Cordless phone RFI


Allan Isaacs
 

I've been plagued by interference on the 80m band for ages and decided to have yet another attempt at tracing its source.
For the test I used an SDR PLay sitting on my desk connected via a USB cable (A plug to B plug) to a computer USB3 socket and a long wire antenna.
I initially looked at narrow spikes every 8KHz from 1.7MHz to 3.752MHz. These were very stable across the range and roughly equal in strength at about -80dBm. I got rid of these by changing the USB lead from a grey one marked Xingong USB Shielded High Speed Revision 2 28AWG/1P + 24AWG/2C to an ancient one, orange in colour with GP on one end. The second cable is fitted with a ferrite ring and reduced the spikes by 18dB to -98dBm. The same ferrite ring had no effect on the first cable.
The next problem was a really annoying pulsing sound producing regular horizontal lines on the waterfall a bit like noise from an electric fence. This was at a rate of 7 pulses per second and I eventually found it was emanating from my cordless phone base unit. Lifting the phone (Siemens Gigaset C430) stopped the pulsing and after replacing it the pulsing restarted after about 30 seconds.
Looking for an RF signal I found this at 1.887GHz. This was relatively stable but slight pulsing was apparent.
A second identical handset in another room was also radiating and pulsing.
The solution was really easy. On the first phone I selected settings, ECO DECT, ticked No Radiation and unticked Maximum Range.
The RF signal disappeared, the trace went completely flat and the pulsing vanished.
I'm now left with just general background mush from countless switching power supplies and the phone but this is now at a fairly low level.
Lifting the phone reduces the level of mush from -98dBm to -104dBm so the next step might be to fit ferrite rings to the base unit power cable and telephone line...
Allan G3PIY

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