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Not so busy 70 cm ISM band
6 January 2014 | by Yesterday’s post entitled “Car keys in the 70 cm band” showed a very busy band around 433.92 MHz with up to 10 simultaneous transmissions. That snapshot was taken on a Sunday afternoon at 16:32 local time. Here is a much less crowded snapshot taken with the USB SDR-RTL dongle under the same conditions as the previous blog post. The difference is that this is from late Monday night at 23:34 local time:
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There are only three bursts of about 1 second length here. This shows that the activity in the band varies a lot and in my mind strengthens the case for believing that the main contribution is from car keys. But of course, one cannot be certain without decoding the bursts. That is possible for weather stations, as shown by Gough Lui in the article “RTL-SDR: 433.92Mhz ASK/OOK Decoding of Various Devices with rtl_433“. The bursts can easily be heard if the receiver is set for Wide FM, as shown in the settings of SDR# in the image above.
Thanks to all viewers who have made the former blog post the most popular on my blog for this week. Thanks also to the RTL-SDR blog which gave it publicity in the blog post “Looking at the 432 to 438 MHz ISM band“.
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I listened to 433.920 Mhz last week as my door chime shows that frequency being used by the portable and plug in chimes. The frequency when listened to on the FT897D in DIG mode, signals are being received all the time using a co-linear antenna in the attic. I haven’t made a point of listening at different times yet but my push on the door post has a range in open air of fifty metres. I have had phantom chime sounds going off in the middle of the night on the cordless chime but not on the plug-in one. They are set to play different tunes so at night I now keep the chime in a metal box and that box in a metal tin so as to try and stifle the spurious signals by using this double Faraday cage effect. But does it work? Well I haven’t been woken up in the night so far.