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The Desert Whooper Beacon 4095.65 KHz

In your August 4th logs a person offered some incorrect information about DW’s transmit format. For your readers’ interest, here are some technical details of the one watt beacon that provides fascinating propagation performance.

The beacon started development in November of 2020. It was built from scratch and has a hybrid controller consisting of two 555 CMOS timers to key the transmitter along with an Arduino Nano controller to generate Morse code and collect data for performance telemetry. The 555 timers will independently key the transmitter if the Nano fails for any reason. (Fortunately the Nano has functioned for 10 months without issue.) The 4.09565 MHz transmitter is crystal controlled and features a MV1403 varactor diode circuit for the upward frequency sweeps of 150 Hertz. The sweeps provide distinctive audio for the SWL’er as well as an easy to spot visual pattern on the waterfall display of Kiwi SDRs. The transmitter has 4 transistors and the final amplifier is a IRF510 MosFet followed by a low pass filter, all inside a shielded box.

The beacon is powered by a sealed lead acid battery which is kept charged with a solar panel. The battery is protected from over discharge by a low voltage disconnect (LVD) circuit that is independent of the Nano. Time has shown that the solar panel size provides good charge current even in cloudy weather and the beacon does not draw much battery power overnight.

The antenna is a 111 foot long, 1/2 wave dipole oriented North/South and it’s about 0.1 wavelength above the dry desert soil to create a more vertical but omni-like pattern. The 1:1 balun is home made and the antenna’s measured VSWR is close to 1.1 to 1

There are six cycles of DW in Morse along with 29-30 “whoops”; then the DW identification is sent along with four different telemetry numbers.

Telemetry is in slow Morse code as follows:

BAT is the battery’s voltage to tenths of a volt using a precision resistor divider and the Nano’s A2D function. Normal values range from 12.8 to 14.4 volts
OTMP is in degrees F with a calibrated 10k NTC thermistor located outdoors, about 100′ from the beacon.
ITMP is in degrees F provided by a digital one-wire DS18B20 microLAN sensor located within the weathertight equipment box. The reading runs 5 to 10 deg warmer than OTMP.
PV is given in milliAmps to monitor solar panel performance and battery charge current. Measurements are based on an INA219 current sensor board. The values of the current can range from 0 to 2000. Note that a value of 4 or 5 is just the idle current consumed by the solar controller because the battery is fully charged.

In case the Nano fails, the number of whoops can decrease from 30 down to 29 as a crude measure of rising temperatures. On some recent days the internal temperature has peaked at 120 degrees F when outside air temperature was measured at 112 degrees! The box is in shade but the electronic circuitry generates waste heat that causes the temperature rise. Monitoring the temperatures during night and day provides for some interesting deltas..

DW was officially deployed to the Western desert on February 21, 2021. It has been heard in Canada, California, Hawaii, Maryland, Utah, Arizona and many other locations. Some SWL logs can be seen here: and eQSLs are being provided.

The DW Team

(via WOR io group)

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