Backyard Meteorological Instrumentation

Software Defined Radio (SDR) Subsystem

Radio System Overview

The objective of the SDR subsystem is to provide a general purpose radio monitoring system with the ability to experiment with tracking aircraft using their ADS-B transponders,

The Software Defined Radio (SDR) subsystem consists of a cheap software defined radio that can cover the frequency range 100kHz to 1.7 GHz. The radio uses two antennas: a HF Whip (5MHz - 45MHz) and a VHF-UHF (25Mz - 1.3GHz) discone antenna. The SDR is connected to, and powered by a USB port on the Raspberry Pi 4.

The SDR and the discone were (well over over a decade ago) used for Amateur Radio (HAM Radio). The HF Whip has a much more illustrious past, as it is part of Australian defence development history, coming from the receiver array  of the Stage A of the Jindalee Over The Horizon Radar (OTHR) outside of Alice Springs back in the late 70's.

The SDR will be  mounted on the DIN rails in the backyard enclosure where the Raspberry Pi is mounted. The antennas will be mounted on poles attached to the enclosures mounting posts.

Software Defined Radio (SDR)

The RTL.SDR is a popular, low cost hardware that can receive wireless signals. The RTL.SDR features the Realtek RTL2832U chip, which provides I-Q samples through the USB interface and a Rafael Micro R820T or similar down converter. You can receive and demodulate various wireless signals across a broad frequency range. Software signal processing can be used to  receive and process wireless signals such as AM radio, FM radio, airplane surveillance signals (ADS-B), signals from smart meters (water or energy metering devices) and signals from Personal Weather Station (PWS) sensors.

The disadvantage of the SDR is that t is essentially a wide-band device which make it susceptible to high-level out-of-band signals. Since the radio signals are converted to digital signals using an 8-bit Analogue to Digital Converter (ADC), the receiver has a limited dynamic range to handle such signals without overloading the receiver,

To mount the SDR on the DIN rail, it will be glued to DIN rail clips to allow the receiver to be mounted to the rail. If the receiver fails, it will be toally replaced and the rail clips are quite cheap, so it isn't really worth taking the effort to make a mounting cradle for the receiver.

The SDR is powered via the USB connector used for data communications with the Raspberry Pi. The two (2) antenna cables run directly from the SDR to the external antennas.

Discone Wide Band Antenna

The D130J Super Discone Antenna is a wideband antenna covering amateur radio, air traffic control plus Personal Weather Station and other devices covered under the Low Interference Potential Devices (LIPD) class licences (433MHz - 434Mz, 915MHz to 928 MHz),

This antenna has not been used for about a decade and it has sustained some damage, such as the loss of one of the horizontal disk element rods and a broken radial element rod.

A stainless rod has been ordered to replace the horizontal disk element. At this stage, it has not been practical to obtain a stainless steel tube to replace this element, but an aluminium replacement pipe has been procured. Currently it is uncertain if the broken radial element rod will be used after the sharp edges of the break have been removed, or replaced with the aluminium tube.

HF Whip Antenna

The whip antenna is showing the signs of sun damage to the insulators and there are rusty nuts and bolts holding the aluminium structure together. Given that the antenna is at least forty (40) years old and spent the early part of its life in the Central Australian sun, it is not surprising. Whether its worth refurbishing the antenna, or using it as-is, is unknown at this time.

Licenced under Creative Commons Attribution Share Alike 4.0 International or better by Mark Little (2022 - 2023)