Review of Nobu’s HF Upconverter, Galvanic Isolator and 14 MHz Low Pass Filter

Back in April we posted about some new products made by Japanese RTL-SDR experimenter and product manufacturer Nobu. Nobu’s new products were a 1:1 galvanic isolator and a low pass filter. The galvanic isolator isolates the antenna from the RTL-SDR and PC, significantly reducing noise. The low pass filter is useful when used with direct sampling modified RTL-SDRs to filter out any strong interfering signals that are above 14 MHz.

Recently Nobu sent us at some samples of his products. He sent us one of his HF upconverters, a galvanic isolator and a low pass filter.

Nobu’s RTL-SDR Products: HF Upconverter, Galvanic Isolator, Low Pass Filter. Placed next to an RTL-SDR for size comparison.

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New EAS SAME Weather Alert Decoder

Over on Reddit and GitHub user cuppa-joe has released a Python based EAS SAME Alert message decoder called dsame which is compatible with the RTL-SDR. EAS is an acronym for Emergency Alert System and is a system that is most commonly used to alert the public to local weather emergencies such as tornadoes, flash floods and severe thunderstorms.

Local EAS weather alerts are encoded with the SAME (Specific Area Message Encoding) protocol. They are transmitted on the local weather radio frequency in the USA and Canada and some weather radio’s are capable of decoding the EAS SAME data. Cuppa-joe’s dsame EAS decoder outputs full EAS weather messages such as:

The National Weather Service in Pleasant Hill, Missouri has issued a Required Weekly Test valid until 12:30 PM for the following counties in Kansas: Leavenworth, Wyandotte, Johnson, Miami, and for the following counties in Missouri: Clay, Platte, Jackson, Cass. (KEAX/NWS)

To use the software you will still need to use a EAS demodulator such as multimon-ng which is available for Windows and Linux, and you will also need Python 2.7+ installed.

An example EAS SAME alert can be heard in the player below:

HackRF Blue Shipped and HackRF One Updates

Back in December of last year we posted about the Indiegogo campaign for the HackRF Blue, a lower cost clone of the HackRF One software defined radio. They were able to achieve a lower cost by better component sourcing and manufacturing choices, thus reducing the cost from $299 to $200. The HackRF Blue Indiegogo campaign was successful and most of the HackRF Blue’s have now been shipped and delivered to backers. If you need help getting started with the HackRF Blue see To get started on Windows, simply use Zadig like with the RTL-SDR, and use SDR# which has built in HackRF support.

The specs of the official HackRF One (and by extension the HackRF Blue) have also recently been changed. They now officially acknowledge that the HackRF One is capable of operating at a frequency down to about 1 MHz. They write:

Now that we’ve seen consistent low frequency performance across multiple manufacturing runs, we’re comfortable changing the official specification: HackRF One operates from 1 MHz to 6 GHz. Try attaching a long wire antenna to listen to shortwave radio!

Recently some people have been considering which mid priced SDR to upgrade to from their RTL-SDR. Our opinion is this: The HackRF has pretty poor RX performance, probably the same as, or even worse than the RTL-SDR, so we suggest you buy it only if you want TX capabilities or need operation above 1.7 – 2 GHz. If you only need RX then we suggest you choose the Airspy or SDRPlay as they have much better RX performance. See our list of SDRs for more possible options. 

The HackRF Blue.
The HackRF Blue.

New ADS-B Filter with Built in Bias Tee Available

Adam who is the manufacturer of the popular LNA4ALL low noise amplifier (LNA) that is commonly used with the RTL-SDR has come out with a new product for ADS-B enthusiasts. The product is an ADS-B filter with a built in bias tee for providing phantom power. Adam previously sold an older version of the ADS-B filter that came without the bias tee.

The bias tee allows you to inject DC power into the coaxial cable in order to easily power an LNA (like the LNA4ALL) or other device that is placed near the antenna. The antenna could be far away from a power source, such as on your roof or up a mast. It ensures DC power reaches the LNA, but at the same time does not enter the RTL-SDR dongle, as DC current on the antenna input could destroy the RTL-SDR. For best performance it is recommended to use an LNA near the antenna, especially if you have a long run of coaxial cable between the antenna and RTL-SDR.

The filter uses Low Temperature Co-fired Ceramics (LTCC) type components as opposed to the seemingly more commonly used SAW and microstrip filters. Adam writes that each type of filter has its tradeoffs, but he believes the LTCC filter is the best for this application.

Comparison between different filter types.
Comparison between different filter types.

The insertion loss of the filter in the pass band is about 2.4 dB and the filter will significantly attenuate broadcast band FM, TV stations, WiFi and 1.8 GHz+ cell phones. However, it does not do so well with 950 MHz cell towers and possible radar on 1.2-1.3 GHz as the LTCC filter is not as sharp as a SAW filter. In Adams own tests he shows that the addition of the filter improves ADS-B decoding performance by about 20%, but the improvement you see will vary greatly with your RF environment.

The filter is currently selling for 20 Euros + 5 Euros shipping (~$28 USD).

ADS-B LTCC Filter with Bias Tee
ADS-B LTCC Filter with Bias Tee

RTL-SDR vs. AIRSPY on ADS-B Reception: Round 2

A few days ago we posted about Anthony Stirk’s comparison between the RTL-SDR and the Airspy on receiving ADS-B signals. In his first test Anthony used an E4000 dongle, which is known to have inferior performance at the ADS-B frequency of 1090 MHz.

Now Anthony has done his test again, but this time with an R820T2 RTL-SDR. His results show that the R820T2 RTL-SDR is better than the E4000 RTL-SDR, but that the Airspy is still better than the R820T2 RTL-SDR. The R820T2 received at maximum distances more comparable to the Airspy, though still fell short of the Airspy by some 50 kms in some directions. Anthony’s writes that his distance seems to be mainly limited by geography so it is possible that in some other location the Airspy could out perform the RTL-SDR by a more significant distance.

The most interesting part of his last experiment was that over a 28 hour period the E4000 RTL-SDR received only a total of 2.9 million messages whilst the Airspy received a total of 10.3 million messages. In the new experiment the R820T2 received a total of 22.3 million messages whilst the Airspy received a total of 31 million messages, which is a little closer. However, with the R820T2 RTL-SDR, 3 million messages were unusable, versus only 31 unusable messages with the Airspy.

From these results it’s clear that the better design and more ADC bits in the Airspy can significantly improve ADS-B reception. However, there is a cost difference at $199 for the Airspy vs <$20 for the RTL-SDR. The Airspy cost may be soon less of a problem we are aware that an Airspy Lite version is in the works and that will probably cost around $99 USD.

In the future Anthony will do another test with no error correction enabled because the current version of the Airspy ADS-B decoder has no error correction whereas the RTL-SDR ADS-B decoder does. Those results may show that the Airspy is even better that shown here.

Update: Anthony ran the test again with a modified version of ADSB# with not error correction and obtained the following results which show that the Airspy receives about double the messages compared to the RTL-SDR:

Total Messages Received:
Airspy 65,150,313
RTL 32,973,049

Airborne Position:
Airspy 4,615,972
RTL 2,270,810

Airspy 533
RTL 635,549

Airspy vs R820T2 RTL-SDR on Maximum ADS-B Distance.
Airspy vs R820T2 RTL-SDR on Maximum ADS-B Distance.

List of all SDRSharp Plugins from

Vasilli, an SDR# plugins programmer has released a list of all his available SDR# plugins on his website (in Russian, use Google translate). Some even which were missing from our own list. The ones we hadn’t seen yet were:

  1. MPX Output plugin. Allows programs like RDS Spy to work with the audio output from SDR#
  2. Aviation band 8.33 calculator. Automatically converts the current frequency input to an aviation one according to the standard 8.33 kHz channel spacing.
  3. Frequency Lock. Simply locks the frequency change settings in SDR# to prevent accidental changes.
  4. SDR Update Script. Not a plugin, but a script that automatically updates SDR# and installs most of Vasilli’s plugins all at once. To use this script, it must be placed in a subdirectory of the SDR# folder.

Here’s an example video of SDR# running the MPX plugin so that RDS Spy can be used.

Determining the Radiant of Meteors using the Graves Radar

With an RTL-SDR or other radio it is possible to record the echoes of the 143.050 MHz Graves radar bouncing off the ionized trails of meteors. This is called meteor scatter and it is usually used to count the number of meteors entering the atmosphere. Amateur radio astronomers EA4EOZ and EB3FRN decided to take this idea further and synchronised their separate receivers and recordings with a PPS GPS signal in order to determine the radiant of the meteors they detected. They write:

The idea was to analyze the Doppler from the head echoes and and see if something useful can be extracted from them.

We detected a meteor from two distant locations and measured Doppler and Doppler slope at those locations. The we tried to find solutions to the meteor equation by brute force until we obtain a big number of them. Then we plotted those solutions in the sky and we see some of them pass near a known active radiant at the time of observation. Then, we checked the velocity of those solutions near the known radiant and found they are quite similar to the velocity of the known radiant, so we concluded probably they come from that radiant.

But they can come from everywhere else in the sky along the solution lines! There is not guarantee these meteors to be Geminids, although probabilities are high. Once all the possible radiants of a meteor are plotted into the sky, there is no way to know who of all them was the real one. Doppler only measurement from two different places is not enough to determine a meteor radiant. But don’t forget with some meteors, suspect to come from a known shower, the possible results includes the right radiant at the known meteor velocity for that radiant, so there seems to be some solid base fundamentals in this experiment.

Initially they ran into a little trouble with their sound cards, as it turns out that sound cards don’t exactly sample at their exact specified sample rate. After properly resampling their sound files they were able to create a stereo wav file (one receiver on the left channel, one receiver on the right channel) which showed that the doppler signature was different in each location. The video below shows this wav file.

Using the information from their two separate recordings, they were able to do some doppler math, and determine a set of possible locations for the radiant of the meteors (it was not possible to pinpoint the exact location due to there being no inverse to the doppler equation). The radiant of a meteor shower is the point in the sky at which the meteors appear to be originating from. Although their solution couldn’t exactly pinpoint the location, some of the possible solutions from most meteors passed through the known radiant of the Geminids meteor shower. With more measurement locations the exact location could be pinpointed more accurately.

Possible solutions for the radiant of the Geminids meteor shower.
Possible solutions for the radiant of several meteors detected during the Geminids meteor shower.

Wireless Door Bell 433 MHz ASK Signal Analysis with a HackRF

Paul Rascagneres, an RF experimenter has recently uploaded a document detailing his efforts at reverse engineering a wireless doorbell (pdf file) with a 433 MHz Amplitude Shift Keyed (ASK) signal with his HackRF software defined radio. The HackRF is a SDR similar to the RTL-SDR, but with a wider available bandwidth and transmit capabilities.

To reverse engineer the doorbell, Paul used GNU Radio with the Complex to Mag decoder block to receive and demodulate the ASK signal. Once demodulated he was able to visually see the binary modulated waveform, and manually obtain the serial bit stream. From there he went on to create a GNU Radio program that can automatically obtain the binary strings from the ASK waveform.

In order to replay the signal, Paul found that the simplest way was to use the hackrf_transfer program, which simply records a signal, and then replays it via the HackRF transmitter on demand. With this method Paul was able to ring his doorbell via the HackRF.

Paul also confirmed his SDR results with an Arduino and 433 MHz transceiver. He then took it a step further and used the Arduino to create a system that could automatically receive and replay signals at 433 MHz and 315 MHz.

Decoding an ASK modulated bitstream.
Decoding an ASK modulated bitstream.