Over on YouTube user Adam Alicajic has uploaded a video showing how coax cable loss affects the frame rate when receiving ADS-B. To do this test Adam uses a precision attenuator in between his ADS-B antenna and RTL-SDR dongle to simulate attenuation from coax cable loss. His results show that for every 1 dB of attenuation the frame rate drops by about 10%.
Coax cable loss for common type of cable can be estimated with calculators available at http://www.net-comber.com/cable-loss.html and http://www.arrg.us/pages/Loss-Calc.htm. RG-6 cable has a low loss at 1090 MHz of about 0.23 – 0.32 dB per meter, whereas RG58 has a loss of about 0.5 – 0.6 dB per meter and RG174 (stock antenna cable on most RTL-SDR units) has a greater loss of about 1.2 dB per meter.
Coax length loss contribution to the bad ADS-B reception
The LightSail is a solar sailing spacecraft that has been launched by the planetary society. It is based on the “solar sail” concept, which uses a large reflective foil to harness the suns energy as a means of propulsion. The planetary society write about solar sails:
Solar sails use the sun’s energy as a method of propulsion—flight by light. Light is made of packets of energy called photons. While photons have no mass, a photon traveling as a packet of light has energy and momentum.
Solar sail spacecraft capture light momentum with large, lightweight mirrored surfaces—sails. As light reflects off a sail, most of its momentum is transferred, pushing on the sail. The resulting acceleration is small, but continuous. Unlike chemical rockets that provide short bursts of thrust, solar sails thrust continuously and can reach higher speeds over time.
The LightSail Concept
Currently a test mission of the LightSail concept is under way. The LightSail is in orbit and expected stay in orbit for about 1-2 months. Initially the mission had trouble with communications, but after an automatic reboot of the on board computers they have now confirmed that the LightSail is transmitting properly.
With an RTL-SDR and appropriate satellite antenna, it should be possible to monitor the LightSail. The LightSail transmits at a frequency of 437.435 MHz with the AX.25 protocol, FSK encoding at 9600bps and with a call sign of KK6HIT. The LightSail can be tracked at http://sail.planetary.org/missioncontrol and the planetary society are also requesting that amateur radio tracking enthusiasts email over any data they capture. Over on twitter some users have confirmed LightSail downlink hits:
Over on the swling.com blog, Thomas has uploaded a review of the TitanSDR Pro that was published in the May 2015 version of The Spectrum Monitor online magazine. Although the TitanSDR pro software defined radio is an expensive (1,380 EUR for the basic / 1,970 EUR for the PRO), it may be of interest to those looking to go further into the short wave listening hobby. The TitanSDR is a high performance 9 kHz to 32 MHz receiver with 16 built in preselectors and a bandwidth up to 2.1875 MHz. It’s main markets appear to be government intelligence and military, but it also finds good use in general short wave listening.
Thomas reviews the TitanSDR positively, liking its easy to use software and its superb sensitivity, but noting that it’s high price may be a bit off putting.
Also, in the current June version of The Spectrum Monitor magazine Thomas also reviews the SDRPlay, a $149 USD HF capable radio, also giving it a favourable review.
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 RTL-SDR.com 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.
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:
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 http://hackrfblue.com/getting-started/. 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.
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.
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).
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
Unusable: Airspy 533 RTL 635,549
Airspy vs R820T2 RTL-SDR on Maximum ADS-B Distance.
