SDR in the Local Newspaper: Investigating an RF Dead Spot for Car Key Fobs
Reddit user [SDR_LumberJack] writes how he was recently featured in his local newspaper [Part2] in Ontario, Canada thanks to his efforts in helping to hunt down the cause of an RF deadspot with an SDR. He began his journey by reading a story in his local newspaper called the [Windsor Star]. The story was about locals having found a ‘dead-spot’ for car key-fobs. In the dead-spot key-less cars wouldn’t start, key-fobs wouldn’t unlock cars, and alarms would go off.
Being intrigued by the story [SDR_LumberJack] investigated by driving around with an RTL-SDR, HackRF and a laptop running SDR#. Eventually he found that there was what appeared to be a WBFM Broadcast radio station interfering at 315 MHz. This frequency happens to fall into the ISM radio band that used by car remotes and key-fobs. The exact source of the interference hasn’t been nailed down just yet though.
While it’s possible a broadcast station is at fault it is also possible that his SDR was just overloading, causing broadcast FM imaging. Perhaps a WBFM filter could be used to prevent signal imaging that could interfere with the investigation.
Hopefully [SDR_LumberJack] will continue his investigation and we’ll get an update on this story.
If you’re interested, back in 2016 we posted a very similar story about the exact same thing happening at a car park in Brisbane, Australia. The conclusion to that story was that the dead-spot only occurred in particular locations in the car park, and this was due to the shape of surrounding building causing the RF signals to reflect off the walls and distort the signal.
Leaky TV cable.
Ive seen problems with FM stations as high as the eight and ninth harmonic. A good low pass filter which is required should stop most high harmonics not always. VHF TV interference is common with the second harmonic and needs extra filtering. Some stations have been found not running a low pass filter with obvious results. Also common is overdriving or mis-tuneing a PA and the filter cannot deal with it and it will radiate more than -60 Db below Fo. In the old days that happened a lot as few FM stations had a spectrum analizer. Today, you can run harmonic and intermod measurments to well over 100 Db below carrier with an RTL-SDR with one or more FM notch filters at least at one mile away(FCC practice) if not at the transmitter sample port. Assuming you have data for the filters, pads and such, you can establish a 0Db level of Fo using pads then then install the filters and remove pads as needed to measure 2Fo, 3Fo and such, adding and subtracting as needed. If your using a directional coupler at the transmitter, you have to add a 6Db per octave correction or the data for it. While professional equipment is in order to respond to an official citation, RTL-SDR with a few filters, pads, couplers and such can get the problem solved.
Just checked with a RSP1 clone (which doesn’t have the FM trap that the RSP1A features) : I do indeed see mirrors of WBFM stations around 300something MHz.
But even if this is a problem of his SDR, it may well be that the car receivers are also plagued by this.
Maybe a local field strength maximum, caused by concentration of the signal by surrounding structures. Such things happen.