Over on his YouTube channel oh2ftg has uploaded two new RTL-SDR related videos. In the first video he does a tear down on the stock standard antennas that are supplied with most cheap RTL-SDR units. He finds that most are just a simple design, with the center conductor of the coax soldered to the whip, and the shield pinched between a metal plate and the base.
In his second video he measures the stock antennas on a Vector Network Analyzer (VNA). He places the antennas on a reasonably sized ground plane and finds that the antennas are as expected and pretuned to the DVB-T TV band at around 500 – 600 MHz.
Generally the included antennas are okay for receiving strong signals but we recommend getting yourself an outdoor discone antenna, or building a planar disk (pdf) for more serious scanning.
Over on YouTube user Veryokay has uploaded a video showing how he was able to receive WSPR (Weak Signal Propagation Report) signals at 14 MHz with his direct sampling modified RTL-SDR. WSPR is a HF mode designed to be received even if the signal is very weak. It is used to help determine radio propagation conditions. Direct sampling mode allows you to receive HF signals on an RTL-SDR without the need for an upconverter, but it is more difficult to implement and get good results with. To get the best results Veryokay built an add on PCB that fits onto the RTL-SDR which contains and LNA and single ended to differential operational amplifier to amplify and get correct impedance matching on the input.
His video mainly shows how to calibrate the receiver correctly to receive WSPR as incorrect calibration is the most common error when trying to receive WSPR for the first time. In the video he also explains that he is transmitting WSPR himself using his Raspberry Pi and a QRPi WSPR filter shield for use with Rpitx.
Receiving WSPR with the RTL-SDR in direct sampling mode and WSPR-X.
Receiving WSPR mode at 20m with RTL-SDR dongle in direct sampling
By cooling the dongle, and especially the R820T chip, Nobu writes that he sees improved ADS-B decoding performance as his range is increased. Without cooling the R820T chip can get quite hot and causes failing reception at around 1.5 GHz. Passive cooling is usually enough to fix reception at those higher frequencies, but active cooling via a fan can take it further and actually improve sensitivity slightly.
To add to his post, we suspect that the sensitivity of the R820T/2 front end reduces by about 0.5 dB at most when it heats up (after a few seconds), so forced air cooling should be able to improve sensitivity by about this much.
The LimeSDR is a $299 USD software defined radio that has RX and TX capabilities, a tuning range of 100 kHz – 3.8 GHz, a 12 bit ADC and up to 61.44 MHz worth of bandwidth. It is currently seeking crowdfunding over at CrowdSupply.com, and there are still 170 early bird units available at a lower price of $249 USD. The funding campaign ends in 14 days at the time of this post.
The IF Average tool is a RTL-SDR compatible plugin for SDR# which allows you to plot an average of the current spectrum shown in SDR#. This is especially useful for radio astronomers who often need to average the spectrum for a long time in order to get a good plot of the Hydrogen Line. Recently the plugin was updated to support newer versions of SDR# and to upgrade some features. Daniel Kaminski, the author of the plugin writes:
I used ultrafast FFT which works on 4k to 512k bit space. With this plugin it is possible to average up to 64000000 samples in real time. XNA allows to shows the calculation results in real time.
To install the plugin you will need to install the XNA Framework 4.0 Redistributable first. Then copy the plugin files over to the SDR# folder and add the “magicline” to the SDR# Plugins.xml file.
Back in April we posted about how KD0CQ found that he could receive signals up to 4.5 GHz with an RTL-SDR by using a $5 downconverter for DirecTV called the SUP-2400. The RTL-SDR can only receive up to a maximum frequency of about 1.7 GHz, but the SUP-2400 downconverter can be modified to convert frequencies at around 2.4 GHz down into a range receivable by the RTL-SDR.
When we first posted the story the instructions for modifying the SUP-2400 to use as a downconverter weren’t uploaded yet, but they are now. The modification requires decent soldering skills as it involves desoldering a few small SMD components and bridging some points with wires.
Over on YouTube user T3CHNOTURK has uploaded a video showing the downconverter in action. With the SUP-2400 downconverter and RTL-SDR he is able to receive some WiFi at 2.447 GHz as well as signals from a wireless keyboard at 2.465 GHz
RTLSDR Receiveing wifi & 2.4 ghz ism band with moded SUP-2400 Downconverter
The Airspy Mini is a recently released $99 USD software defined radio with a tuning range of 24 MHz to 1800 MHz, 12-bit ADC and up to 6 MHz of bandwidth. The Mini is the younger brother of the $199 USD Airspy R2, but despite the $100 USD price difference, both units are very similar, which makes the Mini a very attractive option. The idea is that the Mini is the cheaper version for those who do not need the more advanced features of the R2.
In a previous review we compared the Airspy R2 with the SDRplay RSP and the HackRF. In those tests we found that the Airspy had the best overall RX performance out of the three as it experienced the least amount of overload and had the most dynamic range. The SDRplay RSP was the main competitor in performance to the Airspy R2, and was found to be more sensitive due to its built in LNA. But the RSP experienced overloading and imaging problems much easier. With an external LNA powered by its bias tee, the Airspy gained a similar sensitivity and still had very good dynamic range. The main downside to the Airspy R2 was its higher cost compared to the $149 USD SDRplay RSP, and needing to fork even more for the $50 USD SpyVerter if you want to listen to HF signals.
In this review we'll compare the difference between the R2 and Mini, and also see if the cheaper Airspy Mini ($99 USD), or Airspy Mini + SpyVerter combo ($149 USD) can compete in this lower price range.
Difference Between the Mini and R2
Airspy Mini
Airspy R2
Price
$99 USD
$199 USD
Tuning Range
24 - 1800 MHz
24 - 1800 MHz
ADC Bits
12
12
Maximum Bandwidth (Alias Free Usable)
6 MHz (5 MHz)
10 MHz (9 MHz)
Extras
Bias Tee
Bias Tee, External clock input, Multiple expansion headers
Dimensions (Including USB and SMA ports)
7.7 x 2.6 x 1 cm
6.4 x 2.5 x 3.9 cm
Weight
21 g
65 g
Right now the "early bird" price of the Mini is $99 USD. We are unsure if this price will go up in the future.
The external design between the two units is different. The Mini comes in a USB dongle form factor which is very similar to a standard RTL-SDR, whilst the R2 comes in a larger box with a female Micro USB input. In our tests this metal enclosure appears to provide good shielding from strong signals. One thing that was missing on the unit was a nut and washer on the SMA connector. Adding a nut helps the PCB ground make good contact with the aluminum enclosure. The Airspy team have said that future units will come with this nut provided.
Airspy R2 (top), Airspy Mini (Middle), RTL-SDR (bottom) for size comparison.
Apart from the price and enclosure, the most noticeable feature difference between the two is the smaller bandwidth of the Airspy Mini. Unlike the Airspy R2, the Airspy Mini does not use a Si5351 clock generator chip. The lack of this chip limits the Mini's maximum bandwidth to 6 MHz and eliminates any ability to use an external clock. The main applications that you miss out on from the lack of an external clock input include: coherent clock, passive radar and direction finding experiments.
From the circuit photos below we can see that the Mini consists of mostly the same parts used in the Airspy R2. Missing is the Si5351 clock controller, expansion headers and the external clock input.
The Airspy Mini Circuit BoardThe Airspy R2 Circuit Board
The Odroid C2 is a $40 single board computer with a 2 GHz ARM-A53 quad core CPU and 2GB of RAM. Compared to a Raspberry Pi 3 it is more powerful and costs about the same. The Airspy R2/Mini is a $199/$99 USD software defined radio that can tune from 24 – 1800 MHz, uses a 12-bit ADC and has a bandwidth of up to 10 MHz. The Airspy wesbite generally recommends that a relatively powerful 3rd generation Intel Core i3 2.4 GHz processor is required for the Airspy.
However, recently the Odroid C2 has proved itself powerful enough to run the Airspy in full 10 MHz mode in GQRX, though without demodulation. On his website Michael DG0OPK shows some screenshots that show the Airspy+Spyverter upconverter combo running on an Odroid C2 and demodulating with the WSJT-X software, running GQRX and running as a spectrum analyzer.
Over on YouTube user radio innovation has also uploaded a video showing the Airspy running GQRX on an Odroid C2.
