Tagged: interference

HamRadioScience: Why Apple’s iMac May be the Best PC for SDR Applications

Over on on the HamRadioScience blog, the author has uploaded an article that makes the case on why Apple iMac PC’s may be the best choice for SDR receivers (at least for HF frequencies). In the testing he uses an SDRplay and Elad FM-Duo to show that the plastic case of the SDRplay does not affect the picked up RFI. He shows that when the SDR’s are connected to an iMac the interference from RFI on HF frequencies is minimal. However when connected to a Core i5 PC, there is significant amounts of CPU and monitor noise generated.

The differences in generated noise probably come from the fact that the iMac is probably much better shielded with an aluminum case and that they have high build quality standards for their monitors. The author suggests that an alternative to using an iMac could be to build your own PC, ensuring that dual chamber metal enclosures are used, which ensures that the power supply is isolated in its own separate steel compartment.

RFI is visible with the SDRplay in SDRuno when using the PC. But no RFI is seen with the iMac.
RFI is visible with the SDRplay in SDRuno when using the PC. But no RFI is seen with the iMac.

Solving the Mystery of a Keyless Vehicle Entry RF Deadspot in a Carpark with a FUNcube Dongle

The Brisbane Times ran a story today that discussed an interesting RF phenomenon that was solved using a FUNcube dongle software defined radio. The Funcube dongle is a SDR similar to the RTL-SDR. The issue was that vehicle wireless entry keyfobs would not work at a particular location within an outdoor shopping centre car park.

The story goes like this – First a user on a local Brisbane subreddit message board posted about how he had noticed that his cars wireless entry keyfob would not work when he parked in a certain area of the shopping area car park. The user wrote:

I walked out to my car from Bunnings, and there was a new HSW Maloo parked in front of me with the owner staring at his key fob and shaking his head.

I said “let me guess, car won’t open?” and he said yeah, and he’d been trying for about 5 minutes. I said that I’d had the same thing happen to me a few months back in the same spot, and then went to open my car.

Nothing. No beep, door stayed locked. Looked around and there was another couple trying to get into their car as well (late model C Class).

It took about 5 minutes of me trying the door every 20 seconds or so before it opened. HSV owner was still there when I left. The only thing he and I could think of causing it was the mobile phone tower in front of Aldi.

After reading the post, user u/riumplus decided to go out to the same spot with his Funcube dongle SDR and see if there was any interference that might explain the issues. But he found no such interference. However, when he pressed the wireless entry on his own keyfob he noticed reflections from the main transmission that were coming from the buildings walls. He wrote:

So I pulled out my SDR and I did a complete frequency sweep from 100kHz to 2.2GHz and… also nothing. Everything completely normal. Nothing on that frequency, nor anything odd anywhere else on the spectrum. Couldn’t see any of the usual potential harmonics from RFID or standard WiFi gear. Here’s the output at 433.3MHz(forgot to grab a screenshot centred right at 433.92Mhz but it was also empty, as was 315MHz).

Here’s where it gets interesting – I noticed that that location is almost in the middle of the car park between the three buildings, and they all have large amounts of metal flashing on their fronts. On a whim I watched the output when I pressed my own keyfob. And what do you know, I could see distorted reflections from my own signal bouncing off these buildings right back at me. My guess is that this is what was causing you issues!

It may sound counter-intuitive, but next time it happens try cupping the keyfob in your hand to weaken the signal. It should still be strong enough to trigger your car to open, but then the reflections will be weak enough they won’t cause you trouble.

So it seems that the layout of the buildings caused a focal point for reflections at that particular location which affected some wireless keyfobs.

The location in the carpark of the deadzone.
The location in the carpark of the deadzone.

Updates to Shielding the RTL-SDR with an Aluminium Case

A few weeks ago we posted about will1384 who had bought an aluminium case from Ebay and was using it to shield his RTL-SDR. After running multiple tests, will1384 discovered that the aluminium case was actually not helping with shielding performance at all. Oddly his results showed that the aluminium case was actually increasing the amount of noise received. Will has been updating his imgur album with noise analysis results from rtl_power scans over the entire spectrum.

In his latest tests he tried a metal outlet box as the case and saw improved results over the aluminium case. His conclusions seem to indicate that the aluminium box is not a good EMI shield. We’re not sure why he found these results, but one theory might be that because the aluminium case is anodized, it has a non-conductive surface, which might cause poor grounding.

RTL-SDR in a Metal Outlet Box
RTL-SDR in a Metal Outlet Box

Simple RTL-SDR Shielding with Copper Tape

Last month we posted about an RTL-SDR experimenter who shielded his RTL-SDR dongle using an aluminium case. Shielding the RTL-SDR helps to reduce out of band interference which can enter the device through the PCB itself.

Now Melih, another RTL-SDR experimenter has recently posted on his blog about his simpler shielding method that uses cheap copper tape. The copper tape was purchased from ebay, and is the type that is commonly used for creating a slug barrier in the garden.

Over on the Reddit thread discussing his work, there are some concerns about excessive crystal drift due to there being no ventilation holes. However, it seems that the general consensus is that lack of ventilation will not significantly affect crystal drift and may actually help to stabilise the crystal over time by keeping the internal temperature more constant.

You may also be interested in Melih’s previous post where he replaces the MCX connector on the dongle with an SMA female connector.

Copper Tape Shielding RTL-SDR
Copper Tape Shielding RTL-SDR

Shielding the RTL-SDR

Over on Reddit user will1384 has posted about his imgur photo album that clearly documents some well researched steps that he took in an effort to shield the RTL-SDR dongle from interference. Interference is caused by strong out of band signals that can sometimes show up even when no antenna is connected to the dongle. Shielding the dongle helps to remove this interference.

The main steps he took were the following:

  1. Buy an aluminium case from ebay and put the dongle inside it.
  2. Remove the USB connector and ground the dongle ground to the aluminium casing using a 1M Ohm resistor and a 47nf ceramic disk capacitor.
  3. Connect the USB data lines to a USB extension cable and wrap a toroid around the 5v and GND lines and twist the two data lines together.

There is a discussion about this shielding project on Reddit.

Shielding Wiring Diagram
Shielding Wiring Diagram

Testing a FM Broadcast Bandstop Filter

Over on YouTube user Cameron Conover has been testing a simple FM broadcast bandstop filter with his HackRF. The same filter can just as easily be used with the RTL-SDR to remove broadcast FM interference and images. Cameron uses a MCM Electronics 88 – 108 MHz FM Trap which can be found very cheaply on Amazon or Ebay for around $15 USD. His video shows that the FM trap works very well and significantly reduces out of band FM interference.

Locating an Interfering Signal with Radio Direction Finding and the RTL-SDR

The people at the MIT Haystack Observatory discovered recently that someone was transmitting an interfering signal on their licensed radar band. The interferer was effectively jamming the radar, preventing them from carrying out any experiments.

After checking for local causes of interference and finding nothing, they decided that the interferer must be coming from further away. To find the location of the jamming signal they did some radio direction finding. This involved driving around with Yagi and magnetic loop antennas and RTL-SDR and USRP N200 SDRs and then measuring the signal strength at various points.

For the software they used a custom GNURadio block which calculated the power spectra using the FFTW C library, and averaged the results to disk. They then post processed the data to calculated the RFI power, and correlated the data with GPS coordinates recorded on his phone.

After all the data was processed, they discovered that the interference originated from an FM radio tower which had a faulty FSK telemetry link. They notified the engineer responsible who then replaced the link and the interference disappeared.

RFI strength at various geographic locations
RFI strength at various geographic locations