Category: HF

Decoding Differential GPS Beacons with an RTL-SDR, Speclab and SDR#

Over on his blog “RTL-SDR DX” dewdude has been exploring the reception and decoding of Differential GPS (DGPS) signals. DGPS signals are transmitted by government authorities in the long wave band at around 300 kHz. These beacons are used to dramatically improve the accuracy of GPS (Global Positioning System) devices from their default accuracy of about 15 m down to about 10 cm. Unlike GPS signals which originate from satellites, the DGPS signal is terrestrial based and is broadcast from multiple known fixed positions. The signal itself contains information about the difference between the DGPS stations received GPS position and it’s known exact position. These differences can be used to correct other GPS receivers that receive DGPS signal.

By using his RTL-SDR (with upconverter or HF modification) dewdude was able to receive the DGPS beacon in SDR#. Then by piping the output audio into SpectrumLab’s DGPS decoder he was able to decode the data contained within the DGPS signal. His post contains a tutorial showing how to set up SpectrumLab to decode DGPS. If you’re interested in hearing what a DGPS signal sounds like, dewdude has uploaded a sound sample at the bottom of another post of his.

Decoding Differential GPS (DGPS) signals in SpectrumLab
Decoding Differential GPS (DGPS) signals in SpectrumLab

Building a simple upconverter with a NE612 IC

Upconverters are often used to extend the RTL-SDR dongles minimum receivable range down to the HF, MF and LF bands. They are available for purchase commercially, or you can build your own, which is what Tomasz of mightydevices.com has done.

By using a low cost mixer IC chip called the NE612 and some passive components Tomasz was able to build a low cost upconverter for his RTL-SDR. His upconverter uses a 100 MHz crystal oscillator that brings frequencies between 0-30 MHz up to a range of 100 – 130 MHz, which is in the receivable range of the RTL-SDR. The upconverter circuit was also designed to be able to provide inline power for a active (powered) Miniwhip antenna. Tomasz’s post explains the design choices and theory behind his circuit design.

An NE612 based upconverter
An NE612 based upconverter
NE612 RTL SDR Upconverter

Using a Quantum Phaser to Null Out Interfering Signals

Over on YouTube user kugellagers has uploaded several videos showing how he used two vertical antennas together with an RTL-SDR and ham-it-up upconverter to demonstrate the effect of using a Quantum Phaser to null out strong interfering signals that can cause trouble when DXing.

A Quantum Phaser is a device that combines signals from two antennas in order to create a steerable null. Essentially this means that a strong nearby station coming from one direction that is overlapping a weak remote station coming from another direction can be heavily attenuated, allowing the weak station to come through.

In his videos kugellagers demonstrates the Quantum Phasers nulling effect with splatter from an AM station, an overlapping IBOC hash signal (AM HD Radio) and Non-Directional Beacons (NDBs).

Phasing Out Splatter From a 50 kW Local On Adjacent Channel

Phasing Out IBOC Hash From A Strong Local On Adjacent Channel

Phasing out LF/NDBs With Closely Spaced Vertical Antennas

Comparing the Ham-It-Up Upconverter with the SV1AFN Upconverter

Over on YouTube user Mile Kokotov has uploaded a video showing a comparison between the ham-it-up and SV1AFN upconverters. An upconverter allows reception of VLF to HF signals with SDR dongles such as the Airpsy and RTL-SDR.

Mile shows that both the ham-it-up and SV1AFN upconverters use the same core component, a double balanced mixer ADE-1. However, the ham-it-up comes with the option for a noise circuit to be populated. A noise circuit is useful if you want to measure the response of a filter or antenna for example. With the ham-it-up the noise source components are sold seperately and need to be carefully soldered on.

On the other hand the SV1AFN upconverter comes with a built in selectable LNA and better filtering circuitry. The SV1AFN upconverter also uses the ADE-1 in a slightly different design compared to the ham-it-up which allows for much improved performance at VLF frequencies.

In the results Mile uses his Airspy and shows that the SV1AFN upconverter is significantly better at receiving VLF frequencies, and also better at receiving a 28.205 MHz beacon. The results are summarized in the table captured from the video shown in the image below. In the second video Mile also compares the local oscillator drift of each upconverter.

Upconverter comparison results.
Upconverter comparison results screencap.
RF Upconverters Comparison: Ham it Up vs SV1AFN - part1

RTL-SDR Panadapter Using Hardware Radio Receiver IF Stages

Over on YouTube user Jay Moore has uploaded a video explaining how to connect an RTL-SDR dongle to the IF stage of a hardware radio in order to create a panadapter. In the video Jay briefly explains how a radio with an IF stage works and then shows how he tapped into his Sansui 2000 hardware radio’s IF stage directly from the circuit board. The IF stage then connects to a ham-it-up upconverter which connects to the RTL-SDR.

By connecting the IF stage of a hardware radio to the RTL-SDR it is possible to use the hardware radio as the receiver while using the RTL-SDR to still maintain the benefits of a spectrum display. Most purpose built hardware radios will have better reception than the RTL-SDR.

RTL-SDR on receiver IF stages

PortableSDR now on Kickstarter

Back in November, 2014 we posted about the PortableSDR, a 0 – 35 MHz portable software defined radio transceiver that was the third place winner in the Hackaday Prize competition. The PortableSDR project is gaining traction and now has a Kickstarter campaign. They write:

The Portable Software Defined Radio, or PSDR, is an Open Source, Fully stand-alone HF/Shortwave Software Defined Transceiver. It includes a Vector Network Analyzer and Antenna Analyzer as well as GPS. It’s built for rugged portable use. It is designed to be a flexible platform for development, a learning aid, and and a useful instrument for electronics enthusiasts.

Features:

  • Coverage from 0 to 35MHz
  • Waterfall display that lets you see radio signals
  • Receives AM, USB (Upper Side Band), LSB (Lower Side Band), and Morse code (CW)
  • Modulates USB and LSB signals
  • Variable bandpass filter

The campaign hopes to raise $60,000 USD to aid in the development of the hardware and software and with the manufacturing process. The kickstarter is offering kits at various stages of completion from $250 to $475 and a fully assembled kit at $499. They note that the current PSDR2 that you will receive from the Kickstarter is still a development version, not the final product. The PSDR2 is missing some key features that will be in the final version like filters and output amplifiers.

The PSDR v.1
The PSDR v.1
PortableSDR - 2014 Hackaday Prize Judge Recap

Receiving VLF with a PC sound card, Miniwhip Antenna and SAQrx

Over on YouTube user Mile Kokotov has uploaded a video showing how he is able to receive Very Low Frequency (VLF) signals between 300 Hz to 30 kHz using just his PC, a Miniwhip antenna connected to his sound card input port and a program called SAQrx Panoramic VLF Receiver. This allows reception of signals between 0-48 kHz with a sound card that can sample at 96 kHz.

Using this set up he is able to receive the Alpha navigation system beacons which are at around 10-12 kHz and some other Navy navigation system beacons between 18 and 48 kHz from his home in Macedonia.

Receiving VLF with PC and software only

Receiving WSPR with the RTL-SDR

Recently RTL-SDR.com reader DE8MSH wrote in to let us know about his experiments with receiving WSPR with his RTL-SDR. WSPR is an acronym for “weak signal propagation reporter” and is a software program and RF protocol designed for very weak signal radio communications between ham radio users. With less than 5W of transmitting power, a WSPR signal could potentially be copied all over the world.

To receive WSPR, DE8MSH used a direct sampling modified RTL-SDR dongle together with a 9:1 unun, 10m RG58 coax cable from RTL-SDR to unun and a 12m wire antenna outside his house. Then by using SDR# together with the WSPR software he is able to copy signals from all over Europe and Canada/USA from his home in Germany.

Some Received WSPR Locations
Some Received WSPR Locations
WSPR Report Information Including Distance
WSPR Report Information
The WSPR Software
The WSPR Software