [SWL3/5]

Under contract from a semiconductor manufacturer, GMRR developed a prototype class-D power amplifier for their new MOSFETs. This 250-W amplifier is intended for use at the 13.56-MHz ISM band, but is also useable to frequencies as low as 1.8 MHz. A novel driving circuit using gate-driver ICs and complementary MOSFETs eliminates all transformers except at the output, thus greatly reducing production costs. [APT1]

Instrumentation canisters used in explosives testing can be scattered over distances of several kilometers and buried at depths as great as 30 m. The RF TL concept developed in this Phase-I feasibility study provides a means of locating the instrumentation canisters and recovering their data. The surface equipment interrogates the RF TLs with a directional UHF signal. Upon interrogation, an RF TL responds on either VHF or VLF. The VHF signal allows location at depths of 3 m and distances of 4 km, while the VLF allows location at depths of 30 m and distances of 90 m. The subsurface transmitter is an ASIC that operates in class D and delivers an output of about 1.4 W. [WES1]

Under this set of contracts from the U. S. Army Intelligence Electronics Warfare Directorate, GMRR developed a prototype adaptive-jammer transmitter for the production of CW, AM, FM, SSB, and noise signals at HF and VHF. The Phase-I feasibility study investigated the use of a number of techniques for improving performance, including techniques include class-D operation of the RF PAs, envelope elimination and restoration, amplitude modulation by a class-S modulator, wideband AM by outphasing, harmonic cancellation, feedforward error cancellation, and multicoupler combining.

In Phase II, 100- and 250-W transmitters were developed. The RF PAs can operate in untuned class-D (high efficiency and maximum noise-power output), tuned class D (for clean communication signals with high efficiency), or class B (for wide bandwidth). Other system components include RF signal generators, a programmable waveform generator, AM/SSB modulator, FM modulator, frequency converters, and a class-S modulator. The transmitter uses the Kahn envelope-elimination-and-restoration (EER) technique. Test results show both superior average efficiency (60 percent vs. 20 percent for conventional transmitters) and superior linearity (third-order IMDs at -42 dBc vs. the usual -30 dBc). Multiple PAs can be combined to produce up to nearly 1 kW. [SWL2]

The fields scattered by helmet-mounted CRTs can significantly degrade the performance of a magnetic-field helmet-mounted sight. Under contract to U.S. Air Force Aerospace Medical Research Laboratory, GMRR developed a multipole model of this phenomenon and software for extraction of its parameters. Laboratory measurements showed that the dipole terms of the model predict the scattered fields with no more than a 16-percent error. The residual field-prediction error was shown to be due to noise and multipole components. [LTS3], [LTS4]

This study determined the characteristics of a number of RF-power combiners, including direct-connection, hybrid, zero-phase, directional-coupler, Wilkinson, and quadrature. The efficiency and input-impedance characteristics of each combiner were determined for combining signals of the same and different frequencies. Concepts for adaptive combining were developed. [SWL4]

This feasibility study addressed the capabilities of troposcatter, stratospheric scattering, ionospheric scattering, meteor-burst communication, and data compression to provide nondeniable communications. The use of a roof-top antenna array favors a 3-GHz carrier frequency for troposcatter and stratoscatter, which together provide a range of 1000 km. Ionoscatter and MBC provide relatively rapid recovery after a nuclear detonation. Data compression reduces the transmitted power, hence interceptability of the signals. It is anticipated that speech can be transmitted at 300 b/s, and that graphics and multilevel video can be compressed by ratios from 5 to 10 with existing techniques. [BMO4]

This feasibility study addressed the problem of providing nondeniable communication at moderate rates (75 b/s) between mobile command centers and launchers at ranges of 200 - 400 miles. A transportable low-frequency system was shown to provide the desired capability with realistic power input and a dual-mode (vertical-monopole / horizontal-dipole) antenna. Alternatively, an airborne-relay system can be used. The use of a directional antenna and frequencies in the low-UHF range minimizes interception. Also included in this investigation were means of acquiring information from the downlink signals of various satellite systems (Milstar, DSCS III, SDI). [BMO3]

This two-phase project for the U. S. Air Force Ballistic Missile Office developed advanced signal-processing concepts for through-the-earth communication and tested them in an experimental VLF receiver. In Phase I, a multicomponent noise model was developed and various signal-processing techniques were analyzed and simulated. In Phase-II, an experimental receiver that can receive all six electromagnetic-field components was developed and tested. The analog portion of the receiver a superhet design with a high-side IF. Remote preamplifiers allow the antennas to be located at some distance. Noise cancellation and detection were done in C-language software. The receiver and auxiliary equipment are operated by the computer through an IEEE-488 interface bus. Field tests at three sites produced SNR improvements from 10 to 24 dB. [BMO1/BMO2]

Previous HMS systems estimate position and orientation as if they were in free space and then attempt to compensate the estimates for the effects of nearby metal (such as an aircraft cockpit). Consequently, they can be highly sensitive to noise and sometimes erratic. Under contract from the U.S. Air Force Aerospace Medical Research Laboratory, GMRR has developed a new signal-processing algorithm. This algorithm is based upon a minimum-variance linear estimator and converts magnetic-field measurements directly into the correct position and orientation estimates. Simulations and tests in the GMRR lab have shown it to be stable and its estimates to be almost as accurate as those made in a free-space environment. An auxiliary nonlinear estimator and supervisory algorithm ensure absolute stability. GMRR work on these algorithms includes formulation, simulation, optimization of computations, and development of automatic-mapping software. [SRL8 - SRL13]

As part of a Transport Canada program, GMRR predicted navigation-system performance derived from new VLF-navigation transmitters. Included were field-strength predictions for OMEGA and candidate VLF-NAV stations, conversion of SNRs into rms accuracies, determination of system availability at a specified accuracy, and estimation of the required radiated power. [KW1]

GMRR provides two continuing-education seminars:

"Fundamentals" follows the SSRE textbook, covering linear power amplifiers, tuned power amplifiers, high-efficiency power amplifiers, CW/FM/AM transmitters, and SSB transmitters. "High-Efficiency Power Amplifiers" uses a 500-page seminar notebook and covers power-amplifier fundamentals; classes C, D, E, F, S, and G; and technique such as Kahn envelope elimination and restoration, Doherty, and outphasing.

GMRR owns its office building, which provides 1200 ft (111 m) of floor space as well as additional storage space. Land behind the building provides room for testing antennas.

The GMRR electronics laboratory provides instrumentation for R&D work at frequencies up to 40 GHz and power levels up to 1 kW. Instrumentation include a 500-MHz digital oscilloscope, spectrum analyzers for 1 GHz (IFR A-7550) and 40 GHz (R&S FSP-38), network analyzers for 300 kHz - 3 GHz (HP 8714B) and 40 MHz - 40 GHz (Anritsu 37269), RF signal generators for 9 kHz - 1 GHz (Fluke 6061A), 9 kHz - 3 GHz with I/Q modulation (R&S SM-300), and 40 MHz - 40 GHz with I/Q modulation (Anritsu 67369A). The lab also includes a 1-GHz vector voltmeter (HP 8508A), programmable-waveform generators (WT-195 and WT-275), driver amplifiers, RF-power meters, multimeters, and power supplies. The entire lab is computer-controllable via the IEEE-488 bus, allowing data collection to be accomplished with speed and reliability. Waveforms and spectra can be captured and transferred to text/graphics software for inclusion in reports. GMRR also has a small shop for basic mechanical work.

GMRR makes use of an assortment of Intel-based computers. Both FORTRAN and C are available. An extensive set of libraries provides mathematical, matrix, random, and other functions. Circuit simulation is accomplished via PSpice. Numerical Electromagnetics Codes (NEC2 and NEC3) are available for simulation of antennas. A large number of in-house programs and subroutines are also available for power-amplifier and communication-link analysis.

GMRR has one of the most extensive collections of literature on RF power amplifiers and transmitters. On hand is a variety of engineering journals (IEEE as well as trade). Over 10,000 papers and reports are indexed for rapid access by author or subject. Searches can also be run via IEEE Xplore.

GMRR has worked with a wide variety of clients in the public and private sectors. Most of our work is repeat business from satisfied customers. Most new business originates in response to a query initiated by the customer.