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Software-defined radio

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See also: Ham Radio

This article is written for beginners, higher level material can be found on using the links below. This article is written primarily for use in The United States, under the jurisdiction of the FCC.

SDR# running in Windows 8

Software defined radio is the means of listening to radio waves using the combination of low level hardware, and software processing. While non-SDR techniques use physical hardware to demodulate FM and AM, In SDR raw data from the receiver gets sent into the USB port, and software is used to control the modulation. The only drawbacks of SDR are bandwidth constraints and processing. Since uncompressed unmodulated data is coming through a PC interface (USB mostly), a high speed connection and a computer with decent specs is required. A Single RTL-SDR May use up to 3.4MB/sec of USB bandwidth. CPU usage varies on the tuner software you use. Using software defined radio opposed to regular radio gives you the ability to see signals you would normally only hear. Your eyes can cover a very wide bandwidth of signals on a computer screen, while your ears can only comprehend a single spoken conversation at a time.

If you wish to experiment with SDRs before you build your own receiving station, you may be interested in the WebSDR project.

SDR models

SDRs come in many shapes in sizes, from the $8 RTL-SDR to $2000 USRP(with daughterboards) There are also many SDR's such as the HackRF, USRP, FunCube, etc. When you buy an SDR, you get what you pay for. More expensive SDR's are purpose built for transmitting, and can handle things such as GSM spoofing, where an unencrypted GSM tower can be simulated to capture SMS in a cleartext fashion.


Two common types of RTLSDRs

A great beginner SDR is the RTL-SDR It features a receive frequency range of 25-1700MHz, 2.048MHz bandwidth and is priced low enough that beginners can see whether radio is a hobby they want to get into(R820T model).

RTL-SDRs come with various tuner chips, the most widely used tuner is the R820T, with the E4000 coming in a close second. The E4000 is a discontinued model and is usually priced higher than R820T models. R820T tuners are generally better for reception of trunked radio systems (digital police radio).

The RTLSDR began its life as a DVB-T OTA TV tuner (a tv tuner for europe). China began bulk manufacturing these TV tuners for the european market. It was later found that a hacked driver could be installed that let the user use the dongle as an SDR.

Currently R820T RTLSDRs can be bought for as low as $8 on eBay with free shipping R820Ts on eBay

There is a minor update of RTLSDR named R820T2, which is still compatible with the old one and improve performances of reception. [1]

Funcube Dongle

As part of AMSAT-UK’s FUNcube satellite project, the FUNcube Dongle is the “ground segment”, or a radio receiver designed to allow anyone to try their hand at reception of satellites like FUNcube anywhere on Earth as part of a global educational collaboration project collecting information from space. The original has a frequency range of 64MHz to 1.7GHz. The Pro + has a frequency range of 150kHz to 240 MHz and 420MHz to 1.9GHz.


The HackRF is a DARPA funded project, it aims to provide innovation and research in the SDR field. It's much more powerful than the RTL-SDR, much more sensitive, covers a greater frequency range. The HackRF starts at $300, has both RX and TX functionality from 30 MHz to 6 GHz, bandwidth of 20MHz. During DARPA funding, beta units, the HackRF Jawbreaker were given to hundreds of testers for free. The production version was funded through KickStarter. The campaign was successfully funded on September 4th 2013 and is currently available for purchase

TX functionality on the HackRF is limited in power because RF theory says you must have an antenna length that corresponds to the wavelength you are sending. For example sending 5Watts over the wrong size antenna will cause a buildup of standing waves, and may fry the transmitter. To prevent this, the antenna build into the breadboard of the HackRF is only designed for ultra low power, good enough for experimentation, but not for serious broadcasting.


SDR's have endless possibilities. Because all demodulation is done in software, no new hardware is needed to decode, for example, a new digital voice mode. All the programmer has to do is write code to convert the raw samples from USB to the correct format data.


Full article

  • I hear FM radio stations from 25-86MHz
    • That is your SDR being overloaded by FM broadcast signal, try installing an FM trap or lowering your gain.
  • I see signals that go the opposite way from where i'm dragging
    • That is interference, not an actual signal, lowering your gain solves this
  • I see a spike in the middle of the screen
    • This is the DC spike, a side effect of the RTLSDR, check the box that says Correct IQ
  • I see lots of interference in the 100-150MHz band
    • It's common to see interference when the antenna is placed near networking equipment, wireless routers and computers. Move the antenna away from these sources to lower the effect

Modulation guide

Some frequencies are used by only one type of modulation. For example 88-108 is broadcast FM, the modulation used is WFM. Using the correct modulation can help you decide what type of signals you are listening to. The table below lists the most common modulations.

The radio spectrum of frequencies is divided into bands with conventional names designated by the International Telecommunications Union (ITU):

Frequency Range Wavelength
ITU designation IEEE bands
Full name Abbreviation<ref name="beasley">{{#invoke:citation/CS1|citation CitationClass=book


3–30 Hz 105–104 km Extremely low frequency ELF N/A
30–300 Hz 104–103 km Super low frequency SLF N/A
300–3000 Hz 103–100 km Ultra low frequency ULF N/A
3–30 kHz 100–10 km Very low frequency VLF N/A
30–300 kHz 10–1 km Low frequency LF N/A
300 kHz – 3 MHz 1 km – 100 m Medium frequency MF N/A
3–30 MHz 100–10 m High frequency HF HF
30–300 MHz 10–1 m Very high frequency VHF VHF
300 MHz – 3 GHz 1 m – 10 cm Ultra high frequency UHF UHF, L, S
3–30 GHz 10–1 cm Super high frequency SHF S, C, X, Ku, K, Ka
30–300 GHz 1 cm – 1 mm Extremely high frequency EHF Ka, V, W, mm
300 GHz – 3 THz 1 mm – 0.1 mm Tremendously high frequency THF N/A

Frequencies of 1 GHz and above are conventionally called microwave,<ref name="Kumar">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> while frequencies of 30 GHz and above are designated millimeter wave. More detailed band designations are given by the standard IEEE letter- band frequency designations<ref name=ieee>IEEE Std 521-2002 Standard Letter Designations for Radar-Frequency Bands Template:Webarchive, Institute of Electrical and Electronics Engineers, 2002. (Convenience copy at National Academies Press.) </ref> and the EU/NATO frequency designations.<ref name="BelovSmolskiy2012">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref>

Getting Started with RTLSDR

RTLSDR is a project made by a Finn. After discovering that the common Realtek TV-Tuner for sale to receive broadcast television on the computer is simply a complex SDR, a project was started to create a simple hack that made it able to work as a fully defined SDR with a wide-range of frequencies. You can find many RTLSDR boards for sale. Keep in mind that these are lower-end and often will not have the ability to transieve, and only receive.

AM vs FM

AM was once the most common bandwidth for consumer radio. In the early 1980's, FM took the world by storm due to its higher quality. For you, this is what you need to know.

AM requires more power to receive, but less power to transmit. It also uses a higher frequency-range than FM. FM is low-power and higher quality audio. AM is wholly analog, while FM has the ability to support Digital Audio. This is due to AM having a lower bandwidth, while FM having a higher bandwidth.

FM has the ability to transmit near without reduction of audio quality. AM has the ability to transmit far but can be obstructed easily with natural developments such as mountains. The farther signals on AM can still be received but faintly.

If you tune your conventional radio, you can find the type of quality on AM is significantly poorer than those on FM, even if the station is nearby.

If you want far range, then AM is your go-to as it can be received and heard even through static. If quality of audio is what you want, while not worrying about distance, then FM is what you want.

There are two new modes of AM you may discover, one being LSB and the other being USB. Many ham broadcasters have begun using these modes as they require less power. In LSB mode (short for Lower Side-Band) the signal is halved and the signal broadcasts on only the first-half of the wave. The second, Upper Side-Band, broadcasts on (you guessed it) the second-half of the band. You can still receive these signals in regular AM mode but it may sound garbled without proper fine-tuning.

Slow-Scan Television

An example of a SSTV broadcast, the Apollo 11 Lunar Module
An example of a SSTV broadcast, the Apollo 11 Lunar Module

Slow-Scan Television (or SSTV) is a method of television reception that ham radio users use to share single images. The images are encoded in one of several formats, which are broadcast as audio to a transcoder. The International Space Station transmits SSTV broadcasts on a monthly basis. It was also used on the Apollo 11 mission to send early images back to Earth (if you believe it happened). SSTV is most commonly used to transmit QSL cards identifying the station.

An example of a SSTV calling-Card (Known as a QSL Card) from station H52LRA-599
An example of a SSTV calling-Card (Known as a QSL Card) from station H52LRA-599 (fictional)

You may be interested in sending and receiving SSTV broadcasts. They are allowed on all frequencies except CW-Only. The currently standardized encoding formats are AVT, Martin, Robot, and Scottie, all having differing picture qualities. Modern SSTV software has decoding software for all.

Points of interest

  • Points of interest are bolded

MF(Shortwave Radio)

An upconverter is required to listen to these frequencies. Shortwave bounces off the Ionosphere and has the ability to be internationally received with minimal difficulty. Shortwave includes the whole of the HF band (3-30MHz), and generally extends from 1.7–30 MHz (176.3–10.0 m); from the high end of the medium frequency band (MF) just above the mediumwave AM broadcast band, to the end of the HF band. Some commercial services, such as the BBC, broadcast on Shortwave. The NVS Time Signal is also broadcast on this band. Some frequencies follow:

  • 60 kHz: Universal time signal (UT-1).
  • 135 – 138 kHz: LF Amateur Band.
  • 472 – 479 kHz: MF Amateur Band.
  • 490 – 518 kHz: Navtex (Weather and Warning messages for ships.).

NOTE : Time signals can also be found on 2.5, 5, 10, 15, 20 & 25 MHz on the HF band. Not all time signals can be heard in all time zones.

Citizens Band (26.965 MHz-27.405 MHz)

Citizen Band was once the most common form of non-licensed communication between persons (thanks to the film Smokey and the Bandit), but its own popularity led to its demise as the more people on it, the more congested it became. It is primarily used by Truckers and Taxi-Drivers now, but that doesn't stop you from using it! The FCC Rules state that you are supposed to announce yourself as K-Your first and last initial, followed by your zipcode (example: K-RMS-74629) but no one in the history of CB radio followed these rules. Rather, they chose to use nicknames such as "Rubberduck".

Citizens Band is not considered true Ham Radio, and other radio amateurs laugh at persons who use CB exclusively. CB has support for AM (old-style) as well as SSB and LSB modes (New-Style).

  • Channel 1 - 26.965 MHz: Used by truckers in eastern USA & Canadian maritime provinces
  • Channel 2 - 26.975 MHz: No special purpose
  • Channel 3 - 26.985 MHz: No special purpose
  • Channel 4 - 27.005 MHz: Often used for 4x4's/ Off-roading
  • Channel 5 - 27.015 MHz: No special purpose
  • Channel 6 - 27.025 MHz: Considered the Super Bowl channel. This has nothing to do with the football Super bowl.
  • Channel 7 - 27.035 MHz: No special purpose
  • Channel 8 - 27.055 MHz: No special purpose
  • Channel 9 - 27.065 MHz: Emergency communications or traveler assistance (FCC and Highway Patrol Monitored)
  • Channel 10 - 27.075 MHz: Often used by truckers for regional roads
  • Channel 11 - 27.085 MHz: No special purpose
  • Channel 12 - 27.105 MHz: No special purpose
  • Channel 13 - 27.115 MHz: Considered the Marine/RV channel
  • Channel 14 - 27.125 MHz: Commonly included transmit/receive crystal in many vintage walkie-talkies
  • Channel 15 - 27.135 MHz: No special purpose
  • Channel 16 - 27.155 MHz: No special purpose
  • Channel 17 - 27.165 MHz: Used by truckers on North/Southbound Highways on the west coast (primarily I-5)
  • Channel 18 - 27.175 MHz: No special purpose
  • Channel 19 - 27.185 MHz: Highway trucker channel (Notable as being the center frequency of the band.)
  • Channel 20 - 27.205 MHz: No special purpose
  • Channel 21 - 27.215 MHz: No special purpose
  • Channel 22 - 27.225 MHz: No special purpose
  • Channel 23 - 27.255 MHz: No special purpose
  • Channel 24 - 27.235 MHz: No special purpose
  • Channel 25 - 27.245 MHz: No special purpose
  • Channel 26 - 27.265 MHz: No special purpose
  • Channel 27 - 27.275 MHz: No special purpose
  • Channel 28 - 27.285 MHz: No special purpose
  • Channel 29 - 27.295 MHz: No special purpose
  • Channel 30 - 27.305 MHz: Depending on local needs, channels numbered above 30 or 35 are generally used with SSB operation.
  • Channel 31 - 27.315 MHz: No special purpose
  • Channel 32 - 27.325 MHz: No special purpose
  • Channel 33 - 27.335 MHz: No special purpose
  • Channel 34 - 27.345 MHz: No special purpose
  • Channel 35 - 27.355 MHz: No special purpose
  • Channel 36 - 27.365 MHz: No special purpose
  • Channel 37 - 27.375 MHz: No special purpose
  • Channel 38 - 27.385 MHz: SSB calling channel, LSB mode
  • Channel 39 - 27.395 MHz: No special purpose
  • Channel 40 - 27.405 MHz: No special purpose


  • 30–46 MHz: Licensed 2-way land mobile communication.
  • 30–88 MHz: Military VHF-FM, including SINCGARS
  • 43–50 MHz: Cordless telephones, 49 MHz FM walkie-talkies and radio controlled toys, and mixed 2-way mobile communication. The FM broadcast band originally operated here (42-50 MHz) before moving to 88-108 MHz.
  • 50–54 MHz: Amateur radio 6 meters band
  • 54-72 and 76-88 MHz TV channels 2 through 6 (VHF-Lo), known as "Band I" internationally; some DTV stations will appear here. See North American broadcast television frequencies
  • 72–76 MHz: Radio controlled models, industrial remote control, and other devices. Model aircraft operate on 72 MHz while surface models operate on 75 MHz in the USA and Canada, air navigation beacons 74.8-75.2 MHz.
  • 87.5–108 MHz: FM radio broadcasting (87.5–91.9 non-commercial, 92–108 commercial in the United States) (known as "Band II" internationally)
  • 108–118 MHz: Air navigation beacons VOR/DME
  • 118–137 MHz: Airband for air traffic control, AM, 121.5 MHz is an emergency frequency
  • 137-138 Space research, space operations, meteorological satellite
  • 138–144 MHz: Land mobile, auxiliary civil services, satellite, space research, and other miscellaneous services
  • 144–148 MHz: Amateur radio 2 Meters band
  • 148-150 MHz: Land mobile, fixed, satellite
  • 150–156 MHz: "VHF business band," public safety, the unlicensed Multi-Use Radio Service (MURS), and other 2-way land mobile, FM
  • 156–158 MHz VHF Marine Radio; narrow band FM, 156.8 MHz (Channel 16) is the maritime emergency and contact frequency.
  • 159.81-161.565 MHz Railways 159.81-160.2 are railroads in Canada only and is used by trucking companies in the U.S.
  • 160.6-162 Wireless microphones and TV/FM broadcast remote pickup
  • 162.40–162.55: NOAA Weather Stations, narrowband FM
  • 174-216 MHz television channels 7 - 13 (VHF-Hi), known as "Band III" internationally. A number of DTV channels have begun broadcasting here, especially many of the stations which were assigned to these channels for previous analog operation.
  • 174–216 MHz: professional wireless microphones (low power, certain exact frequencies only)
  • 216–222 MHz: land mobile, fixed, maritime mobile,
  • 222–225 MHz: 1.25 meters (US) (Canada 219-220, 222-225 MHz) amateur radio


  • 225–420 MHz: Government use, including meteorology, military aviation, and federal two-way use
  • 420–450 MHz: Government radiolocation and amateur radio (70 cm band)
  • 432.000 – 432.025 MHz: Most Commonly used frequency for Moonbounce (Bouncing signals off of the Moon for DX)
  • 433 MHz: Short range consumer devices including automotive, alarm systems, home automation, temperature sensors
  • 450–470 MHz: UHF business band, General Mobile Radio Service, and Family Radio Service 2-way "walkie-talkies", public safety
  • 470–512 MHz: TV channels 14–20 (also shared for land mobile 2-way radio use in some areas)
  • 512–698 MHz: TV channels 21–51 (channel 37 used for radio astronomy)
  • 698–806 MHz: Was auctioned in March 2008; bidders got full use after the transition to digital TV was completed on June 12, 2009 (formerly UHF TV channels 52–69)
  • 806–824 MHz: Public safety and commercial 2-way (formerly TV channels 70–72)
  • 824–851 MHz: Cellular A & B franchises, terminal (mobile phone) (formerly TV channels 73–77)
  • 851–869 MHz: Public safety and commercial 2-way (formerly TV channels 77–80)
  • 869–896 MHz: Cellular A & B franchises, base station (formerly TV channels 80–83)
  • 902–928 MHz: ISM band, amateur radio (33 cm band), cordless phones and stereo, radio-frequency identification, datalinks
  • 929–930 MHz: Pagers
  • 931–932 MHz: Pagers
  • 935–941 MHz: Commercial 2-way radio
  • 941–960 MHz: Mixed studio-transmitter links, SCADA, other.
  • 960–1215 MHz: Aeronautical Radionavigation
  • 1090 MHz: ADS-B Commercial airplane beacons
  • 1240–1300 MHz: Amateur radio (23 cm band)
  • 1452–1492 MHz: Military use (therefore not available for Digital Audio Broadcasting, unlike Canada/Europe)
  • 1710–1755 MHz: AWS mobile phone uplink (UL) Operating Band
  • 1850–1910 MHz: PCS mobile phone—order is A, D, B, E, F, C blocks. A, B, C = 15 MHz; D, E, F = 5 MHz
  • 1920–1930 MHz: DECT Cordless telephone
  • 1930–1990 MHz: PCS base stations—order is A, D, B, E, F, C blocks. A, B, C = 15 MHz; D, E, F = 5 MHz
  • 2110–2155 MHz: AWS mobile phone downlink (DL) Operating Band
  • 2300–2310 MHz: Amateur radio (13 cm band, lower segment)
  • 2310–2360 MHz: Satellite radio (Sirius and XM)
  • 2390–2450 MHz: Amateur radio (13 cm band, upper segment)
  • 2400–2483.5 MHz: ISM, IEEE 802.11, 802.11b, 802.11g, 802.11n Wireless LAN, IEEE 802.15.4-2006, Bluetooth, Radio-controlled aircraft, Microwave oven, ZigBee


Upconverters allow you to listen to HF bands with a converter box and your existing RTL hardware. Normally this wouldn't be possible because the lower limit of an RTL-SDR is 24MHz. This allows you to receive AM, mediumwave, longwave, CB, and shortwave radio on the cheap.

How upconverters work

Upconverters essentially resample everything from 0-50MHz, and retransmit it, using low power, at 100-150MHz.

So for example, if you wanted to listen to a channel at 7MHz, and your offset is +100MHz, you would tune your SDR to 107Mz to listen to it. This math calculation is made easier by the offset feature found in many different types of SDR software. So once you set your offset in the program you are using, you don't need to think about adding 100MHz.

Radio signals on HF are very small, so although 0-50Mz doesn't sound like it would have lots of signals and stations inside it, they contain a majority of advanced ham radio operators, worldwide news networks including North Korea and Russia, as well as stations like the buzzer(UVB-76)


Software is available for Windows, GNU/Linux and OSX.

SDR Software (The stuff you listen with)

Software Supported Modes OS Cost/License Features/Limitations
SDR Touch AM, WFM, NFM, SSB, DSB, CW Android Freemium, Closed supports RTL_TCP and USB OTG, FFT times out after 30 seconds on free version
glSDR AM, WFM, NFM, SSB, DSB, CW, DIG(?), SPEC(?), SAM(?), DRM Android Free, GNU/GPL UI for ghpsdr servers
SDR-Radio Windows Free, Closed Feature packed, harder to get set up
SDR-Radio v2 Windows Free possibly paid out of beta, Closed Beta, Feature packed, supports RTL_TCP
SDR# AM, WFM, NFM, LSB, DSB, USB, CW Windows Free, MS-RSL/MIT Most popular and easiest to use
SDR-J DAB Win,Lin Free, GNU/GPL2 Digital Radio app (dabstick-radio in the AUR)
HD-SDR Windows Free, Closed
PowerSDR Windows Free, GNU/GPL Does not support RTL-SDR
GQRX AM, WFM, NFM, SSB, CW, AFSK1200 Win,Lin,Mac Free, GNU/GPL Runs on GNU Radio
GNU Radio Win,Lin,Mac Free, GNU/GPL Tools to build your own receiver. Versatile, larger learning curve.
Linrad Win,Lin,Mac Free, OSS
Studio1 Paid,Closed Source Formerly WRPLUS

Plugins for SDR#

Purpose Software Cost/License
Scan and log various frequencies Frequency Manager + Scanner plugin Free, OSS
Tune SDR# to the approprate voice channel Trunking plugin for SDR# Free, OSS

HF decoders (shortwave 0-30MHz)

Purpose Mode Software OS Cost/License Common Frequency
Digital decoding on HF CW, Domino, Hell, MFSK, PSK, OLIVIA, RTTY, Throb, Thor, WWV Fldigi Win,Lin,Mac Free, GNU/GPL 0-30MHz AM
HF amateur radio codec FreeDV, Codec2 FreeDV Win,Lin,Mac Free, GNU/GPL 0-30MHz AM
Digital broadcast shortwave DRM Dream Win,Lin,Mac Free, GNU/GPL 0-30MHz AM
Sending images over HF, commonly QSLs Robot, AVT, Scottie, Martin MMSSTV Windows Free, OSS 0-30MHz AM

ADS-B Airplane tracking software

Purpose Mode Software OS Cost/License Frequency
Plane-spotting, decoding ADS-B ADS-B ADSB# Windows Free, MIT 1090MHz AM
Decodes ADSB# data + Plot planes on a map AVR-BEAST VirtualRadar Server Windows Free, OSS 1090MHz AM
Alternative to ADS-B# ADS-B RTL1090 Windows Free, Closed 1090MHz AM
Decodes ADSB (S Mode) ADS-B Dump1090 Linux Free, BSD 1090MHz AM

APRS (like GPS, hams use it)

Purpose Mode Software OS Cost/License Frequency
Amateur packet radio incl. GPS location AX.25, AFSK, APRS AFSK1200 Win,Lin,Mac Free, GNU/GPL 144-145MHz FM, Varies

Trunked Radio (police fire and EMS in urban areas)

Purpose Mode Software OS Cost/License Frequency
Decode control channel data P25, EDACS48, EDACS96, Motorola, MPT1327 Unitrunker Windows Free, Closed Usually 400MHz, 760,860,933MHz FM
Decode control channel data LTR, MPT1327, Passport, P-25 Phase 1, Fleetsync II, LJ1200, MDC-1200, Tait 1200 SDRTrunk Win,Lin,Mac Free, GPL3 Any
Decode digital voice P25p1, ProVoice, X2-TDMA, DMR/MOTOTRBO, NXDN DSD Win,Lin,Mac Free, BSD Usually 400MHz, 760,860,933MHz FM
Decode LTR data LTR LTR Analyzer Windows Free, Unknown ~400-500MHz

NOAA weather satellite

Purpose Mode Software OS Cost/License Frequency Other
Decode NOAA WX images APT, WEFAX WxtoIMG Win,Lin,Mac Freemium, Closed ~137MHz FM Track Live
Decode NOAA Satellite APT Atpdec Linux Free, GPL ~137MHz FM

Television (non digital kind)

Purpose Mode Software OS Cost/License Frequency
Analog television decoder, no sound NTSC, PAL TVSharp Windows Free, OSS Any AM (including ISM security cameras)


Purpose Mode Software OS Cost/License Frequency
Decode various pager formats POCSAG, FLEX PDW Windows Free, GNU/GPL 35-45MHz, 152-160MHz, 455-463MHz, 928-932MHz FM
Decode various including pager POCSAG multimon-ng Linux Free, GPL various

Audio routing software

Purpose Software Cost/License
Routes audio from the receiver to a decoder software above Virtual Audio Cable Shareware, Closed $35
Routes audio from the receiver to a decoder software above VB Audio Cable Donationware, Closed ($5 donation gets you more cables)


The WebSDR project allows ham radio operators to put their station on the internet and allow persons to tune the radio and receive what the host-station can receive. Different stations have different receiving capabilities, so not all WebSDR websites have the same capabilities. It is an international project. You can find more information, as well as the list of participating WebSDRs here.

Video Tutorials

SDRSharp (Windows)

GnuRadio (Linux)

Trunking police scanner (Windows)

Other SDR resources

/r/RTLSDR on Reddit

Scanner centric SDR discussion forums on RadioReference

FCC website, every RF transmitter registered registered with the FCC resides in this database

An online SDR receiver that covers the HF band (shortwave, worldwide transmissions)

2011 US frequency allocation chart PDF

/hamradio/ on 8chan

/ham/ on 4chan, with a FAQ