April 29, 2015

Building a 145 MHz Low Loss Helical Bandpass Filter

Over on YouTube user Mile Kokotov has uploaded a video showing his home made low loss helical bandpass filter for 145 MHz, but also tunable from 110 MHz to 160 MHz. Bandpass filters are useful for the RTL-SDR as often strong out of band signals can cause overload, causing poor reception. A bandpass filter blocks all signals outside of its pass band. A helical bandpass filter is simply a coiled wire enclosed in a conductive container that can be tuned with a variable capacitor made out of two plates.

In his video Mile shows the inner construction of his helical filter, explains the parts and shows what calculations he used for construction.

145 MHz Low Loss Bandpass Helical Filter by Mile Kokotov

Watch this video on YouTube

Vote

April 27, 2015

Hooking up an Si5351A Voltage Controlled Oscillator to the Local Oscillator Input on an RTL-SDR

With most low cost RTL-SDRs the provided 28.8 MHz local oscillator is of low quality and will often have a sizeable frequency offset and temperature drift. To fix this the oscillator can be replaced with a higher quality one.

Over on his blog Milan has showed how he used a Si535A voltage controlled oscillator on a breakout board from etherkit.com to replace the RTL-SDR’s low quality oscillator. The Si535A is a clock generator circuit which is capable of generating almost any frequency up to 200 MHz with 0 ppm offset from a 25 MHz TCXO oscillator. To connect the Si535A break board to the RTL-SDR, Milan soldered an edge mounted SMA connector to the RTL-SDR and connected it to the RTL-SDR’s clock input with a 10nF capacitor.

Milan also writes how the Si535A has options for different drive currents and writes that by lowering the drive current from 8 mA to 2 mA the spurious signals found at multiples of 28.8 MHz in the RTL-SDR are reduced. He also writes the the Si535A also allows you to detune the frequency a bit which can help to tune around any spurious signals. This could be useful for example when using rtl_power as you could create a spurious free plot.

Using a Si5351A to replace the local oscillator on an RTL-SDR.

Tweet10

Vote

10 Shares

April 27, 2015

SDRRecorder – A Linux Script for Recording an IQ Stream at a given Date and Time

Programmer MichelinoK has just released another one of his projects. This one is called SDRRecorder and is a Linux bash script that can be used to automatically record an IQ stream of any frequency and length at a given date and time. MichelinoK writes that he uses this script to automatically record International Space Station (ISS) passes at 145.8 MHz on his Raspberry Pi which is always powered on.

The script can easily be run by first editing the script to specify the frequency, dongle number, gain, PPM offset and destination folder. It can then be run by calling is with the start time, date, length and output file name. He also writes that you must be careful to not record for too long as long IQ files can use up a lot of disk space. To overcome this he uses a networked attached storage (NAS) device which is similar to an external hard drive.

Vote

April 27, 2015

A Crude Skew Planar Wheel Antenna for Receiving GPS L1 with an RTL-SDR

Over on his blog /dev/thrash RTL-SDR experimenter Elia has been attempting to build an antenna to receive Global Positioning System (GPS) signals with his RTL-SDR. After doing some research he decided to build a Skew Planer Wheel antenna which he tuned for the GPS L1 frequency at 1575.42 MHz. A Skew Planar Wheel antenna is circularly polarized omnidirectional antenna which can be built out of wire. It is well suited to receiving signals from low earth orbiting (LEO) satellites such as the GPS satellites.

Elia later tested his antenna with a commercial GPS receiver circuit and was able to obtain a GPS fix.

Skew Planar Wheel Antenna on the RTL-SDR for receiving GPS.

Vote

April 24, 2015

RTL_Power Frequency Display Software

Programmer MichelinoK has recently posted about another piece of RTL-SDR related software that he has just released. His latest software is called RTL_POWER Display and is a Windows program that allows you to simply use your mouse to determine the frequency of a signal within a waterfall plot that was previously produced by rtl_power.

Rtl_power is a command line program that can be used to create very wideband spectrum scans over a long period of time with an RTL-SDR. It comes with the official RTL-SDR release, but a more modern version can be downloaded from keenerds Windows builds.

Vote

April 21, 2015

Remote RTL_UDP: RTL-SDR Remote Control Android App

Over the past few weeks programmer MichelinoK has been working on an Android app that allows the RTL-SDR to be listened to and controlled remotely. The app is called “Remote RTL_UDP” and allows you to control the parameters such as frequency, modulation type and gain settings on a remote rtl_udp server. It also allows you to store frequencies in a database and import frequencies from a SDR# database. The server can be run on a Raspberry Pi, Linux PC or any Linux device that is capable of running rtl_fm.

The post contains a full tutorial on the set up which involves the installation of ezstream and icecast for streaming audio as well as instructions for the installation of the modified version of rtl_fm called rtl_udp which allows remote control of the parameters via udp.

Tweet68

Vote

68 Shares

April 21, 2015

AISRec for Android: New AIS Decoder

Back in December last year we posted about Jane feverlay’s work on his software called AISRec, which was a Windows AIS decoder for the RTL-SDR that he had developed.

AIS is an acronym for Automatic Identification System and is a system used by ships to broadcast position and vessel information. By monitoring AIS transmissions with the RTL-SDR we can build a boat radar system. We have a tutorial on this here.

Now from YouTube it looks as though Jane has developed a new Android version of AISRec which he describes as a high performance dual-channel AIS receiver for use with a single RTL-SDR dongle. The features of AISRec include:

1. Functions include AIS and GPS NMEA I/O, from/to USB devices and serial ports (special android devices with serial ports).
2. It supports TCP server and UDP forwarding jointly or independently for AIS and GPS.
3. It supports steaming in WIFI in AP or station modes and Ethernet.
4. It supports auto run on boot for especially rooted devices.

He also writes that AISRec also has companion implementations for Windows desktop and metro, Linux on Raspberry Pi 2 and the Radxa pro.

Unfortunately, at the moment there does not seem to be a link available to download the software and the video may only be a preview. The video does link to his Taobao shop at http://shop114459024.taobao.com/ (in Chinese use Google Translate), which is where he might release the software when it is ready.

Watch this video on YouTube

Vote

April 21, 2015

Sigimira Decoder now supports the RTL-SDR Directly

Sigmira is a popular free Windows software program which supports decoding of radio modes such as PSK31, RTTY, CW, STANAG 4285, Japanese Slot Machine, FSK, HFDL and M141 as well as standard modes such as NFM, AM, USB and LSB.

Previously Sigmira could be used with the RTL-SDR by piping audio into it from another program such as SDR#. In the latest version that was just released Sigmira now directly supports connecting to the RTL-SDR, allowing for a spectrographic display and point and click tuning. Sigmira also now includes a database with frequencies of over 2000 known signals.

Screenshot of the Sigmira decoding software which now supports the RTL-SDR directly.

Vote