Category: Other

Transmitting FM, AM, SSB, SSTV and FSQ with just a Raspberry Pi

Previously we posted about the Raspberry Pi’s ability to modulate one of its pins to produce FM transmissions with PiFM. A developer (F5OEO) has recently expanded on this idea, and now the Raspberry Pi is capable of modulating and transmitting FM, AM, SSB, SSTV and FSQ signals anywhere between 130 kHz to 750 MHz.

To transmit with the Raspberry Pi all you need to do is plug in a wire antenna to Pin 12 (GPIO 18) on the GPIO port and run the PiTx software by piping in an audio file or image for SSTV. 

Important Disclaimer: While the output power is very small, you should still take great care as the carrier is a square wave, and there is no filtering on the antenna output. So any transmissions will cause harmonics all across the spectrum – possibly interfering with life critical devices. A filter *must* be used if you actually plan on transmitting with any sort of range further than your room. The predecessor PiFM has been reported to have a range of 10cm without an antenna, so it may be best to not connect an antenna to the pin if just testing. With a simple wire antenna the range is increased to 100m which could affect your neighbours. There are also strict laws and licences governing transmitting in most countries so make sure you follow them carefully. In short, get your ham licence and understand what you are doing before transmitting with any sort of amplification/range.

The code for PiTX can be downloaded at https://github.com/F5OEO/rpitx. Also see the authors (@F5OEOEvariste) Twitter account at https://twitter.com/F5OEOEvariste for some more info about PiTX.

PiTX transmitting SSTV and received in HDSDR. From PiTX's author's Twitter @F5OEOEvariste
PiTX transmitting SSTV and received in HDSDR. From PiTX’s author’s Twitter @F5OEOEvariste

Over on YouTube the author of PiTx has also uploaded a video showing a wireless doorbell being replayed with PiTx. On the video description he writes:

PiTx is a software which permit to transmit HF directly through a pin of Raspberry Pi GPIO. Unlike PiFM which transmit only in FM, PiTx is able to perform multi modulation (FM,AM,SSB,SSTV,FSQ) : it has an I/Q input to be agnostic.
The demonstration here is done in several steps :
– Record an I/Q file from a doorbell transmitter on 434MHZ (first part)
– Playing it with the Raspberry Pi using Pitx on HF on same frequency
– Listen to the doorbell receiver which recognize the signal

Conclusion : Pitx is now a real TRANSMIT SDR at very low cost. Be aware that it generate lot of harmonics and never compete with USRP or HackRF.
Goal is to popularize the transmission as rtlsdr popularize the reception.

Tutorial on Properly Positioning a Preamp (LNA) in a Radio System

Radio blogger Anthony Stirk has made a post on his blog explaining some critical concepts behind understanding why it is important to position a low noise amplifier (LNA) near the radio antenna, rather than near the radio. In the post Anthony explains how the Noise Figure (NF) and linearity (IP3) of a radio system affect reception.

Using the free AppCAD RF design assistant software, Anthony explains how the noise figure of a system increases with longer coax cable runs, and how it can be reduced by placing an LNA right next to the antenna. He also explains why the sensitivity of the radio won’t increase if the LNA is placed close to the radio instead.

In addition to this, he also explains why adding more LNA’s to a system decreases the linearity (IP3) of the system and that if the receiver has a built in LNA that the system linearity can be severely degraded by adding extra LNA’s, causing easy overloading and intermodulation. In conclusion Anthony writes the following:

In summary, a setup with a good antenna system connected to a receiver with a built in LNA:

  • May not benefit from having a preamp at the antenna.
  • The presence of a built in LNA is detrimental to the linearity and may degrade the signals.

So in conclusion:

  • Put the preamp as close to the antenna as possible.
  • Receivers with a built in LNA may not get the most out of an antenna system or preamp.
  • Proper gain distribution guarantees better performance than one-size-fits-all solutions, both in terms of sensitivity and strong signals handling.

Optimal Setup: Antenna -> LNA -> Coax -> Receiver
Optimal Setup: Antenna -> LNA -> Coax -> Receiver
NF and Linearity Calculations
NF and Linearity Calculations in AppCAD

DesktopSDR MATLAB RTL-SDR Text Book Released

Back in August we posted about an RTL-SDR related text book called DesktopSDR that was due to be released later in the month. The text book discusses technical SDR topics, with the RTL-SDR used as the radio receiver and MATLAB used as the digital signal processing tool. It looks to be very useful to students of radio or communications engineering. There were a few delays with the release, but it is now out at www.desktopsdr.com. The eBook version is free whilst the print version is soon to be released on Amazon for about $68 USD for the paperback and $89 USD for the hard back. 

To go along with the book they have also released several accompanying videos that are available at desktopsdr.com/videos.

The books blurb reads:

The availability of the RTL-SDR device for less than $20 brings software defined radio (SDR) to the home and work desktops of EE students, professional engineers and the maker community. The RTL-SDR can be used to acquire and sample RF (radio frequency) signals transmitted in the frequency range 25MHz to 1.75GHz, and the MATLAB and Simulink environment can be used to develop receivers using first principles DSP (digital signal processing) algorithms. Signals that the RTL-SDR hardware can receive include: FM radio, UHF band signals, ISM signals, GSM, 3G and LTE mobile radio, GPS and satellite signals, and any that the reader can (legally) transmit of course! In this book we introduce readers to SDR methods by viewing and analysing downconverted RF signals in the time and frequency domains, and then provide extensive DSP enabled SDR design exercises which the reader can learn from. The hands-on SDR design examples begin with simple AM and FM receivers, and move on to the more challenging aspects of PHY layer DSP, where receive filter chains, real-time channelisers, and advanced concepts such as carrier synchronisers, digital PLL designs and QPSK timing and phase synchronisers are implemented. In the book we will also show how the RTL-SDR can be used with SDR transmitters to develop complete communication systems, capable of transmitting payloads such as simple text strings, images and audio across the lab desktop.

Download the book at desktopsdr.com
Download the book at desktopsdr.com

4NEC2 The Definitive Guide Book

4NEC2 is a freeware antenna simulation software program. The program can be used to model an antenna, and then determine through simulation properties such as the radiation pattern and SWR of the modelled antenna. It is very useful for those designing home made antennas for their RTL-SDR or other radio, or for those just trying to understand how antennas work. 

4NEC2 can be a complicated piece of software and to get the most out of it a guide is needed. We’ve recently been informed that Mark Schoonover KA6WKE, is writing an eBook guide on this topic. His guide is currently only 20% complete, but a free sample of his book in its current state can be downloaded from leanpub. The guide already shows what the main windows of 4NEC2 do, and shows you how to model and simulate a simple dipole antenna.

4NEC2: The Definitive Guide
4NEC2: The Definitive Guide

Building an active wideband antenna for your SDR

As the RTL-SDR and other SDRs are wideband, we recommend that people use a wideband antenna if they want to scan over all the frequencies. Normally a discone or a scantenna is suggested as the wideband antenna of choice, however there are alternative designs such as the Dressler ARA-2000 which is a type of log spiral antenna that can receive from 50 – 2000 MHz. This particular product is no longer for sale, but an article detailing the breakdown of its construction can be found online. 

The dressler ARA-2000
The dressler ARA-2000

The article shows how to build the antenna from scratch. The active element consists of a thin copper sheet (or copper foil) in the shape of a wedge which is rolled into a cylinder around a plastic sheet. It is then connected to a low noise amplifier (LNA), which is powered through a bias tee. For the LNA something like the LNA4ALL could be used if building it yourself.

This antenna may be something to consider if you are thinking about building your own wideband antenna. It has the advantage of being much smaller than a discone and it can also be fully enclosed in a plastic radome to that fully protects it from weather elements. The author of the article also writes that he saw better performance from this antenna than when compared with a discone.

Inside the ARA-2000
Inside the ARA-2000

Frequency Manager Suite Plugin for SDR#

Recently the popular Frequency Manager + Scanner plugin set for SDR# has been upgraded and renamed to the “Frequency Manager Suite”. The plugin can be downloaded from their new website at www.freqmgrsuite.com. The plugin suite includes a frequency scanner and manager, a scanner metrics recorder, a scheduler, an activity logger and a frequency entry plugin.

Apart from plugins the suite also now includes a plugin manager program called “Pluginator” which can help you to install and delete plugins without needing to edit the Plugins.xml file directly. There is also a new database manager tool which can help you to import frequency databases from online or other sources.

We have posted the full feature release below:

The Frequency Manager Suite (FMS) adds 4 more plugins to the previous set of 3:

  • Scheduler – allows you to schedule listening activities by date and time.

  • Activity Logger – records scanner activity to a file for later use.

  • Scanner Decisions – now a first-class plugin and also available as the classic stand-alone window. And you can change the position of the plugin without restarting SDR#.

  • Frequency Details – displays details about a frequency from your database. And you can change the position of the plugin without restarting SDR#.

New standalone applications also come with FMS:

  • Data Tools – this significantly upgraded import/export application lets you import data from 6 popular internet databases as well as generically-formatted files, and permits you to export your FMS databases to standard file format.
  • The Pluginator – an application that lets you add, delete, and change the order of plugins without ever having to hand-edit the SDR# file Plugins.xml. Just fill in the blanks.

New features in Frequency Manager + Scanner:

  • The new Preferences dialog allows customization without hand-editing a configuration file.
  • FMS configurations are now kept in a file separate from those of SDR#, permitting easier upgrades to SDR#.
  • You can have multiple frequency databases, and can change to a different database without restarting SDR#.
  • When tuning manually you can optionally change the radio settings (mode, BW, etc.) according to what’s in the database for the tuned frequency.
  • Scan resolution – high-res or low-res to favor speed over accuracy or vice versa.
  • Adjacent Frequency Rejection now displays its bandwidth on the spectrum analyzer.
  • Plus many other improvements and bug fixes.

fms_1v2   fms_3   fms_2   

pluginator

GNU Radio Conference 2015: Presentations

The GNU Radio conference (GRCon15) is a yearly conference discussing all matters related to GNU Radio, an open source graphical block based DSP programming application that is compatible with most SDR’s, including the RTL-SDR. The conference started on August 24 and is due to close this Friday August 28, however many of the presentation slides are now available for viewing on their website.

This year there are many interesting talks, including a speech by Balint about radio direction finding, RF sniffing and digital FPV on drones. There are also several tutorial presentations that show how to install GNU Radio, how DSP sampling works, an intro to analog RF concepts and how to build a software radio from scratch.

gnuradio

Seeing through walls with WiFi signals and USRP software defined radios

Researchers at the University College of London have found a way to use WiFi signals to see through walls, using a USRP software defined radio and software written in LabView. The researchers have shown that they are able to utilize local WiFi signals to detect and monitor moving objects such as people behind a wall in a similar fashion to how radar systems work. The advantage over traditional radar is that their system is completely passive, requiring no transmitter, other than the already ubiquitous WiFi signal.

In a demonstration the researchers showed how they were able to not only detect the presence of a person behind a wall, but also detect small hand gestures that were made.

Detecting body gestures from WiFi signals in LabView.
Detecting body gestures from WiFi signals in LabView.

It appears the researchers are patenting their work and are looking to market their technology towards military and security surveillance operators as well as towards other applications such as traffic monitoring and the monitoring of children and the elderly.

We aren’t sure what type of radio accuracy is required for a system such as this, but it may be possible that SDR’s that cost less than the USRP may also work, assuming the software technology can ever be replicated/licensed.

wifi_hostage
A proposed application of the technology: Allowing police to see through walls in a hostage situation.