Author Topic: Broadband spectrum recording and analysis - SigInt for the layman  (Read 1171 times)

Offline PrepareSmarter

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Broadband spectrum recording and analysis - SigInt for the layman
« on: September 05, 2019, 07:14:05 PM »
This is one of my first posts here, so I wanted to share something useful for the community. This is taken from a post I made earlier on ARFCOM.

"Think no one is listening to your comms? Think again."

Radio traffic is everywhere; amateur radio, commercial radio, police/military radio, cellular data, wifi, ect.. One thing is certain; if you think no one is listening in, you are wrong.
Governmental and private organizations have had sophisticated systems for monitoring, recording, and analyzing bulk radio data for a long time. However, it wasn't until recently that hardware and software was cheap and available to the average citizen.

You may ask, why should I care about this?
Broadband spectrum recording has many uses for preppers and hobbyists alike.
You can use this technology to record local VHF/UHF radio traffic (FRS, Marine, MURS, amateur, commercial, etc..) in order to find out what is going on around you. These techniques have been used by the Border Patrol and US military with great success.

You can also use this technology to record one or more HF bands to record nets, listen to news and disaster relief traffic, and most importantly to record and analyze digital traffic that may be missed if you could only listen to it live.
I don't know how many AmRRON nets I've been on where an unexpected frequency shift or an issue with an operator's settings required a repeat to be transmitted once or more. If the operator could rewind the transmission, much of this could be avoided.

You can even setup automated recording. This allows you to use a frequency and time as a blind drop-box, even when you are away from the system. Combined with one time pads or PGP encryption (post SHTF of course), these techniques provide a secure and convenient way to pass information.

The Hardware:
I won't get too deep into the specifics of which brand or model of each piece of hardware I use. This will only be an overview of the components in the system.

Software Defined Radio (SDR) Receiver or Transceiver
The receiver/transceiver is the heart of any radio listening system. Cheap models are available (RTL-SDR) but the amount of bandwidth you can monitor is limited. For our purposes, I prefer a SDR that supports 10MHz of bandwidth or larger (RSP1a, hackRF, etc..)

Feed line is the run of 50ohm connecting your SDR to the antenna.

Keeping two antenna systems, one for VHF/UHF and one for HF, is important if you want to listen to all the radio traffic in your area.
A discone is a good listening antenna for VHF/UHF/6m, but it may pick up more noise than other antenna systems.

A maypole is my go-to antenna for HF bands, for field expediency and its intrinsic broadband capabilities.

The computer

A modest computer can be used for narrow-band recording, but broadband spectrum recording works best on a computer (laptop, desktop, or even a tablet PC like the Surface Pro) with certain properties:
A strong processor, as recording is very processor intensive.
A decent amount of RAM, 4+ Gb reccomended
A SSD (or less advisably, a fast HDD) with a large amount of storage. Broadband spectrum recording can consume 40+ Mb per second.

The Software:

SDR Console V3
SDR-Console V3 ( is a free (donations accepted) software package created by Mawnan Smith. This amazing software creates a great interface to your SDR.
The software has always been able to record raw capture data, called IQ files. Version 3 added the Analyser module, is the special tool that civillians had been lacking for years.

If you want to route your recording to a digital modem program, such as FLDigi or HRD, you will need a software audio router such as VB-Cable.
This is only necessary if you want to analyze digital traffic (text or packet) or demodulate digital voice traffic.

In Action..

The first image (below) shows the system recording 6MHz at 23.8Mb/s, capturing the entire 40m and 80m amateur bands.

This image (below) shows the software displaying a waterfall representing 20 minutes of recording the 40m and 80m bands.
Each vertical yellow band represents a separate signal or conversation.

This image shows a snapshot of an UHF spectrum recording that has been analyzed.

I hope this post has been informative and interesting.

« Last Edit: September 05, 2019, 07:19:04 PM by PrepareSmarter »

Offline patriotman

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Re: Broadband spectrum recording and analysis - SigInt for the layman
« Reply #1 on: September 13, 2019, 05:40:06 AM »
Welcome, and an excellent first post!

I think this is incredibly important for SIGINT purposes
Blessed be the LORD my strength, which teacheth my hands to war, and my fingers to fight: My goodness, and my fortress; my high tower, and my deliverer; my shield, and he in whom I trust; who subdueth my people under me.

Psalm 144:1-2

Offline JoJo

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Re: Broadband spectrum recording and analysis - SigInt for the layman
« Reply #2 on: September 13, 2019, 11:05:45 AM »
  :welcome: and I agree with patriotman, great first post.
In principle, no less than in practice, socialism is the ideology of thieves and tyrants.

Offline reconprepper

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Re: Broadband spectrum recording and analysis - SigInt for the layman
« Reply #3 on: February 25, 2020, 12:19:19 PM »
think no one is listening? wrong I am LOL :) Thanks for the post I have some of the equipment mentioned in the post and use it for those reasons. :)
mid Georgia area here