05 January, 2014

Car keys in the 70 cm band

The 70 cm amateur band covers from 432 to 438 MHz in Norway and radio amateurs have primary status. Secondary users are among others remote controls for keyless entry systems for cars since it is an ISM (Industrial, Scientific, Medical) band also.

I wanted to see how much traffic the secondary users generate. I used my RTL-SDR dongle with RTL2832U and R820T chips that I bought on Ebay for less than 10 US$ almost a year ago. The antenna was a roof mounted HL-B61N vertical (1.7 m long). This is the output of the SDR# program:

Press image for a larger view

It is clear that this band is pretty busy! No wonder that amateur repeaters have had to move their input frequency away from this frequency range. 

The waterfall covers 10-12 seconds and there are up to 10 transmissions simultaneously. The nominal frequency is 433.92 MHz and there are emissions from 433.75 - 434.05 MHz. I live in a suburban area with about 1 million people, but I imagine that I only pick up a small part of the remotes in this area since the car key transmitters are very weak. Anyway it demonstrates both the versatility of the cheap software defined receiver dongle, as well as how busy the band is. 

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  1. Sverre, did you do the direct conversion mod to RX on the HF bands below 24 MHz?
    I saw your twitter post about 3524 kHz so I think you did, right?
    If so, is it easy to do the mod? Any advice?

    1. Hi, yes I did the direct conversion mod by soldering two wires to the capacitors on pins 1 and 2 of the RTL2832U chip. When the front-end is the R820T only the I input is used (pins 1 and 2), so I really wanted to have my HF input on the unused Q input on pins 4 and 5, but I don't have soldering iron with a fine enough tip for soldering directly on the chip's pins.

      The two wires went to a MCL T1-1T transformer and then to my antenna via my QRP antenna tuner to get a minimal amount of filtering. But it needs more filtering as there is aliasing. I have tried it on the 3.5, 7 and 25 MHz bands and it works well.