Boeing Employees Amateur Radio Society

10 Meter Beacon Project

The K7NWS beacon is an entirely homebrew affair. The transmitter outputs approximately 1 watt at 28.265 MHz. As of May 2000 it is operating at the BEARS Shack in Kent, Washington at the Kent Space Center which is located in an area outside the fence south of Bldg. 18-03. (E-42 on Map, "Washington - Space Center, Kent").

The unit consists of four boards, RF Exciter, RF Power Amp, Controller, and CW Message Generator, which are described below. The boards are installed in a pair of plastic boxes with see-through lids, which are then mounted to a wooden base. Power is provided by a 12 volt wall-plug type AC adapter. Here is a picture. Click it for a larger image.

The RF exciter includes a unique overtone crystal oscillator, three buffer amplifiers, and a class A amplifier and low pass filter capable of providing about 100 milliwatts output. The oscillator uses an old crystal marked 9.417 MHz, which just happens to run at 28.265 MHz on its 3^rd overtone. The oscillator uses the crystal in its series mode and is very tolerant of "mistuning"; in fact there are no tuning adjustments in this oscillator. The oscillator output is buffered by a simple low-gain amplifier stage. The 2^nd and 3^rd buffers and class A amplifier are keyed for CW, with the oscillator running continuously. The board is made by cutting 0.25 inch square pads in double-sided circuit board material with a Dremel Tool. Component leads are soldered directly to the square pads for connection and mechanical support. Some chip bypass capacitors are used in spots on the board since they were available and provide superior RF performance. The lower level stages use 2N3904 transistors and the class A amplifier uses a junkbox transistor similar to a 2N4401. The entire exciter is broadband with no tuning. Separate keyed +12 volts is provided to the last buffer and class A stage, and a "timed" +12 volts is supplied to the oscillator and buffer. Total power drain is about 70 mA.

The RF power amplifier is a small board with a push-pull 1 watt amplifier and 5 element low pass filter. It uses the same board construction as the exciter. Two junkbox transistors, plastic TO-92 type similar to 2N4401's, are used to provide approximately 1 watt output power. Ferrite toroids are used in broadband transformers which are employed for impedance matching and coupling. The power amplifier operates directly from the +12 volt supply. Power drain is approximately 150 mA. The power amplifier board includes an RF detector that is used to detect a "stuck key" condition. A feedback resistor value on the exciter board is adjusted to provide proper drive to the power amp.

The control board contains the "stuck key" timer circuit and the CW keyer. This circuitry is built on perf board with part leads bent and soldered together. Except for the 555 chip in a socket, its not very maintenance friendly. The "stuck key" circuit uses a 555 timer chip with some discrete transistor logic (2N3904's) to latch off the +12 volts to the oscillator stage on the exciter board if continuous RF output is detected for longer than about 15 seconds. This ensures the transmitter cannot put out an unmodulated carrier with no identification. Removing +12 volt power for several seconds resets the timer. The CW keyer circuit uses a PNP transistor to switch the +12 volts and includes waveshaping so that the CW keying is hard but does not have key clicks. There are monitor LEDs on the control board for primary +12 volts, timed +12 volts, and keyed +12 volts.

The CW message generator board is an actual printed circuit board that was adapted from a repeater CW ID'er cooked up by Glen N7UIG. The board uses a PIC microprocessor to generate the CW message string. This board always comes up and starts sending the message when power is applied. The only other functionality is a control line that is grounded to select a 5 second delay time between message strings rather than 10 seconds. The CW message string is encoded "the hard way" as that was most expeditious. The processor is non-reprogrammable, meaning a new part must be "burned" and installed to change the CW message.