Category: Amateur Radio

Locating Various HF Transmitters and Number Stations with KiwiSDRs

If you weren't already aware, the KiwiSDR is a US$299 HF SDR that can monitor the entire 0 - 30 MHz band at once. It is designed to be web-based and shared, meaning that the KiwiSDR owner, or anyone that they've given access, can tune and listen to it via a web browser over the internet. Many public KiwiSDRs can be found and browsed from the list at sdr.hu.

One thing that KiwiSDRs have is a GPS input which allows the KiwiSDR to run from an accurate clock, as well as providing positional data. Time Difference of Arrival (TDoA) is a direction finding technique that relies on measuring the difference in time that a signal is received at over multiple receivers spread out over some distance. In order to do this an accurate clock that is synchronized with each receiver is required. GPS provides this and is able to accurately sync KiwiSDR clocks worldwide.

Over on his blog Christoph Mayer has been steadily documenting his work on getting Time Difference of Arrival (TDoA) direction finding to work with KiwiSDRs. This is not an easy task with HF signals, as they tend to bounce around and propagate through various means, meaning that signals can be delayed if not received directly. So far it appears that he's been most successful in locating signals received by ground wave, but he is also working with an ionospheric ray-tracing model and electron density data to take into account propagation delays from skywave propagation.

Skywave and Groundwave Propagation
Skywave and Groundwave Propagation

In one post from late last year Christoph shows that he was able to pinpoint the location of the German DCF77 longwave time station by using three KiwiSDRs spread out around Europe. The actual location of DCF77 is already known, so this shows that the technique actually works. Other posts show him locating transmitters for STANAG 4285, some unknown frequency hopping signals, OTH radar from Cyprus, CODAR, DRM, VOLMET and more.

Christophs' code can be found at https://github.com/hcab14/TDoA. According to users gathering the data and running the code is still a fairly elaborate process. But there is talk over on the KiwiSDR forums about eventually creating a server that would allow users to more easily request a location computation for a particular signal. 

Pinpointing DCF77 with KiwiSDRs
Pinpointing DCF77 with KiwiSDRs (Bottom right image shows pinpointed location)

Also related to this topic, priyom.org has been using KiwiSDRs to try and locate numbers stations. Numbers stations are mysterious voice stations on the HF bands that when transmitting read out a string of numbers. Most speculate that the numbers are some sort of code intended for international spy agents. Using a simpler method of just noting which KiwiSDRs in the world receive a particular numbers station more strongly, they've been able to determine the likely country of some well known stations.

Tutorial: Setting up a Low Cost QRP (FT8, JT9, WSPR etc) Monitoring Station with an RTL-SDR V3 and Raspberry Pi 3

QRP is amateur radio slang for 'low transmit power'. QRP digital modes such as FT8, JT9, JT65 and WSPR are modes designed to be transmit and received across the world on low transmit powers (although not everyone uses only low power). The special design of these modes allows even weak signals to be decodable by the receiving software. Released in 2017, FT8 has shown itself to now be the most popular mode by far with JT9 and JT65 taking a backseat. WSPR is also not as active as FT8, although WSPR is more of a beacon mode rather one used for making contacts. 

Apart from being used by hams to make contacts, these weak signal modes are also valuable indicators of the current HF propagation conditions. Each packet contains information on the location of the transmitter, so you can see where and how far away the packet you've received comes from. You also don't need to be a ham to set up a monitoring station. As an SWL (shortwave listener), it can be quite interesting to simply see how far away you can receive from, and how many countries in the world you can 'collect' signals from.

This tutorial is inspired by dg0opk's videos and blog post on monitoring QRP with single board computers. We'll show you how to set up a super cheap QRP monitoring station using an RTL-SDR V3 and a Raspberry Pi 3. The total cost should be about US $56 ($21 for the RTL-SDR V3, and $35 for the Pi 3).

With this setup you'll be able to continuously monitor multiple modes within the same band simultaneously (e.g. monitor 20 meter FT8, JT65+JT9 and WSPR all on one dongle at the same time). The method for creating multiple channels in Linux may also be useful for other applications. If you happen to have an upconverter or a better SDR to dedicate to monitoring such as an SDRplay or an Airspy HF+, then this can substitute for the RTL-SDR V3 as well. The parts you'll need are as follows:

  • RTL-SDR V3 (or upconverter, or other HF & Linux capable SDR)
  • Raspberry Pi 3 (or other SBC with similar performance)
  • Internet connection
  • Band filter (optional but recommended)
  • HF antenna (this could be as simple as a long wire)

Examples of QRP Receivers with an RTL-SDR

Monitoring FT8, JT9, JT65 and WSPR simultaneously with an RTL-SDR V3 and Pi 3
Monitoring FT8, JT9, JT65 and WSPR simultaneously with an RTL-SDR V3 and Pi 3

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Using the RTL-SDR as a tool to Measure Crystals

Over on YouTube OM0ET has shown how he uses his RTL-SDR for measuring crystals. While working on his home made HF 6-band SSB transceiver, OM0ET needed a way to measure the frequency of some 8 MHz crystals that he needed for his IF filter.

To perform the measurement he simply inserts the crystal into a homemade oscillator circuit, and measures the output with an RTL-SDR V3 operating in direct sampling mode. With the measurements he's able to figure out if the crystal is actually working in the first place, and secondly determine an accurate frequency measurement.

RTL-SDR USB receiver - cheap tool for matching crystals

Chasing Cubesats on a $25 Budget with an RTL-SDR and Homemade Antenna

Cubesats are small shoebox sized satellites that are usually designed by universities or amateur radio organizations for basic space experiments or amateur radio communications. Typically they have an orbit lifespan of only 3-6 months.

Cubesats typically transmit signals at around 435 MHz, and they are powerful enough to be received with a simple home made antenna and an RTL-SDR. To help with this Thomas N1SPY has created a YouTube video where he shows exactly how to construct a cheap eggbeater antenna made out of a few pieces of copper wire and an SO-239 UHF connector. Later in the video he demonstrates some Cubesats being received with his antenna, an RTL-SDR and the SDR-Console V3 software.

2018: Thomas N1SPY chases mini satellites on a budget

Creating a Linear Transponder with an RTL-SDR, HackRF and Raspberry Pi

A linear transponder is essentially a repeater that works on a range of frequencies instead of a fixed frequency. For example, a normal repeater may receive at 145 MHz, and repeat the signal at 435 MHz. However, a linear transponder would receive a wider bandwidth, and add a set frequency offset to the received signal. For example a signal received by a linear transponder that receives from 145 - 145.5 MHz, may receive a signal at 145.2 MHz and it would translate that up to 435.2 MHz. Another signal received at 145.4 MHz would translate up to 435.4 MHz. Hence the received frequency linearly translates to the transmitted frequency.

Over on his blog ZR6AIC has shown that it is possible to create a linear transponder using an RTL-SDR for receiving, a Raspberry Pi for processing the signal, and a HackRF for re-transmitting the signal. 2M and 70cm band bandpass filters are also used. For software he uses a GNU Radio flowchart that simply moves the IQ data from the RTL-SDR to the HackRF.

In the video below he demonstrates the linear transponder in action with two handheld radios.

A Linear Transponder made with HackRF, Raspberry Pi and RTL-SDR.
A Linear Transponder made with HackRF, Raspberry Pi and RTL-SDR.
Building a Linear Transponder with Gnu Radio, rtl dongle and hackRF module..

Monitoring FT8, JT65, JT9 on Multiple Bands with Low Cost Single Board PCs

Thank you to Michael (dg0opk) who wrote in and wanted to share details of his full SDR monitoring system for weak signal HF modes. His setup consists of nine ARM mini PCs (such as Banana Pi's, Raspberry Pi's, and Odroid's), several SDRs including multiple RTL-SDR's, an Airspy Mini, FunCube Dongle and SDR-IQ, as well as some filters and a wideband amp. For software he uses Linrad or GQRX as the receiver, and WSJTx or JTDX as the decoding software, all running on Linux.

Michael also notes that his Bananapi FT8, JT65 and JT9 SDR monitor has been up and stably running continuously for half a year now. Bananapi's are lower cost alternatives to the well known Raspberry Pi single board computers, so it's good to note that a permanent weak signal monitoring system can be set up on a very low budget. Presumably even cheaper Orange Pi's would also work well.

With his setup he is able to continuously monitor FT8, JT65 and JT9 on multiple bands simultaneously without needing to tie up more expensive ham radios. His results can be seen on PSKReporter. A video of his RTL-SDR Raspberry Pi 3 decoding FT8, JT65 and JT9 can be found here.

Weak Signal Receiver Setup
dg0opk's weak signal receiver setup

Creating a High Altitude Balloon Telemetry System with Raspberry Pi, RPiTX and RTL-SDR

The 2M TX Filter by ZR6AIC
The 2M TX Filter by ZR6AIC

Over on his blog ZR6AIC explains how he's created a full HAB (high altitude balloon) telemetry transmit and receive system using RPiTX and an RTL-SDR dongle running on a Raspberry Pi 3.

RPiTX is software that enables the Raspberry Pi to transmit any modulated signal over a wide range of frequencies using just a single GPIO pin. However, the transmission contains multiple harmonics and thus requires sufficient filtering in order to transmit legally within the 2M ham band. To solve this ZR6AIC uses a 2M Raspberry Pi Hat kit which he designed and created that contains two low pass filters as well at the option for an additional power amplifier.

The rest of ZR5AIC's post explains how his HAB telemetry system combines the Raspberry Pi 3, RPiTX 2M Hat, RTL-SDR, a GPS unit, battery, temperature sensor and optional camera into a full HAB transmitting system. He also explains the software and terminal commands that he uses which allows him to transmit via RPiTX a CW beacon and GPS and temperature sensor APRS telemetry data with the Direwolf software. Full instructions on setting up the alsa-loopback audio routing is also provided.

Launching the High Altitude Balloon.
Launching the High Altitude Balloon.

Thomas N1SPY Demonstrates Receiving SSTV Images from the ISS

Earlier in the month the International Space Station (ISS) was transmitting SSTV images down to the earth for anyone to receive an decode. The ISS does this several times a year to commemorate special space related events, such as the day Yuri Gagarin (first man in space) was launched.

In the video Thomas explains why the ISS does this, how to track the ISS, and then he demonstrates actually receiving and decoding the signal. Thomas uses an Airspy HF+ to receive the signal on 145.8 MHz, however an RTL-SDR could do the same job. For decoding he uses the MMSSTV software.

For new on when the ISS might transmit SSTV again, keep at eye on the ARISS Blog, and the ISS Ham Twitter page.

2018: Thomas N1SPY receives SSTV signals from the ISS