October 20, 2015

Another L-Band Antenna Build and comparing L-Band reception on the RTL-SDR, HackRF and SDRplay

Over on Reddit user killmore231 has made a post showing his comparison of L-Band reception with RTL-SDR, HackRF and SDRplay software defined radios. killmore231 built the L-band patch antenna which Adam 9A4QV showed how to build on his YouTube channel late last month.

When testing the antenna on his RTL-SDR he saw no reception of any L-band signals at all. The RTL-SDR requires an external LNA to properly receive signals at this frequency range, which he did not have. Next he tried it on his HackRF and saw that some signals were weakly visible. When he tried it on his SDRplay the L-band satellite signals were clearly visible, probably due to the SDRplay’s good sensitivity at this frequency range and the fact that it has a built in LNA. His results show that the SDRplay is a good SDR for receiving L-band satellites as it does not need an external LNA for decent reception. An external LNA may still be needed if a long run of coax cable is used however.

SDRplay reception of L-band satellite signals with no external LNA.

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October 20, 2015

Passively Cooling the RTL-SDR with a Thermal Gap Pad

John Mills recently wrote in to us at RTL-SDR.com to show us how he cools his RTL-SDR by using a thermal gap pad stuck to the entire bottom of the RTL-SDR PCB. A thermal gap pad is a soft pliable material that is often used to interface between electronic chips and heatsinks. The gap pad forms into tight hard to reach spaces and conducts heat towards the heatsink. It is not electrically conductive, so the entire bottom of the RTL-SDR can be stuck to the thermal gap pad, which is then stuck to a metal heat sink.

John uses a thermal gap pad made by Bergquist, with part number GP5000S35-0.100-02. This gap pad is 0.1 inches thick, is easily cut with a craft knife and is tacky so it easily sticks to the heatsink and RTL-SDR PCB. It has a thermal conductivity of 5W/m.k. John uses the pad to help to cool the R820T, RTL2832U and voltage regulator chips. It has been shown in some previous posts that by cooling the R820T chip increased sensitivity can be obtained, especially at frequencies above 1.2 GHz.

He writes that if there is sufficient interest then he may consider selling strips of it on eBay. You can contact him at sdr_AT_milairuk.co.uk.

Below we’ve posted images of Johns thermal pad cooled RTL-SDR’s, along with his comments on them in the captions.

Inside latest SDR / Latest SDR - "This is my latest version using a R820T2 version, and I have also fitted this with a TCXO. In this version I also used a 1Mohm and 47nF to ground the USB shield wire as in a previous post. This version only uses one metal spacer and the end of the PCB is secured by two M2 nylon screws / nuts. Case from China RF on Ebay." — Open SDR – “Just held onto a heatsink with two pieces of string ! Then this sits on another larger heatsink using another piece of Gap Pad to hold it – this has been working in my garden shed now for over 2 years feeding ADSB data”

Diecast Box SDR – "in this one I have made two small threaded metal clamps, lined with gap pad and tightened just enough to keep the PCB in good contact with the gap pad underneath and the diecast box. I use small BNC to MCX pigtails off Ebay to connect to the antenna socket. I also remove the LED and place through the box as can be seen." — Diecast Box SDR – “in this one I have made two small threaded metal clamps, lined with gap pad and tightened just enough to keep the PCB in good contact with the gap pad underneath and the diecast box. I use small BNC to MCX pigtails off Ebay to connect to the antenna socket. I also remove the LED and place through the box as can be seen.”

Latest SDR - Outside — Latest SDR – Outside

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October 13, 2015

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.

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October 13, 2015

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.

SDRplay RSP FM Dynamic Range Demo

Watch this video on YouTube

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October 13, 2015

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.

QRM mitigation with antenna phasing

Watch this video on YouTube

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October 12, 2015

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

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October 7, 2015

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.

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

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October 6, 2015

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

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