Category: Amateur Radio

Understanding Direction Finding on the KiwiSDR

Earlier this month we posted about the KiwiSDR direction finding update, which now allows anyone with internet access to utilize public KiwiSDR's for the purpose of pinpointing the physical location of a transmitter that transmits at a frequency below 30 MHz.

A few people have had trouble understanding how to use the direction finding feature, so KiwiSDR fan Nils Schiffhauer (DK8OK) has written up a KiwiSDR direction finding usage guide. Nils' guide explains the basic technical ideas behind the TDoA (Time Difference of Arrival) direction finding technique used, and highlights some important considerations to take into account in order to get the best results. For example he discusses best practices on how to choose receiver locations, how many receivers to choose, and how to properly take into account the time delaying effects of ionospheric propagation with HF signals.

Finally at the end of the document he shows multiple case studies on HF signals that he's managed to locate using the discussed best practices. Looking through these examples should help make it clear on how receiver locations should be chosen.

DK8OK Locates Radio France at 15320 kHz
DK8OK Locates Radio France at 15320 kHz

An Opensource Mini-Whip Antenna and Upconverter Design for RTL-SDRs

Thank you to Igor Yatsevich for submitting news about an open source Mini-Whip and Upconverter design that he's created and released for free on GitHub. An upconverter converts HF frequencies into VHF frequencies so that they can be received by RTL-SDRs in their quadrature mode, and a Mini-Whip is a small active antenna for receiving HF signals.

The designs include the PCB Gerber files for manufacturing, the components list and assembly and usage guides. Also both through-hole and SMD designs are provided.

The Mini-Whip design has a frequency range of 10 kHz - 30 MHz and to power it you'll need a 5 - 13V bias tee. You will need to install it up high and preferably away from the house as Mini-Whips are quite susceptible to local noise pickup. Another very important point is that Mini-Whips need to have a good ground connection. The upconverter is based on the ADE-1 mixer, and uses a 125 MHz local oscillator.

Igor's documentation on the project is excellent, and is a good read for getting more information about upconverters and Mini-Whips. He has noted that he is sending us some samples of units that he's built, so when we receive them we'll post again with test results. It looks as if he's put a lot of research into these designs so we're looking forward to seeing how well they work. 

Diagram on how to ground a miniwhip connected to a metal mast.
Diagram from Igor's documentation about how to properly ground a Mini-Whip connected to a metal mast.

Video Tutorial on Receiving ISS Astronaut Amateur Radio Conversations with RTL-SDR

Over on his YouTube channel Crazy Danish Hacker has posted a new video that shows how to pick up amateur radio voice signals from the International Space Station (ISS).

Often astronauts on the ISS will schedule times to chat with schools via amateur radio frequencies. This provides an opportunity to learn about radio whilst at the same time allowing kids to talk directly to an astronaut.

If you live in an area that can 'see' the ISS at the same time as the school then you can easily pick up the downlink (astronaut to ground) portion of the conversation while the ISS passes over. The downlink signal is fairly strong, so only a simple antenna is required. In his video Crazy Danish Hacker uses a telescopic whip attached directly to his RTL-SDR which is placed outside with a view of the sky.

International Space Station - Software Defined Radio Series #29

Using an SDRplay RSP2 to Measure IMD

Over on YouTube channel Rate My Radio has uploaded a set of three videos showing how to use an SDRplay RSP2 as a low cost spectrum analyzer to measure the inter modulation distortion (IMD) performance of lower end hardware TX capable radios. The test can only be performed on radios that have IMD performance less than that of the RSP2, so very high end amateur radios cannot be tested.

The process is to use audacity to play two audio tones into the transmitting radio under test, and then the SDRplay is used to receive the output. On the SDRuno software you're then able to see the third order and higher IMD products. Later he also performs white noise IMD tests as well. Below is the video description:

We cover 2 Tone Testing, White Noise Testing, and how the later can be particularly useful in terms of station monitoring. Naturally, we show the effects of 'all knobs to the right' :)

Jarrad also covers how with just an SDR Play and a 'rubber ducky' antenna, station performance can be monitored in real time.

Why would a Ham want to do this? The answer is simple: To defend their station performance against that on air Expert, who got their ticket when you needed to send CW at 50WPN, who served in the military radio unit for 20 years, has 3 engineering degrees and worked as a professor at both MIT and Havard, not to mention the times they lectured at Cambridge & Oxford.

With an SDR Play and a bit of simple math, any OM can put such experts in their place.

Below we only post the third video of the three part series. Links to Part 1 and Part 2 are available in those links, or on his channel.

Part 3: IMD Testing & Realtime Station Monitoring

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

RASPBERRY PI3 SDR Monitor 40m FT8/JT65/JT9 (RTL-SDR/LINRAD)

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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.

Thomas N1SPY chases mini satellites on a budget