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Identifying interference


AllanIsaacs
 

Things are really quiet and I decided to take a break from sorting out my Murphy 62B and make an active receiving aerial to cover 25KHz to 8MHz mainly to listen without lots of interference from household appliances etc.
The aerial is based on a ferrite rod (rather than a large frame carrying a loop) with coils tuned by a varicap diode whose control voltage is varied remotely.
I've just been bench testing a prototype with a coil covering around 40KHz to 120KHz and it's still early days but I found a very useful application that might prove useful to anyone willing to spend an hour or two making something similar.
The ferrite rod is extremely directional at VLF and can pinpoint interference sources so that maybe these can be sorted out, perhaps by adding filters or ferrite rings.
It's a lot better than using a portable long wave receiver, but I suppose an SDR is necessary, albeit a really cheap one would suffice.
Those not familiar with SDRs will not be aware that you can use one as an exceedingly cheap spectrum analyser. Not for precise measurements but for simple comparative checks.
I'm recording progress here. Hopefully I can finish the task and fit the receiver on a 20 foot pole without getting distracted.
http://www.radiomuseum.co.uk/active.html
Allan G3PIY


AllanIsaacs
 

If anyone is following this, I’ve tested a breadboard and will soon be erecting the ferrite rod aerial on a pole in the garden.

It’s looking promising and should give me interference free reception and with the benefit of pinpointing sources of noise.

Whether the ferrite rod will give me decent 40m and 80m reception remains to be seen.

The circuit is very simple and should be reproducible if you purchase a few 1S149 varicap diodes (ten for £2.39)

It’s a lot smaller than the old 1 meter loop aerial I was using previously.

http://www.radiomuseum.co.uk/active.html

Allan G3PIY

 


AllanIsaacs
 

Progress is proceeding on my active aerial and interference identification.

http://www.radiomuseum.co.uk/active.html

The good news is that the design does actually work and is easy to reproduce.

It works quite well from 24KHz up to around 10MHz and reasonably well on 40m and 80m.

I listened to a net this morning on 3612MHz with AM stations up to S7 (the aerial is only 5 or 6 inches long).

 

My first attempt to investigate some really annoying long-standing interference resulted in pointing an accusing finger at my cordless phones.

Interference is spread over loads of frequencies and has numerous signatures ranging from white noise to rhythmic pulsing.

I already discovered these Gigaset phones mess up wireless connections to my keyboard and mouse but that’s something over 2GHz.

This pulsing occurs at 100KHz and every 50KHz from there in the form of a group of at least 10 pulsing spikes. It’s clearly carrying information and not just random noise.

Turning off my phone reveals at least one other set of pulses (maybe two sets) at slightly different frequencies. This is either another handset in my own  system of 3 phones or quite possibly from a neighbour.

The next step is to see if I can reduce the noise by using ferrite rings over the power supply lead. Presumably the noise is not being radiated as a proper long wave radio signal as it should be using 2.4GHz?

 

Has anyone noticed interference from cordless phones I wonder?

Allan G3PIY