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Welcome to the World of Software Defined Radio (robertputt.co.uk)
300 points by robputt796 on Dec 4, 2016 | hide | past | web | favorite | 82 comments

SDR is fun and the way of the future. I'd say we're getting close to a very low cost SDR that also brings together usability and excellent RX/TX and filtering capability. We're not there yet and so my Kenwood TS-2000 and Yaesu 857D and 817ND still kicks the butt of low cost SDR's.

You'll find as you set up the base station your focus quickly moves to antennas. And then you become obsessed with antennas.

Definitely get into amateur satellites. http://amsat.org/status/ Go get a cheap handheld arrow dual band antenna with built-in diplexer to get started. I use macdoppler and also Ham Radio Deluxe's Satellite module. They are Mac and Windows respectively - I use them both for different things. Mostly I use macdoppler to make the doppler corrections automatically on my Yaesu 817ND and I use HRD for my TS-2000.

Also, build yourself a HF monoband dipole antenna tuned to 14.3MHZ and then listen in to the ham band from 14.00 to 14.150 for digital/cw and from 14.150 to 14.300 for voice. It'll cost you about 20 bucks for the coax and a few bucks for the wire. You don't need a balun if you're using low enough power for TX.

If you want to be a bit naughty (this is HN afterall) google around and learn about FLTSATCOM 7 and 8. It's an old US milsat that is basically an open repeater. They're geosynchronous birds that are used for encrypted comms but Brazilian criminal gangs periodically use them for free communication. They build antennas out of spare parts and use down-converters/up-converters to deal with the 250mhz frequency which is outside normal VHF/UHF. With your SDR you can build your own cheap home depot antenna and listen in without a converter. Just do absolutely not broadcast because you'll get arrested - someone already has. You can sometimes pick up used special forces directional portable antennas for a reasonable price on ebay for 250mhz.

I'm a radio amateur, callsign WT1J. If you're into this stuff and find yourself able to TX on HF or satellite, drop me a line and we can set up a schedule.


The Pirates using FLTSATCOM aren't Brazilian criminal gangs, they are truckers, families, and others who have used rudimentary technology to keep in touch with one another. Think CB radio from the 70's and 80's in the United States.

I've only heard casual conversation between truckers and families on UHF Satcom.

I'm a dabbler and I came across a very cool project that seemed a bit out of my league at the moment - but perhaps you and others on this thread will get a kick out of:


Basically, making your own radar system with Coffee cans. As a newbie, I'm a bit surprised by all the modular components that were just fitted together. I knew about Amps and attenuators but didn't know you could get VCOs and mixers like that too. Any books/website recommendations/beginner projects would be appreciated!

I'm a noob and yeah antennas seem crucial. I wish there was some book I could read on the subject or some recommended websites. Maybe HN can recommend something. There are many tutorials how to build an antenna for specific purpose, but I'd like to gain some basic understanding of how do you design it.

Take a look at the ARRL Antenna Book

What he said. It's sitting on a desk next to me.

So are you referring to American arrests? Because you make it sound like the Brazillians don't get arrested.

A reply comment says that truckers are using it - what's their legal status?

However the high cost SDR will kick the butt of high cost analog rigs.

> criminal gangs

or unregistered companies operating under hostile conditions created by the government?

SDR has been an endless source of fun for me. I have a half a dozen different rtl dongles hooked up to raspberry pis and my laptop at any one time. Most recently I wrote a program to read the 900MHz broadcasts from my electric and water meters [1]. I'm also using an existing program to monitor our gas meter [2]. rtl_433 is another fun one [3].

[1] https://github.com/shaunhey/ea_receiver [2] https://github.com/bemasher/rtlamr [3] https://github.com/merbanan/rtl_433

This is a super cool application for this, unfortunately our water meters and stuff in the UK are very basic mechanical devices in most cases still which sucks so none of this fun for me... This does remind me I probably should check out a few other things though, such as ZigBee protocol on my home automation stuff.

The SMETS1 and SMETS2 smart meters that you will be getting soon will have ZigBee.... but secured and tied down so although you will be able to sniff the comms... everything will be encrypted.

...We're far too privacy and security conscious in the UK to allow plaintext personal data go over the airwaves...

Been meaning to look into the smart meters here in the U.K. after having them fitted recently. I had horrible thoughts of it all being clear text, so it's nice to know that no one can snoop on my gas and electric usage. I had assumed they were at least working on the 2.4Ghz frequency as it'd dropped out a few times due to interference with other kit.

All comms are end-to-end encrypted, each meter has unique certifications and keys and the data also tends to be stored encrypted on FLASH. The certification also requires the standards authorities to view the source and have strict guidelines on security, robustness, sanitisation of data, etc.

> All comms are end-to-end encrypted... The certification also requires ...strict guidelines on security...

So what you're saying is that, in the UK, your water meter and electrical appliances are required to have a higher level of security than your online browsing is allowed to have?



hehe... effectively, yes, oh the irony....

You have to consider tho, that the smart meters don't operate on the public internet, its all on a proprietary WAN (government specified).

There are many, many wacky design decisions regarding the UKs smart metering design but they do at least make an effort regarding security and robustness.

The flashing light on an electric meter (assuming you don't have the really old dial type) flashes once per watt-hour, which can be used for measuring.

433mhz is used in place of the 900mhz broadcasts in Australia

From that project you linked, it decodes "Oregon Scientific Weather Sensor" protocol - but there's several different versions of that. I'll have to have a play with it one day - thanks!

What do your electric/water meters sound like?

If you like SDR, you should definitely have a look at GNU Radio Companion (GRC) [1]. A nice introduction into the methods of GRC is given by Michael Ossmann [2], which is really worth checking out.

[1] http://gnuradio.org/redmine/projects/gnuradio/wiki/GNURadioC... [2] http://greatscottgadgets.com/sdr/1/

I'll add some other software recommendations along the more visual route. For those on Windows, SDR# [1] is pretty great. On Linux and Mac, I really enjoy using GQRX [2]. And finally, when you're digging deep down into a received signal, inspectrum [3] can't be beat.

[1] http://airspy.com/download/ [2] http://gqrx.dk/ [3] https://github.com/miek/inspectrum

I can't recommend those videos enough, great stuff!

Pardon me, what videos?

"I’ll be honest I have always thought radio related stuff is a bit of a strange and boring hobby, the electronics side fine, but sitting and chatting to people in a slow and often poor quality conversation didn’t really seem that interesting to me"


You can send data over radio links and do other very interesting things... All without using a network! Just two nodes transmitting and receiving data via antennas at the speed of light. You don't have to talk at all. A lot of people don't get this when they think about radio.

Radio is not just old, retired guys talking about health problems on VHF/UHF repeaters. Rather, it's a world-wide network, that has no infrastructure dependencies, transmitting data at the speed of light.

What many people don't realize is that right now, as you're reading this, you're probably using radio. Either wifi or 4G cellphone. Seems like an obvious statement, but RF engineers have done such a great job at making IP over the air reliable with 802.11X and 4G/LTE that we can forget about the fact that we're using it.

The problem with this though is security. We tend to focus as security engineers too much on stuff riding on top of IP and forget about the underlying layer 1 protocols. And so that's for me what makes radio really interesting - especially dark corners that haven't been looked at for some time because they've been forgotten about.

If you have proper security on a higher abstraction layer you don't care at all how the data gets transmitted, who can listen to it or modify it.

Note that on the Amateur frequencies, it's illegal to "Obfuscate" your transmissions. You can't even use coded language.

Didn't know that. Surely some people tried steganography? Or is there too much noise to try something like that? (and thanks for the antenna book recommendation)

That would count as "obfuscation".

It's really hard to prove steganography if you are hiding encrypted data (which looks like noise if it's properly encrypted)

Depends on what you're trying to protect. Seeing encrypted traffic flows at Tor entry and exit nodes for example can expose identity.

Being able to monitor encrypted traffic on the wire or air can yield other useful and potentially compromising metadata.

Nope, good old CAT6 for me ;)

...which also broadcasts RF.

...and can be quite easily sniffed with the right equipment.

Yeah, it's full of cool stuff. Like old guys talking about emergency systems over VHF/UHF repeaters. And younger people, too.

A fun way to use one of these is to decode images sent from weather satellites. We built a system to decode images from a Russian Meteor-M2 satellite by using a RTL-SDR, home made antenna and GNU radio (image gallery : http://meteor.amphinicy.com/). Unfortunately, we are located in the city center with a lot of RF noise and interference so images are not as clean as they could be.

I really wish there were cheaper Tx capable SDRs, as they look like a lot of fun! The cheapest ones I've seen at the moment seem to be around $200 hackrfs from aliexpress.

Isolated Audio card & PPT signal + HT can probably be done for less than $100 if you go with one of the cheaper HTs.

Heck you can pick up a Mobilinkd[1] for ~$60 if you just want to stick with KISS/AX.25 over Bluetooth. They're really awesome little devices for what they do.

[1] - http://www.mobilinkd.com/

While not SDR, there are some cheaper alternatives if you're just looking to mix code and radio transmitting.

[1] https://www.adafruit.com/products/3076 [2] http://www.ebay.com/itm/162086931088

Yeah that's a good point I've got a module which uses http://www.silabs.com/products/wireless/EZRadioPRO/Pages/si4...

The chip itself covers 119–1050 MHz, but the modules I have, have filters on to allow only narrower ranges.

I got excited there for a second, I thought you linked the Si468x, which I have been thinking about tinkering around with. It is just a receiver, but it does HD radio which interests me for some reason (it certainly isn't the audio quality :))

Funny that, I've played all week-end with a RTL stick[0], and a £30 'weather station' thing from amazon[1]. Using rtl_433[2] it took no time at all to be able to receive what the sensor pods are sending over, it's really pretty cool!

I've also ordered some 433mhz wall/light switches, I hope to be able to receive/transmit and log all of that stuff, from what I can see, with a bit of tweaking you can integrate quite a few vendor's kit, and even integrate it with amazon Alexa for voice control.

Also started to 'explore' the spectrum using gqrx/gnuradio, I can see how it could become addictive :-)

[0]: http://www.ebay.co.uk/itm/152142033580

[1]: https://www.amazon.co.uk/gp/product/B00SIZZBDK [ whoops sorry went up in price, cyber week is over :( ]

[2]: http://www.rtl-sdr.com/using-rtl-sdr-rtl_433-decode-various-...

...You can totally listen in to HAMs using RTL-SDR. 10m, 6m, 2m, 1.25m, 70cm, 33cm, and 23cm all fall within the ranges mentioned.

As much as the HAM radio is traditionally an HF affair, the easiest to obtain licence (Technician class) actually pretty much only allows for VHF and UHF transmission. Not only that, but even among Generals and Extras, 10, 6, and 2 are very popular, especially for local communication (usually through a repeater), whereas HF is more commonly used for DXing.

You can even listen in to APRS, which is message stream that carries a lot of interesting data (mostly GPS stuff, but you'll catch a few text, email, or general messages through it every once in a while).

Yes that's about right. For technician's 2 meters and 70cm are by far the most popular. Just tune into a local repeater using a $50 handheld and you can chat all day. You can also use echolink and IRLP to chat world-wide via your local repeater or even connect to a repeater from your smartphone or PC.

I will leap as soon as I can get something like the https://xtrx.io/ with a mini PCIe interface or expresscard interface. I want to do the processing on my laptop without the limitation of a slow bus.

Also I would love some solution to nicely expose the ufl connectors. expresscard would seem a better choice to me that mini PCIe, to connect antennas on the side of the laptop or the back of the desktop when using a PCIe to expresscard adapter.

I could only dream of having this in my thinkpad able to do some cool mesh networking/APRS strait in my machine.

Think this bookmark of an intro to SDR is appropriate


How do Ham radio operators feel about SDR? (In general)

Do they think it's interesting and cool, or do they reject it for being different and inferior vs. more expensive and sensitive equipment?

It depends on who you ask.

The younger generation who have ECE/CS degrees love SDR. So does the military and the commercial/corporate radio industry. Some of the older generation frown upon SDR as voodoo magic with too many computer/network dependencies.

There is a general distrust (among older hams) of computer/network technology and especially anything that depends on a non-human (computer program) to modulate or demodulate a message.

For example, you can send Morse Code with a Carrier Wave and a simple switch (on and off) that you built from wood scraps and copper. You don't need an external thing (program, software, device, etc.) to do that for you. So there are less dependencies and in general it is much simpler to reason about and use. When the shit hits the fan, you want simple, reliable things.

The problem is, most humans don't know Morse Code. So they can't decode a message anymore. Thus the reliance on computers and software.

The efficiency gains are hard to argue with as well. You can do things with SDR that would be impossible or far too costly in hardware... Like resurrecting and communicating with a 36 year-old satellite http://www.rtl-sdr.com/rebooting-isee-3-usrp-software-define...

Pretty much. There's a lot of people doing cool stuff over the radio, but the conventional wisdom is that if it wouldn't work in an emergency, you can't rely on it. So while you can rely on computers somewhat, be prepared to jump on SSB/CW/FM in the worst cases (ie, you only have power for your radio, nothing else).

And relying on the internet for anything is an absolute no-no, at least among the emergency people.

Good SDRs have overwhelmingly better performance than all but the most expensive traditional superhet receivers. Even then, the exotic superhet receivers match rather than beat equivalent SDRs. Direct conversion receivers are exceptionally good at isolating weak signals that are close in frequency to strong signals, which is a particularly important trait in amateur radio operation. This is widely understood and largely uncontroversial.

Some amateur SDRs are essentially computer peripherals, but they increasingly look just like traditional transceivers. The Icom IC-7300 was the most talked-about transceiver of 2016; It operates like a traditional transceiver and costs about the same, but has exceptional receiver performance and a host of DSP-based bells and whistles. I expect that many Elecraft customers are completely unaware that they bought an SDR rather than an analog transceiver.

Personally, I would never go back to a superhet receiver. If you use a good SDR for half an hour, you're spoiled for life. Superhet receivers still have a place at the low-end of the market, but they're being slowly squeezed out.

Except for noise. Computers are noisy beasts with spurious radiation (birdies) from DC to UHF. Trying to run a sensitive receiver anywhere near a computer, means that you have to put up with an unholy amount of interference. And nothing in the design of the SDR can fix this.

Old-time hams would never have a computer in the same room as their radios because of this interference.

FWIW, I was designing and building my own SDRs ten years ago, and am currently running one of the high-end SDRs.

Are there SDRs where you "remote" the receiver and antenna? Or antenna and low-noise amplifier?

I found another site that reviews some units, and mentions the issues you noted: http://www.rtl-sdr.com/tag/sensitivity/

As above, I have a RF Space NetSDR+ which is connected to the computer via ethernet. So you can remote it via WiFi or any similar networking technique.

>Are there SDRs where you "remote" the receiver and antenna?

Yes. The FlexRadio and Apache Labs SDRs are equipped with ethernet. There are numerous standalone SDRs that can be used without a computer.

The rtl-sdr isn't really comparable to a proper radio. It is, after all, just a repurposed TV tuner. It's a great little toy for exploring radio, but the performance is exactly what you'd expect for a $20 dongle.

I've used rtl_tcp on a raspberry pi before, but that was mostly to get the receiver close to the transmitter (in which case I unhooked the antenna as well).

It all boils down to shielding. A Rigol spectrum analyzer will never have the performance of an HP due to the lack of EMI mitigation. I have a Flex 6500 with no noise issues due to the computer, then again my RX antenna is 100 feet away.

Yeah. But I don't think your Flex 6500 tunes above 70MHz, which is where the birdies from modern computers, modems, video cards, and high-end displays are worst.

I have a RF Space NetSDR+, and it's wonderful below about 30 Mhz, but there's a horrifying racket on 6m and 2m, and the FM broadcast band is pretty much unusable for weak signal work.

I'm playing with a optical link to the top of the hill behind my place. We shall see.

I'm surprised that the computer interference issue isn't more widely discussed. I suspect that it's because newbies have no idea of how much of the junk they see on their screens doesn't actually exist. Plus living in city environments they are so swamped by RFI that they think it is normal.

Just looking at the screen on the OP's web page makes me shudder. It's a whole mess of computer interference.

Does using a long usb extension cable offer any relief for these small tv-tuners? Inverse-square law and all?

There is no unity, which is good. Its just like the conversion from AM to SSB or incentive licensing or FM repeaters on 2M in the 70s or no-morse-code licensing or dstar or thru-hole to SMD or rtty to PSK31 or pretty much everything that's ever happened to ham radio ...

Never heard of it. That's crazy. I think its interesting or it'll be the death of the hobby (depending solely on if the participant is personally involved). Its the same stuff looked at differently. We've always all been a fan of that since its start. So obviously I've dropped the docs on me being eligible for QCWA and 3rd gen ham.

I will say as having fooled around with it, latency can sometimes be truly awful which has quite an impact on your normal half duplex operation, ops almost willfully don't understand scalability of the software with bandwidth or real time RTOS desires. Nothing funnier than a PC crash in the middle of a QSO. Ability to "do ham radio stuff" does not necessarily come with basic computer competence, although it often does...

There's a big impedance jump going from "I downloaded HPSDR and it worked" to "I pkg install'd gnuradio and made my own broadcast FM receiver and it worked". Its like using Scratch to write hello world vs porting Emacs to Haskell. The first time you get gnuradio to work its a rush. I imagine the jump from gnuradio to FPGA based boards is similar rush if you can do it. In my infinite spare time...

Generally favorably, most of the mid-high end HF rigs are about 50% SDR based anyway.

As far as I can tell the average user doesn't tinker with them much however.

The other side of this coin is the reason that high-end Receivers don't use SDR techniques in their front end is because the intermodulation characteristics of current A/D converters fall a long way short of conventional Superhet technology.

It will be a long time before an A/D converter chip can equal the performance of a high-level mixer. In fact it will probably never happen, as any advances will apply equally to both technologies.

But yes, I agree with you that few hams are building SDRs. This was my biggest disappointment with SDR. Five years ago I thought that SDRs would usher in a new area of home-brew, but it's evolved into yet another bunch of appliance operators who have little interest in writing the software. So sad, because it's such a fascinating field..

I ignore it as my escape from the daily grind of computing is digging the soldering iron out and building some very minimal kit out of only discrete components. There is something fascinating about extreme minimalism. The fact it transmits and receives is a byproduct of the journey. I'm unsociable enough to confirm a couple of contacts and then build something else.

I have no problem with SDR, it just doesn't interest me. You need a physical connection with the universe sometimes.

Ham here. WT1J. I chat with quite a few hams that run the entire age spectrum. Young and old are into SDR. The older hams use everything from super expensive commercial SDR's that come with rack mountable hardware to Ham Radio Deluxe. The younger hams use RTL-SDR and HackRF along with the pricey gear.

It really depends what you're doing. If you're contesting on RF for example, you'll find younger european hams that have HUGE budgets and build enormous multi-band yagi antenna towers and use very expensive SDR's to monitor the whole band and for excellent RX and signal discrimination capability.

Then you've got the SIGINT crowd that hang out at defcon and they'll generally use cheaper SDR's with bleeding edge software.

The HF band ragchew crowd tend to use boat anchors (old ham radios) with big amplifiers up to legal limit (1.5kw) and hang out especially on the lower frequencies like 7mhz, 3mhz and 1.8mhz. (40m, 80m, 160m in ham speak).

But generally once you get into radio you realize it's less about the base station and more about the antenna.

I just got my Ham license a couple weeks ago and picked up an updated version of the SDR dongle he's using a few days ago[0][1]. I've just started playing with it but even if you're not that into radio, it's worth $20 and a couple hours with to explore the invisible radio frequency world around us.

[0] Be sure to get one with a TCXO (Temperature Compensated Crystal Oscillator) which reduces frequency drift.

[1] His link actually goes to the newer, improved, Mini 2+.

I am interested in using a SDR for exploring the PHY in software. This includes having to sniff the GSM, bluetooth etc. I have a Nooelec SDR USB dongle but I am yet to experiment with it. I have used to listen to FM radio and that's it.

I am thinking of starting decoding FM radio by myself to understand the basics.I do not want to use GNU radio except for better understanding. I would like to learn and have an understanding of things from scratch so that I get an intuitive feel when I am developing sniffers. Is this a good way to start? I really love wireless protocols but have worked from link layer upwards and would love to understand wireless PHY implementations.

For those interested in ADSB#, I believe it was renamed ADSB Spy and is now included with SDR# available at airspy.com.

SDR# has a bit of a messy history and at one time was completely open source but was later closed after some dispute between the developers (to the best of my knowledge).

If you look hard enough you can still find the source code of the older versions of SDR# and ADSB# which is handy if you want to learn how they work, although its all C# .NET and there is some wild stuff going on to try and squeeze enough performance out of the code. This is where I started learning though being a .NET developer, and from there moved on to GNU Radio and now learning C++.

Fascinating subject.

I have been through a number of SDRs - Softrock, RTLSDR, HackRF, and more recently I have a Red Pitaya (http://redpitaya.com/) and am ICOM 7300 (http://www.icomamerica.com/en/products/amateur/hf/7300/defau...), which is a traditional looking radio with SDR at its core.

The flexibility offered by doing all the work in software is ground-breaking... really only limited by imagination.

Apparently antirez is a radio enthusiast.


Yes, and contrary to the article it has a google maps display, too. Try "./dump1090 --net --interactive" and open localhost:8080. Would be neat if it could query some API to find out the flight's origin and destination, etc (flightradar24 maybe? do they have an API?)

The web interface on the dump1090 fork shipped by FlightAware has links to lookup flight information for each aircraft:


Google started blocking the mechanism used by the dump1090 map display. It might work if you have used it previously but not for new users: http://discussions.flightaware.com/ads-b-flight-tracking-f21...

The FlightAware fork now uses an OpenLayers map instead: https://discussions.flightaware.com/ads-b-flight-tracking-f2...

Thanks for the link, that looks nice! I used dump1090 for the first time this weekend and simply ran it from localhost, it worked fine.

I look forward to seeing some end-user reviews of this device when it finally ships. I realise that there are plenty of pre-production reviews, but I believe that the final version will have broader matching networks, facilitating better sensitivity on HF and low VHF.

On Linux there's Cubic SDR (among others). http://cubicsdr.readthedocs.io/en/latest/

Make sure to use it with a supported device. The device in the article is supported I think (by the SoapyRTLSDR module).

I have a KiwiSDR going here on the HF band:

Also wrote a GNU Radio based scanner that will work with the SDR dongles:


Strange that he couldn't find ADSB# - I used it just a few months ago. Perhaps it's been merged with another repository. If he wants the zip I used for installation I can send it to him, just give me an email address.

The University of Twente SDR is pretty neat too: http://websdr.ewi.utwente.nl:8901/

Can't you decode OTA TV? I know GNU radio was doing that years ago, but not in real time yet. I would think that's possible now.

The cheap RTL_SDR dongles are actually sold as OTA TV tuners. It's how I watch TV.

Edit: Apparently I'm wrong, and the DVB-T decoding is actually done in hardware.


Nonetheless, you can receive TV with a higher-end SDR, ala https://medium.com/@rxseger/receiving-atsc-digital-televisio...

Snapshots depict sub -100dBm noise floor...on a $21 DVB-T receiver...mkay, sure.

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