Comparing RTL-SDR’s on L-Band Reception, Tuner Temperatures and Passive Cooling

Over on Reddit user MaxWorm has been doing some experiments with comparing various RTL-SDR dongles on L-band (1 – 2 GHz) reception. Previously we wrote a tutorial on decoding Inmarsat signals which are at around 1.5 GHz and noted that the R802T/2 dongles can have some trouble at these frequencies.

It is known that the R820T/2 is not as good as the older now rare and expensive E4000 tuners at frequencies above 1.5 GHz, and it is also known that sensitivity decreases as the temperature of the R820T/2 increases, especially at frequencies above 1.5 GHz.

MaxWorm tested an R820T, R820T2 and two E4000 sticks at receiving L-band frequencies. He found that one of the E4000’s performed the best, but surprisingly the other E4000 dongle was totally deaf in the L-band. The R820T and R820T2 dongles performed similarly – not as good as the best E4000, but not as bad as the worst. All tuners exhibited reduced signal strength when warm.

In another post MaxWorm also measured the temperature of the tuner chips in each of his units, and created a simple heatsink for one of his R820T2 dongles. His results show that the heatsink passive cooling works well, significantly cooling the R820T2 chip. His measurements are copied below:

R820T2 in Plastic case:
R820T2: 77°C top / 74°C bottom
RTL2832: 56°C top / 54°C bottom

R820T2 bare PCB:
R820T2: 62°C top / 63°C bottom
RTL2832: 43°C top / 42°C bottom

R820T2 in Alu-Case with Alu “L-Bridge” on Tuner:
R820T2: top 37°C / bottom 47°C
RTL2832: top 49°C / bottom 40°C

E4000 in plastic case:
E4000: 37°C top / 37°C bottom
RTL2832: 46°C top / 40°C bottom

bare E4000 PCB:
E4000: 37°C top / 32°C bottom
RTL2832: 40°C top / 37°C bottom

Other experimenters have previously applied fan cooling and oil cooling to RTL-SDR dongles to cool them and increase sensitivity.

RTL-SDR with heat sink to aluminium case.
RTL-SDR with heat sink on the R820T2 chip connectoed to the aluminium case.
L-Band Reception Results for an R820T, R820T2 and two E4000 dongles.
L-Band Reception Results for an R820T, R820T2 and two E4000 dongles.

Demonstrating the Dynamic Range on the SDRplay RSP

The SDRplay radio spectrum processor (RSP) is a $150 USD software defined radio that can be considered as a next stage level up from the RTL-SDR dongle. We also consider it a competitor to the $199 USD Airspy SDR.

Over on YouTube the SDRplay designers have posted a video that demonstrates the dynamic range that is possible with their SDR. Dynamic range is a measure that defines the range between the strongest and weakest signal that can be received. So for example, if you have two signals near to each other on the frequency spectrum, dynamic range defines how much stronger can one signal be compared to the other before the weaker signal disappears into the noise.

In the experiment they use two frequency generators to generate a simulated wanted signal at 98.4 MHz and an unwanted blocking interferer at 98.7 MHz. They show that by reducing the IF bandwidth in their configuration screen and thus tightening the internal filters that the dynamic range can be increased to about 70 dB.

Previously Leif sm5bsz performed some similar tests, comparing many SDRs against one another, but did not utilize the programmable IF filters in the SDR Play RSP perhaps undervaluing the best possible dynamic range by about 5-10 dB.

Mitigating QRM (Interference) with an Antenna Phaser

Over on YouTube user London Shortwave has posted a video showing his antenna phasing system in action with a Funcube Dongle Pro+ and SDR# running on a tablet. An antenna phaser reduces unwanted noise by using two antennas and positioning one “noise” antenna so that it receives the unwanted noise strongly, and positioning the main antenna to receive the desired signal as best as possible. Then the signals are combined by a phaser unit in such as way that the unwanted noise is subtracted from the desired signal.

In his experiments London Shortwave discovered that an ethernet over Power adapter used by one of his neighbours was causing the shortwave spectrum to get completely obliterated by noise. His video shows the effect of turning his phaser unit on and off when trying to reduce this noise. London Shortwave has also done a very nice writeup on dealing with urban interference on shortwave, and includes a section that discusses antenna phasing.

The antenna phaser set up
The antenna phaser set up

Tutorial on Properly Positioning a Preamp (LNA) in a Radio System

Radio blogger Anthony Stirk has made a post on his blog explaining some critical concepts behind understanding why it is important to position a low noise amplifier (LNA) near the radio antenna, rather than near the radio. In the post Anthony explains how the Noise Figure (NF) and linearity (IP3) of a radio system affect reception.

Using the free AppCAD RF design assistant software, Anthony explains how the noise figure of a system increases with longer coax cable runs, and how it can be reduced by placing an LNA right next to the antenna. He also explains why the sensitivity of the radio won’t increase if the LNA is placed close to the radio instead.

In addition to this, he also explains why adding more LNA’s to a system decreases the linearity (IP3) of the system and that if the receiver has a built in LNA that the system linearity can be severely degraded by adding extra LNA’s, causing easy overloading and intermodulation. In conclusion Anthony writes the following:

In summary, a setup with a good antenna system connected to a receiver with a built in LNA:

  • May not benefit from having a preamp at the antenna.
  • The presence of a built in LNA is detrimental to the linearity and may degrade the signals.

So in conclusion:

  • Put the preamp as close to the antenna as possible.
  • Receivers with a built in LNA may not get the most out of an antenna system or preamp.
  • Proper gain distribution guarantees better performance than one-size-fits-all solutions, both in terms of sensitivity and strong signals handling.
Optimal Setup: Antenna -> LNA -> Coax -> Receiver
Optimal Setup: Antenna -> LNA -> Coax -> Receiver
NF and Linearity Calculations
NF and Linearity Calculations in AppCAD

SDR# updated to revision 1400 & SDR Touch updated to V2.6

The popular SDR# software which is often used together with RTL-SDR dongles has recently been updated to revision 1400. This new revision brings an interesting new feature which automatically estimates and displays the peak, floor and signal to noise ratio (SNR) values of the currently tuned bandwidth. Watching the SNR metric is very useful when tuning the RF gain settings, as best reception is obtained when the SNR value is maximised. The author also writes that there have been several radical changes to the code that leverage the latest .Net 4.6 framework which should improve the signal processing quality, CPU usage, user experience and hardware support. The changelog is pasted below:

Enhanced the Center tuning mode and extended it for RTL-SDR;
Enhanced the spectrum display;
Changed the frequency labelling to use multiples of 2.5/5/10 or frequency steps;
Added Peak, Floor and SNR estimation for the selection;
Enhanced the defaults for better user experience;

We note that some plugins may break with this update so be sure to make a backup if upgrading. Vasili, one of the most active SDR# plugin programmers has updated most of his plugins to work on this new version now.

Revision 1400 of SDR# with SNR estimation.
Revision 1400 of SDR# with SNR estimation.

In addition to this update, over on the Android OS the popular mobile app SDRTouch has been updated to version 2.6. This new version brings the following features and improvements:

  • Baseband recording and file playback
  • Direct sampling support for full-band receivers
  • Improved SSB image rejection
  • Fixed tuning step
  • Manual filter bandwidth
  • Improved accessibility
  • Bug fixes

Reverse engineering a wireless thermostat with an RTL-SDR

When Tom Taylors home heating boiler was replaced the builders also replaced the old wired rotary thermostat with a digital wireless one. It sounds good, but Tom soon discovered that the thermostat UI was terrible and that the buttons were horrible to press, making him prefer to shiver in the cold. So Tom decided to see if there was a smarter way to control the heating.

When Tom investigated the thermostat, he discovered that the wireless unit transmitted in the unlicensed 433 MHz band and that the thermostat only transmitted two commands, turn on or turn off. By using his RTL-SDR and the CubicSDR software on his Mac he was able to detect the short blip of the thermostat wireless signal. Next he recorded the on and off signals and opened the sound files in Audacity, an audio processing software tool. In Audacity he was able to compare the sound waveforms of the on and off signals.

From his analysis he discovered that each signal consisted of a preamble and then an on or off command which is repeated twice, presumably to reduce the likelihood of interference. Tom also discovered that the commands were encoded with pulse width modulation.

From this knowledge Tom was then able to use a cheap 433 MHz transmitter together with an Arduino microcontroller board and a short script to create identical on or off transmissions that control the boiler. Tom writes that his next steps are now to create a heating schedule based on his families shared calender, make a thermostat control loop and create a web connected interface with a Raspberry Pi.

The 433 MHz thermostat on/off signal detected with an RTL-SDR in the CubicSDR software
The 433 MHz thermostat on/off signal detected with an RTL-SDR in the CubicSDR software

Building an L-band helical antenna for Inmarsat

Previously in August of this year we wrote an article showing how to decode Inmarsat satellite STD-C NCS EGC messages with an RTL-SDR. Inspired by this article, reader Mario Filippi, N2HUN has written in to show us how he built an L-band helical antenna to receive these signals. A helical antenna is one of the better choices for receiving Inmarsat signals as it will provide higher gain when compared to a patch antenna, however the disadvantage is that it is much larger. Of related interest, Adam 9A4QV also recently showed us a video detailing the correct dimensions for building an air gap patch antenna.

Mario’s Inmarsat antenna consists of a 90cm Ku band dish, a homebrew L-band LHCP helical antenna and an inline amplifier. He used the assembly instructions found on UHF Satcom’s page at and scavenged most of the parts from his junk box. To help others with the construction of a similar antenna Mario has also created a document detailing the construction of the antenna with several useful build images (.docx file).

Helical Inmarsat antenna feed for a 90cm Ku band dish
Helical Inmarsat antenna feed for a 90cm Ku band dish

Mario has also recently given a presentation about the RTL-SDR to the Mid Atlantic States VHF Conference entitled “SDR Dongle for VHF/UHF Reception”. The presentation is an overview of the RTL-SDR dongle and many of its interesting applications, including several screenshots of software in action (dropbox) (mega mirror).

Reverse engineering a public parking electronic display to play Tetris

Recently we received an email from reader @Ivoidwarranties about his latest project which involved using a HackRF to reverse engineer the RF protocol used by a public parking electronic display. Once reverse engineered @Ivoidwarranties used a XR-2206 monolithic function generator, hybrid RF amplifier and an Arduino to create a device that overrides the public parking display and plays a game of Tetris on it.

We don’t have any details on the HackRF reverse engineering side of things, but he has uploaded a video to YouTube showing the hack in action.