RF Design

Component Derating

In most small-signal analog and digital designs, component derating is not usually a concern. However, in the design of power circuits, such as power supplies, motor drivers, and power amplifiers, component voltage, current, and power specifications are very important. Using a component at less than its voltage, current, and power specified maximums to increase the circuit’s reliability is call component derating.

My first engineering experience in the 1970s and 80s was designing switching power supplies for the aerospace industry and later for commercial manufacturers of printers, monitors, and computers. Some of our customers provided very detailed specifications for their power supply requirement, and a few customers included with the specification their internal component derating guidelines. I recall that IBM was one of those customers that provided detailed derating guidelines. These Component Derating Guidelines published by IBM are much more detailed than what I remember from almost 40 years ago.

Small Loop Antenna

Ever since I learned about it, my favorite antenna has been the small loop (magnetic loop) antenna. This antenna takes up much less space than most other HF antennas. It can be mounted vertically to get some directivity or horizontally for lower noise, omnidirectional operation. The high Q of the small loop acts like a preselector, reducing interference.

About 20 years ago, I lived in a condo in Waikiki and purchase one of the original AEA Isoloop antennas (made with aluminum tubing). Given the size of my lanai (balcony), I did not have many options for operating HF. The loop sat horizontally on a tripod and appeared to work very well with my Icom IC-735.

After retiring, I decided to get back into Amateur Radio after being inactive for a number of years. I no longer had any HF (which is where my interest lies) equipment. So, I thought about building a small loop antenna and started doing some research.

The best information I have found is on AA5TB’s page for Small Transmitter Loop Antennas, which includes detailed technical information and a spreadsheet calculator. I built a 6 foot (octagonal) loop for 40 – 10 meters. The loop is tuned with a Trombone Capacitor as described by KD7S in his November 1994 QST article. The maximum capacitance does not tune 40 meters, so I added a high voltage relay and capacitor in parallel with the Trombone Capacitor.

Advanced but Inexpensive Transceiver

When it came to finding a new HF transceiver, I had two goals in mind. First, I wanted something that took advantage of more recent advances in transmitter and receiver design. Second, I did not want to make a large investment, at least initially. I even toyed with the idea of building my own transceiver from scratch.

I do not remember where, but I came across information about the QRP Labs QCX transceiver, which seemed to fit the bill. This single-band CW transceiver is microcontroller controlled and has a direct-conversion receiver that uses a Quadrature Sampling Detector, instead of a superheterodyne receiver. And, the QCX uses a Class E amplifier in the transmitter. As a kit, the QCX also contributed to my desire to build the transceiver myself.

I first built a QCX-40 (QCX for the 40 meter band). The QCX-40 worked after only a minor adjustment (documented in the manual) to get the receiver to align properly. About a year later, I built a second QCX for the 20 meter band. The QCX-20 also worked well, except for low transmit power, which seemed to be related to the output low-pass filter.

Low-Pass Filters

Transmitters incorporate a low-pass filter in the output stage to attenuate harmonics, which could otherwise interfere with other radio communications. The harmonic of greatest concern is the second harmonic, because it is only twice the transmitted frequency, making it the hardest to attenuate. My experience with the QCX low-pass filter and cookie-cutter low-pass filters for transmitters in general is documented in a two part analysis. Part 1 deals primarily with my analysis of the QCX-20 low-pass filter and the origin of the filter design. Part 2 continues the analysis of these basic filters, including recommendations for future applications.

Power Amplifier Efficiency

Power efficiency is important for a number of reasons. First, most of us want to conserve energy. The higher the efficiency, the less power wasted. Second, and more important for electronic equipment, the wasted power becomes heat. This heat increases the temperature, which increases the stress on components. Efficiency in power amplifiers plays an important role in maintaining this stress at an acceptable level.

QRP Labs produces an inexpensive 50W amplifier to complement the QCX transceiver. Although not a linear amplifier, it works great for CW. The package is small, but has a relatively large heatsink. Some concerns raised regarding the efficiency of this amplifier prompted me to write this analysis of the Amplifier Efficiency.

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