Add streaming audio to OP25 RPi scanner

Thanks to Graham Norbury, here is how to set up a Raspberry Pi OP25 scanner to stream audio to an icecast or other streaming server. I have updated my plug-and-play op25-rpi.img image at to include this capability (look for the or later-dated file). 1. Copy the following into a new file /etc/asound.conf on the Raspberry Pi (you’ll need to do this as root): # output device pcm.loopout { type dmix ipc_key 328211 slave.pcm “hw:Loopback,0,0″ } # input device […]

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Thoughts on a Wideband SDR Recording Tool

As part of the HamSci Solar Eclipse experiment, I recorded wideband data from my SDRs on the theory that capturing the data for later study might be more productive (and less stressful) than trying to do measurements in real time. I think that theory played out very well. I used a simple Gnu Radio flowgraph to do the recording from an HPSDR Hermes receiver. When I say simple, I mean simple. Here’s the whole thing, which records four 384 kHz […]

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Update on Update on P25 Scanner


Since my updated post on building a P25 trunking scanner with a Raspberry Pi and an RTL-SDR dongle, I’ve created a copy-and-play image file that gets you up and running in minutes. I’ve put it (along with a couple of other RPi image files) at The filenames are somewhat long, but modestly descriptive so you should be able to figure out which to download. Once you have the file, unzip it and copy to an 8GB SD card. Put […]

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Shrink/Expand Raspberry Pi Filesystem Images

This post documents a few tricks I found useful when making SD card images for Raspberry Pi projects, to minimize file size and download times while making full use of the SD card space. These instructions work for Raspbian Stretch images; not sure about older or newer versions. The first time a Raspbian system boots, it runs a program to expand the filesystem to use the full size of the SD card. So, when you are using a Raspberry Pi […]

Read More Here! 0 Dongle Measurements (Part 2 — Results)

In Part 1 I described some background about SDR dynamic range as an introduction to the measurements I did on a v3 USB “dongle” SDR. Now for the results. I wrote a Gnu Radio program to help measure the dongle. It tunes the dongle, sets its front-end gain, shows a spectrum display, and also shows the power in a 500 Hz bandwidth around the tuned frequency. That power reading is what I used for these measurements. The flowgraph is […]

Read More Here! 1 Dongle Measurements (Part 1 — background)

I’m using several USB “dongle” SDR sticks from These nifty $20 devices can receive from 100 kHz to above 1700 MHz. They have a low noise amplifier in the front end that is adjustable from 0 to about 50 dB gain. I wanted to determine the impact of the gain setting on receiver performance, and the results are below. First, some introduction. The dongles use an 8 bit analog-to-digital converter (“ADC”). The number of bits sets the dynamic range, […]

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Update on P25 scanner — new version, and new installation instructions


Since I posted my original articles on configuring the Gnuradio-based op25 P25 trunking scanner, there’s been a lot of progress in the code. In particular, there is a new fork that changes the program to a command-line version, which saves a lot of CPU resources and also offers added features. The new version will run nicely on a Raspberry Pi 3, and probably on an RPi 2 as well. Following are instructions to install it from scratch on the PI. […]

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Eclipse — 80M Waterfall

Here’s a waterfall showing the 80M CW band from a bit after 1600 UTC (noon) and 2000 UTC (4:00PM EDT) on eclipse day. There was clearly enhanced propagation for about an hour, starting ~15 minutes before the local maximum (73% obscuration) at 1819 UTC. There aren’t many signals shown at any time, but that’s to be expected at mid-day in this frequency range. In order to show this long time period, the waterfall only updated every 60 seconds and I […]

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More Solar Eclipse Analysis (HF Propagation)

I’ve continued processing the data generated from software-defined-radio recordings I made during the 2017 solar eclipse at my fairly isolated and low-RF-noise location on Beaver Island, Michigan (45 38 38 N, 85 29 24 W, grid square EN75gp). The nearest AM station is about 28 miles away, so it’s a pretty good site for monitoring the high frequency bands. The data was recorded over the eight hours from 1400 – 2200 UTC (10AM to 6PM EDT) on Monday, August 21, […]

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First data from Solar Eclipse 2017

I set up a smaller version of my mad-scientist stuff at our cottage on Beaver Island, Michigan (45 38 38 N, 85 29 24 W, grid square EN75gp), to record the RF spectrum before, during, and after the eclipse on August 21, 2017. The recordings were successful, and I captured just under 1 TB (!) of data. Fortunately, disk drives are cheap these days. I’ve done the very first step of analysis on one of the data sets — a […]

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