Software-defined radio (SDR) is a radio communication system where components that have been traditionally implemented in hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by means of software on a personal computer or embedded system.
Experimenting with software defined radio used to be expensive, but now it is cheap. Nowadays it is very cheap to start experimenting with SDR. Most receivers use a variable-frequency oscillator, mixer, and filter to tune the desired signal to a common intermediate frequency or baseband, where it is then sampled by the analog-to-digital converter. Cheapest wide receiving range well working device is to use suitable DVB-T receiver stick (10-20 Euros/Dollars) and suitable software (very many alternatives, for example SDRsharp and Gnu Radio).
My article Software defined radio with USB DVB-T stick started the long list of SDR related postings. The newest postings now are Filter measurements with RF noise source and Antenna measurements with RF noise source.
432 Comments
Tomi Engdahl says:
https://qsl.net/vk4jzz/
Tomi Engdahl says:
What is the difference between SMA and 3.5mm connectors?
https://www.keysight.com/main/editorial.jspx?ckey=1000003325:epsg:faq&id=1000003325:epsg:faq&nid=-11143.0.00&lc=fin&cc=FI
SMA and 3.5mm are matable connectors. The SMA connector is an economical, lower performance interface. Mechanical tolerances, stability, temperature performance, SWR, repeatability, loss, and wear are all much better in the 3.5mm interface.
It is easy to see which type a connector is by looking at the material surrounding the center pin. SMA type will have a foam bead, while 3.5mm uses an air interface.
Due to cost, the 3.5mm connector is most often used in metrology applications, whereas the SMA connector is used for production. Because of this, it is not uncommon to see wear and tear degrade the SMA to the point of damaging mated connectors. Extra care must be used when mating SMA onto 3.5mm due to the cost of the 3.5mm connectors. Be sure to gauge SMA as well as 3.5mm to prevent protruding SMA pins from propagating damage.
Tomi Engdahl says:
An Upcycled Radio Telescope Brings the Universe Closer — on the Cheap
Built from salvaged and dumpster-dived parts, saveitforparts’ radio telescope is a low-cost entry point into the hobby.
https://www.hackster.io/news/an-upcycled-radio-telescope-brings-the-universe-closer-on-the-cheap-17e05572fd17
Tomi Engdahl says:
RF Signal Generator DDS AD9959 4-Ch 225MHz Arduino Shield © TAPR-OHL
DDS (Direct Digital Synthesis) Analog Devices AD9959 4-Synchronized Channels DDS Arduino Shield for Arduino MEGA 2560
https://create.arduino.cc/projecthub/GRA_AND_AFCH/rf-signal-generator-dds-ad9959-4-ch-225mhz-arduino-shield-89517b
Tomi Engdahl says:
Ray Ring’s High-Efficiency Amplifier Design Drives a QRP Ham Transmitter From a 4V Lithium Battery
PIC18-based transmitter design offers a claimed 75 percent efficiency using cheap, compact MOSFETs.
https://www.hackster.io/news/ray-ring-s-high-efficiency-amplifier-design-drives-a-qrp-ham-transmitter-from-a-4v-lithium-battery-538fc7d01ef0
Tomi Engdahl says:
NanoVNA To Test The Loss & Length Of Coax Cables by Jim W6LG YouTube Elmer for Ham Radio Basics
https://www.youtube.com/watch?v=R5iYuGLvlas&feature=youtu.be
Tomi Engdahl says:
The Latest Update to the Flipper Zero Lets It Flip-Flop Between Any of Its Three Frequency Bands
https://www.hackster.io/news/the-latest-update-to-the-flipper-zero-lets-it-flip-flop-between-any-of-its-three-frequency-bands-d42b08204738
The Flipper Zero is blazing through its campaign checklists, and has a few neat new tricks to tell us about!
Tomi Engdahl says:
https://hackaday.com/2021/01/20/a-one-transistor-ham-transmitter-anyone-can-build/
Tomi Engdahl says:
https://hackaday.com/2021/02/09/3d-printer-makes-ham-antenna-portable/
Tomi Engdahl says:
https://reference.digilentinc.com/reference/programmable-logic/b205mini/getting-started
Tomi Engdahl says:
https://www.edn.com/evaluating-reduced-size-emi-antennas-part-1/
Tomi Engdahl says:
https://interferencetechnology.com/diy-near-field-probes-preamplifiers/amp/
Tomi Engdahl says:
https://pysdr.org/content/intro.html
Tomi Engdahl says:
https://hackaday.com/2021/02/19/3d-print-your-next-antenna/
Tomi Engdahl says:
Ham Radio Forms a Planet-Sized Space Weather Sensor Network
For researchers who monitor the effects of solar activity on Earth’s atmosphere, telecommunications, and electrical utilities, amateur radio signals a golden age of crowdsourced science.
https://eos.org/features/ham-radio-forms-a-planet-sized-space-weather-sensor-network
Tomi Engdahl says:
New quantum receiver the first to detect entire radio frequency spectrum
https://phys.org/news/2021-02-quantum-entire-radio-frequency-spectrum.html
Tomi Engdahl says:
Review: Com-Power’s comb generator for conducted emissions
https://www.edn.com/review-com-powers-comb-generator-for-conducted-emissions/
Tomi Engdahl says:
https://hackaday.com/2021/02/26/spectrogram-drawing-for-fun-and-coding/
Tomi Engdahl says:
The Evolution of RF Signal-Observation Tools
https://www.mwrf.com/technologies/test-measurement/article/21155581/spectra-lab-the-evolution-of-rf-signalobservation-tools?oly_enc_id=7211D2691390C9R
Tools used to visualize RF signals have evolved over time from the spectrum analyzer to today’s RF recorders. However, each era’s tools have had limitations. This article shows how the modern approach builds on the best aspects of what’s come before.
Tomi Engdahl says:
Ka-band #ADCs and #DACs offer the potential to extend software-defined radio to software-defined microwave for #satellite communication
https://buff.ly/3va55cP
Tomi Engdahl says:
https://hackaday.com/2021/03/11/monitor-spacex-rocket-launches-with-software-defined-radio/
Tomi Engdahl says:
10kHz to 225MHz VFO/RF Generator with Si5351 – Version 2 © GPL3+
For use in DIY homebrew radio equipment such superheterodyne receivers, SDR, HAM QRP transceivers or RF generator.
https://create.arduino.cc/projecthub/CesarSound/10khz-to-225mhz-vfo-rf-generator-with-si5351-version-2-bfa619
Tomi Engdahl says:
How to Build an Airplane Tracker with Raspberry Pi
By Ryder Damen 21 hours ago
Project a map of current aircraft onto your ceiling.
https://www.tomshardware.com/how-to/raspberry-pi-airplane-tracker
Tomi Engdahl says:
River’s Educational Channel Controls an RF Fan and Light By “Abusing” the Raspberry Pi’s GPIO Pins
Using the rpitx software, River’s Raspberry Pi now controls his home’s lights and fans — and with only a low-pass filter and antenna.
https://www.hackster.io/news/river-s-educational-channel-controls-an-rf-fan-and-light-by-abusing-the-raspberry-pi-s-gpio-pins-910b452573e5
Tomi Engdahl says:
Web Pages (and More) Via Shortwave
https://hackaday.com/2021/03/13/web-pages-and-more-via-shortwave/
If you are a ham radio operator, the idea of sending pictures and data over voice channels is nothing new. Hams have lots of techniques for doing that and — not so long ago — even most data transmissions were over phone lines. However, now everyone can get in on the game thanks to the cheap availability of software-defined radio. Several commercial shortwave broadcasters are sending encoded data including images and even entire web pages. You can find out more at the Swradiogram website. You can also find step-by-step instructions.
https://swradiogram.net/
https://wiki.radioreference.com/index.php/Shortwave_Radiogram_Gateway
Tomi Engdahl says:
SDR DECODED
How Amateur Radio Fans Decoded SpaceX’s Telemetry & Engineering Video
https://m.youtube.com/watch?v=74_N163HyhA
Tomi Engdahl says:
Track aircraft with a Raspberry Pi
Explore the open skies with a Raspberry Pi, an inexpensive radio, and open source software.
https://opensource.com/article/21/3/tracking-flights-raspberry-pi
Tomi Engdahl says:
SDR-RTL DONGLE SHORTWAVE UP CONVERTER
https://m.youtube.com/watch?v=0xPFbU0Q7ek
Got an old gen 1 or 2 SDR-RTL Dongle in the drawer looking for something to do?
Tomi Engdahl says:
A homemade receiver for GPS & GLONASS satellites
http://s53mv.s56g.net/navsats/theory.html
Tomi Engdahl says:
Gabe Emerson Upcycles an Old TV Dish, Cardboard, and Foil Into a GOES Ground Station
https://www.hackster.io/news/gabe-emerson-upcycles-an-old-tv-dish-cardboard-and-foil-into-a-goes-ground-station-4001bf8e4c2c
Rather than turn to a commercial dish Emerson took an old DirecTV dish, some aluminum foil, and cardboard to pick up weather satellite data.
Gabe Emerson has turned a salvaged satellite TV dish into a receiver for GOES weather satellite data via a software-defined radio — using nothing more than upcycled cardboard and foil tape.
Emerson’s complete ground station includes a commodity laptop, the upcycled dish mounted to a pan-tilt-zoom (PTZ) system from an old security camera, a SAWbird low-noise amplifier (LNA), and a low-cost receive-only RTL-SDR software-defined radio capable of picking up a 1.700 MHz signal.
https://m.youtube.com/watch?v=EK8mFrxxSbY
Tomi Engdahl says:
https://hackaday.io/project/11056-waffle-a-pocket-sized-digital-radio-system
Tomi Engdahl says:
https://electronics.stackexchange.com/questions/38838/powering-an-led-from-rf-cellphone-signals
Tomi Engdahl says:
CELL PHONE RADIATION POWERS LED OR Mobile Detector
YouTube · Madni Mobiles
13 Feb 2019
https://youtu.be/LJ6wGqeYrj0
Tomi Engdahl says:
https://hackaday.com/2021/04/03/satellite-ground-station-upcycles-trash/
Tomi Engdahl says:
Next-generation replacement for the KerberosSDR adds a new radio and a range of improvements for direction-finding and passive radar.
RTL-SDR.com’s KrakenSDR Offers Five Software-Defined Radios in a Single, Passively-Cooled System
https://www.hackster.io/news/rtl-sdr-com-s-krakensdr-offers-five-software-defined-radios-in-a-single-passively-cooled-system-2a6c51ed2b4a
Next-generation replacement for the KerberosSDR adds a new radio and a range of improvements for direction-finding and passive radar.
RTL-SDR.com has confirmed that its KrakenSDR, a software-defined radio (SDR) combining five RTL-SDR receivers with a common clock in a single unit, is launching on Crowd Supply in the very near future.
“A coherent radio allows for very interesting applications, such as radio direction finding, passive radar, and beamforming,” the company writes of the project. “You might be interested in KrakenSDR if you are a ham radio operator participating in a fox hunt or monitoring repeater abuse, a professional RF engineer tracking down sources of noise and illegal broadcasts, an instructor demonstrating RF fundamentals, an amateur radio astronomer looking to increase image resolution, a search and rescue team locating a disaster victim, an owner of a very large property monitoring its use, or a conservation group tracking tagged animals outside of network coverage.”
Tomi Engdahl says:
https://hackaday.com/2021/04/06/fan-tastic-misuse-of-raspberry-pi-gpio/
https://riveducha.onfabrica.com/decode-wireless-signal-with-usb-tv-tuner
Tomi Engdahl says:
https://hackaday.com/2021/04/06/fan-tastic-misuse-of-raspberry-pi-gpio/
Tomi Engdahl says:
Fun While It Lasted, Falcon 9 Telemetry Now Encrypted
https://hackaday.com/2021/04/07/fun-while-it-lasted-falcon-9-telemetry-now-encrypted/
A few weeks back we brought word that Reddit users [derekcz] and [Xerbot] had managed to receive the 2232.5 MHz telemetry downlink from a Falcon 9 upper stage and pull out some interesting plain-text strings. With further software fiddling, the vehicle’s video streams were decoded, resulting in some absolutely breathtaking shots of the rocket and its payload from low Earth orbit.
Unfortunately, it looks like those heady days are now over, as [derekcz] reports the downlink from the latest Falcon 9 mission was nothing but intelligible noise. Since the hardware and software haven’t changed on his side, the only logical conclusion is that SpaceX wasn’t too happy about radio amateurs listening in on their rocket and decided to employ some form of encryption.
https://www.reddit.com/r/amateursatellites/comments/mm8fcz/spacex_vehicle_decoding_and_encryption_part_2_its/
Tomi Engdahl says:
RF Monitor
http://www.nd6t.com/bitx/RF%20Monitor.htm
In response to several requests for a VSWR and output power monitor I have developed a simple circuit to be easily added to the BITX (or any transceiver) and some software to get it working. I used a 20 dB directional coupler design, sometimes called a “Stockton bridge”, that is simple, broadband, and requires no adjustment.
Tomi Engdahl says:
Pseudonymous reverse engineer “Hash” has published a series of videos demonstrating how to capture data from wireless smart meters using a low-cost software-defined radio — and how to process that data to retrieve information including meter identification tags and outages.
Reverse Engineer Hash Tackles Smart Meter Monitoring — with an SDR and a Faraday Cage
https://www.hackster.io/news/reverse-engineer-hash-tackles-smart-meter-monitoring-with-an-sdr-and-a-faraday-cage-61573a1a96cf
RECESSIM’s Hash is currently working to reverse engineer smart meter mesh networks — and you can follow along on YouTube.
“who can access that information” turns out to be “Hash,” thanks to a low-cost USRP B200 software-defined radio (SDR), the GNU Radio software package, and some ingenuity — plus a series of meters acquired on the second-hand market and a Faraday cage testing area, so the meters can be allowed to transmit without fouling up anyone’s real-world energy readings.
In the video series, which is currently three parts long, Hash details how the smart meters set up a mesh network, how to receive their signals, how to process them to retrieve useful information, and tears down a few examples purchased from an online auction house.
https://m.youtube.com/channel/UCVa4o0P6xhhSDi3rgLm2SBw
https://wiki.recessim.com/view/Advanced_Metering_Infrastructure
Tomi Engdahl says:
https://hackaday.com/2021/04/15/the-50-ham-a-simple-wspr-beacon/
Tomi Engdahl says:
DIY SDR DSP Radio with Raspberry pi and RTLSDR dongle
Relatively cheap and simple to build modern SDR DSP radio “scanner” with big touch screen and rotary dial. Also, it can decode all digital radio signals.
Build instructions at:
https://youtu.be/Gtp3XfqgIfc
Tomi Engdahl says:
10 Watt 50 ohm 40 dB Attenuator
This 10 Watt 50 ohm Terminator (Dummy Load) provides 40 dB Attenuation for frequency and power measurements up to 200 MHz
https://www.qsl.net/kp4md/qrpload.htm
Tomi Engdahl says:
SpaceX Encrypts Falcon 9 Telemetry After Amateur Radio Operators Download Data
https://www.extremetech.com/extreme/321671-spacex-encrypts-falcon-9-telemetry-after-amateur-radio-operators-download-data?utm_campaign=trueAnthem%3A+Trending+Content&utm_medium=trueAnthem&utm_source=facebook
Tomi Engdahl says:
A Superheterodyne Receiver With A 74xx Twist
https://hackaday.com/2021/04/19/a-superheterodyne-receiver-with-a-74xx-twist/
In a world with software-defined radios and single-chip receivers, a superheterodyne shortwave radio might not exactly score high on the pizzazz scale. After all, people have been mixing, filtering, and demodulating RF signals for more than a century now, and the circuits that do the job best are pretty well characterized. But building the same receiver using none of the traditional superhet trappings? Now that’s something new.
In what [Micha] half-jokingly calls a “74xx-Defined Radio”, easily obtained discrete logic chips, along with some op-amps and a handful of simple components, take the place of the tuned LC circuits and ganged variable capacitors that grace a typical superhet receiver. [Micha] started by building an RF mixer out of a 74HC4051 analog multiplexer, which with the help of a 2N3904 phase splitter forms a switching mixer. The local oscillator relies on the voltage-controlled oscillator (VCO) in a 74HC4046 PLL
a 74xx-defined radio
https://acidbourbon.wordpress.com/2021/04/11/a-74xx-defined-radio/
I built a shortwave radio receiver from scratch using only cheap and easily available components, i.e. standard transistors, op-amps and 74xx logic chips. No typical radio parts – no coils, no variable capacitors, no exotic diodes. This project is easy to build and gives you a hands-on experience with radio technology which you won’t get from a fully integrated SDR.
For example: We have a radio station broadcasting at 10 MHz and we tune our local oscillator to 10.1 MHz, then the signal after the mixer will have a signal component oscillating at 10.1 MHz – 10 MHz = 100 kHz. The catch: The 100 kHz signal still has the same envelope function as the the 10 MHz AM station, i.e. the same audio information that was modulated onto the carrier. We basically only “tune down” the carrier wave, without changing the information that is carried. The beat frequency is often called IF – intermediate frequency.
Why do we do that? Because the lower the frequency of the signals, the easier it becomes to amplify and filter them! So in order to listen to our station we have to have a bandpass filter and filter out and amplify only the intermediate signal at (in our example) 100 kHz. Be aware that all the other stations have also been mixed with the local oscillator and have landed at some other intermediate frequency (that we don’t want to listen to). Because we already have a tunable local oscillator that determines which station lands where after the mixing, the IF band pass filter can be set to a fixed frequency (here 100 kHz).
The most humble mixer
Okay, so the heart of a heterodyne receiver is a RF mixer. I did some googling and found the classical implementations:
There is the famous diode ring mixer, which is technically identical to the famous ring modulator sound effect you might know from pop music. It relies on signal transformers to generate galvanically insulated input and output signals. It did not fit my design dogmas.
There is the Gilbert Cell mixer, a clever discrete analog circuit comprising six transistors to implement an analog multiplier. The Gilbert Cell, too, has differential signal inputs and outputs. Although six transistors look like an okay amount of complexity, the prospect of biasing all of them properly turned me off a bit. There is a very nice integrated Gilbert Cell IC, the NE612/SA612, which is sort of straightforward to use, but it is not really a convenient off-the-shelf component, that you find in every hobbyist distributor’s online shop. Also it is rather priceous (around 2€) and I was only able to get an SMD package.
I was reading some more about RF mixer theory and I came across this document from analog devices which seems to be a course about RF/IF circuits. In the theoretical introduction to RF mixers they discussed an ideal (switching) RF mixer
An ideal switching mixer
I have never viewed it like this before, but it sort of makes sense: Make two copies of the RF input signal with opposite polarities and switch between these two at a fixed frequency f_LO. This is effectively the same as multiplying (mixing) the RF input signal with a square wave of frequency f_LO. Well, mixing with a square wave is not exactly the same as mixing with a pure sine wave (because you also mix with the higher order harmonics of the local oscillator frequency) but for building our heterodyne short wave receiver it does not make that big of a difference.
It might not be the best mixer ever. But it was cheap and straightforward and seems to work up to 50 MHz until you begin to see noticeable injection loss. Over the entire short wave band (3-30 MHz) the mixer works with a gain of more or less exactly 1. I am impressed.
Okay, we have our mixer. Now we need a local oscillator that provides a stable rhythm by which to flick the switch. Desired frequency range: The entire shortwave band of 3-30 MHz. Luckily the toolbox of 74xx chips provides exactly what we are looking for! It is called 74HC4046 and it is a Phase Locked Loop (PLL) IC.
A caveat: Please do use an actual 74HC(T)4046 and not it’s older and slower cousin the MOS4046 (aka HEF4046, CD4046). I got good results with a Texas Instruments CD74HCT4046 (with a T). Funny enough I got bad results (too slow, too low frequency) with a Texas Instruments CD74HC4046 (without the T), even though the datasheets claimed both chips have similar performance.
Mixer – check, local oscillator – check, now we need a narrow filter! Remember, we want to pick out one narrow frequency band which we want amplified, all other mixing products which don’t land near our target frequency (here 100 kHz) shall be suppressed. Once again: no tuned LC circuits! So we go for a classic op amp band pass
Demodulator
Okay. After the IF filter we have a single 100 kHz sine wave with audio information modulated onto it. To get to the audio we “only” have to rectify the waveform and smoothen it a bit with a low-pass filter.
A lot of classical AM demodulator circuits that I found on the web use a germanium or Schottky diode as a half-wave rectifier (to cut away the negative half of the IF wave). They perform better than your regular silicon diodes, because they have a lower threshold voltage and thus can rectify smaller input signals. But what if we don’t have and don’t want to buy special magic radio diodes?
Fear not, there is a nice OpAmp circuit that goes by the name of “active half wave rectifier” and makes the use of special diodes obsolete.
Tomi Engdahl says:
The Evil Crow Is Ready To Cause Some RF Mayhem
https://hackaday.com/2021/04/27/the-evil-crow-is-ready-to-cause-some-rf-mayhem/
Tomi Engdahl says:
MIGOU “Hybrid Radio” Combines the Best of SDR and Hardware Radio for IoT Experimentation
https://www.hackster.io/news/migou-hybrid-radio-combines-the-best-of-sdr-and-hardware-radio-for-iot-experimentation-626fec212538
Designed to combine the best of high-power software defined radio and low-power hardware communications, MIGOU is impressive.
“I’m the main developer of the MIGOU platform. This platform uses the Microchip AT86RF215 transceiver (like TinySDR and iotSDR) and a Microchip SmartFusion2 flash-based FPGA SoC,” lead developer Ramiro Utrilla Gutiérrez writes in a submission to RTL-SDR.com. The particularity of our work is what we have called the hybrid radio approach, which proposes to provide low-resource devices with the ability to operate both as a current mote, using a hardware transceiver, and as an SDR system.”
“This is possible using only the AT86RF215 transceiver. With these capabilities, hybrid radio end-devices can exploit the SDR hardware flexibility for those sporadic tasks that strictly require it, and still benefit from the energy efficiency of hardware transceivers for all other tasks.”
The team has no plans to release the MIGOU as a commercial product, but is instead making it available as an open hardware project. Full details, including test results, schematics, bill of materials, and manufacturing files, are available on the project website.
http://elb105.com/migou/
Tomi Engdahl says:
Adam Łoboda Clones a 433MHz Garage Key with the Universal Radio Hacker and a Simple Arduino Sketch
https://www.hackster.io/news/adam-loboda-clones-a-433mhz-garage-key-with-the-universal-radio-hacker-and-a-simple-arduino-sketch-b583e4dd82ac
Rather than shelling out for an official copy, Łoboda opted to make his own — for less than $5 in parts.
Adam Łoboda needed a spare key for his remotely-openable garage door — and rather than buy one from the manufacturer, decided to clone his original using the tiny DigiSpark and an FS1000A transmitter alongside the Universal Radio Hacker (URH) software suite.
“My garage keys do not have any rolling code and encryption so the reverse engineering was fast and simple (something like two hours of work),” Łoboda explains in the introduction to his step-by-step tutorial video on the subject. “In the video I have used Linux PC with RTL SDR dongle installed and [the] set of tools Universal Radio Hacker to record and decode garage key fob transmission on [the] ISM band.”
“[A] Linux PC is not required, you can do all these steps on Windows PC or Mac (requirement is to have RTL-SDR dongle, URH and Arduino IDE installed). After decoding the signal I was able to replicate 1:1 set of pulses and pauses within my simple Arduino sketch.”
The replica transmitter is built around the Arduino-compatible DigiSpark development board and a low-cost FS1000A transmitter.
Tomi Engdahl says:
Michael Wiebusch Builds a Shortwave Receiver with a Difference: It’s Driven by 74xx Logic
Challenged to create a shortwave-based guitar effects pedal, Weibusch creates a “74xx-defined radio.”
https://www.hackster.io/news/michael-wiebusch-builds-a-shortwave-receiver-with-a-difference-it-s-driven-by-74xx-logic-8fb2c5350667
Tomi Engdahl says:
Technical Challenges in Building Repeatable Wi-Fi Testing
https://www.eetimes.com/technical-challenges-in-building-repeatable-wi-fi-testing/
Wi-Fi testing can present engineers with any number of challenges. With the Broadband Forum’s TR-398 Issue 2 testing, the primary focus of the work is around testing the access point (AP) devices under test (DUT) since they would be deployed in the field. This approach is taken to allow results and measurements to better correlate with how the DUT will perform in the field. In this testing scenario, creating a stable, well-defined, and controlled RF test environment is critical.
In the test setup we use at the University of New Hampshire InterOperability Lab (UNH-IOL), we control the RF environment of the DUT inside a semi-anechoic faraday chamber. The setup has equally important characteristics. First, the typical role of the faraday chamber is to prevent transmission of RF signals from inside to out of the chamber and vice versa. Second, the construction of the chamber with a radio absorbent material (RAM) lining helps to greatly reduce the reflection of the RF signals transmitted by the DUT. Added into the chamber is a number of near-field antennas, establishing the RF link with the DUT and the test setup. The semi-anechoic nature of the chamber ensures the signals captured by the antennas are the direct path (non-reflected) signal from the DUT. The same is also true in the reverse, with signals transmitted from the near field antenna also not being reflected inside the chamber.