The January/February issue of QEX is coming soon, and it is full of theoretical and practical technical articles that you won't want to miss.

Tom Dean, KB1JIJ, describes "A Digital Frequency Hopping Spread Spectrum Transmitter." Dean designed this transmitter using an Altair DE2 Development and Education Board with an Altair Cyclone II field programmable gate array (FPGA) as a digital logic class project at the United States Military Academy, West Point. Rudy Severns, N6LF, wraps up his seven-part series on "Experimental Determination of Ground System Performance for HF Verticals," wrapping up with "Ground Systems with Missing Sectors." If you have buildings or property lines that prevent you from running radials in all directions from the base of your vertical antenna, you will want to read Severns' study results.

Paul Wade, W1GHZ, continues his series on "Waveguide Filters You Can Build and Tune" with "Part 2 -- Waveguide Post Filters" and shows us how to build filters for a variety of bands using different sizes of waveguide. Maynard Wright, W6PAP, shows us how to calculate FM sideband amplitudes and zero crossings using Bessel functions. Of course he uses his favorite free mathematical analysis tool, the GNU Octave computer program!

Ray Mack, W5IFS, presents another installment of his software defined radio column. In this installment of "SDR: Simplified," he looks at the real world operation of digital to analog converters (DAC) and their limitations. In the "Tech Notes" column, Mike Hamel, WO1U, teams up with Dave Cripe, NM0S, to discuss some problems with his September/October 2009 QEX article, "Phase Controlled Differential Drive for EER Amplifiers." Hamel discovered that the correct terminology for his technique is "Outphase Modulation of High Efficiency Amplifiers." He presents some new circuit ideas and an LTSpice model for his revised amplifier project. The model file is available for download from the ARRL Web site.

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