Mains noise

Useful information regarding antennas for SDR products.
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Mains noise

Post by vk7jj » Thu Aug 02, 2018 5:50 am

There are excellent discussions already happening with excellent input from a number of people about electrical noise.

In my own location I have determined by experimenting over a year or three that most of my unwanted interference is mains borne and is conveyed to me by

a) being radiated from mains wiring from afar and
b) being fed directly into my radio shack, feed lines and antenna systems by what I have come to call "the mains feed line"

There is very little to be done about a). There is a lot that can be done about b).

Because at my location the principle source is b) I have over time learned how to get rid of it.

The first step is to appreciate the principle of "the mains feed line" so here goes with that. This post is an attempt to provide a really simple picture of what is happening to me. The second part - what to do about it - is also pretty simple.

If this post generates any interest I'll have a go at what I did about it but before that I'd appreciate the thoughts of others.

Cheers, Phil VK7JJ
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Re: Mains noise

Post by pnoelw » Thu Aug 02, 2018 10:42 am

This has to be the bane of every listener's life! I would be most interested in how you have approached and resolved this in your environment.
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Re: Mains noise

Post by CaptainNemo » Thu Aug 02, 2018 1:49 pm

Please don't keep us in suspence... :o :shock:

Very interesting subject with fine exposition by vk7jj... please go on!!!


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Re: Mains noise

Post by DanubeBCL » Fri Aug 03, 2018 7:49 am

The standard situation is type a) (noise radiated from mains wires in the houses that surround you).
You can do nothing against that type of noise, because...
... the neighbours do not understand why they are the jammers. You cannot persuade them to switch off the noise sources or use line filtering, grounding ... They do not understand that "the old man with the antennas" near them wants to have noisefree radio reception (or they are ignorant and do not care).
... the regulations in most countries are very lax. They allow noise radiation at highest levels (e. g. for so called homeplug powerline modems) and the authorities do not care much about RF noise.

73s, Heinrich

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Re: Mains noise

Post by de_WY6K » Mon Aug 06, 2018 3:40 pm

And the rest of the story is....
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Re: Mains noise

Post by MesaMike » Mon Aug 06, 2018 5:03 pm

Don't ground your radio/coax to the mains ground wire! If you do (even if you don't) it's important to use a balun/unun to connect the feedline to the antenna so that common mode currents on the feedline don't couple into the antenna.
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Re: Mains noise

Post by vk7jj » Tue Aug 07, 2018 1:25 am

Hi guys and thanks for the comments! Instalment number two of this breathtaking thread is at hand.

We can't look towards "what we can do about it" until we sus out more of the problems and then visualise a possible goal we might work towards. So here goes with that.

We are our own worst enemies because we send lots of electrical crap into the mains feed line.
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Antennas provide the greatest single opportunity for us to have fun with our receivers and improve our signals.

Your little SDRPlay black box can only deliver to you what you put into it!
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PS. No brownie points will be awarded for finding the mains short circuit. There was also a technical error in the first post, thank you all for not pointing it out and totally embarrassing me.

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Re: Mains noise

Post by vk7jj » Sun Aug 12, 2018 7:41 am

Hi again.

Sorry about the large graphics in the previous post caused by my screen grabs being 144 dpi. I forget to resample them. Likewise forgive the delays between posts.

OK. There here are three obvious parts of the mains interference problem to think about ranked in probable ease of doing something useful about.

Perhaps you're way ahead of the curve so maybe this is a check list to tick off!

1. Checking out known interference generators like switch mode power supplies
2. Isolating the antenna and feed line from "the mains"
3. Acquiring / building a more suitable antenna and feed line

Let's look at number 1 first. Here are the devices in my radio shack that were obvious candidates to check out.

- small switch mode mains power plug packs for a number of Raspberry Pi's
- same for an ethernet switch, an 8 camera DVR security system, a cheap video monitor for the DVR, two old laptop computers
- a large 13.8 volt 30 Amp switch mode bench supply used for amateur transceivers, an AA/A/9v NiCad recharger
- an old fashioned two tube fluorescent light
- an iMac, an old headless MacMini
- a 5KW solar system with mains inverter

I checked them out for noise using the following methods:

a) Tune a small battery powered transistor radio to a vacant spot on the AM broadcast band or if you can, to a band of interest to you. Make sure it is an AM radio, not FM. Maybe you have a hand held ham radio that can tune to AM with a broad band receiver that can actually be tuned to most of your frequencies of interest.

Walk around your candidate suspect devices and wiring, push the radio up as close as you can to each device and the wires coming out of it. You will be in no left in doubt when you find an offending device because as you approach it more closely it will give an increasingly loud buzz or other noise. Sometimes you can follow the buzz even along a piece of coax or an ethernet cable or a power cable or an earth lead.

b) Turn off all the suspect devices then turn them on one at a time and listen for or watch to see any known offending signals in your waterfall.

c) Buy or scrounge a cheap old fashioned ferrite rod to make an RF sniffer, the type used in old transistor radios is great. Wind ten or so turns of any suitable wire on one end of it and connect / solder the ends of the wire to a small audio plug that will fit in the microphone input of your mobile phone. Connecting an inexpensive small signal diode like a 1n4148 in series with the wire from the ferrite rod that goes to the active pin on the audio jack will allow you to see even more. Download and install an FFT app onto your phone and run it. You can poke the ferrite rod right into suspect spots and see the resulting crap.

d) Using a similar ferrite rod sniffer connected to your SDRPlay's antenna input via a spare coax lead also works a treat particularly if you have existing known interference and you suspect it's coming from something close by in your room.

Obviously the worst offenders for me were switch mode power supplies.
I had to give up using the big 13.8 volt bench supply and replace it with an older fashioned linear heavy duty supply, linear supplies invariably have large heavy mains transformers in them so they are easy to identify and painful to pay for the postage of. I spent quite a while trying to tame it but had to give up, my supply was hopeless and removing it and using a linear supply made a huge difference.

Crappy switch mode supplies usually inject interference onto their low voltage wires and also onto your mains wiring and that is often overlooked. Your ferrite probe will usually make it blindingly obvious.

In the next post hopefully without much delay we'll look at some alternatives for replacements, some fixes, and at some simple home-brew filters for those who'd like to have a go at construction.

In the mean time if people have come across particular devices or causes of interference in or about the home or workshop that are relevant that would be great, my gear is quite mundane.

Cheers, Phil

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Re: Mains noise

Post by vk7jj » Sat Aug 18, 2018 6:22 am

Hi guys, I've temporarily (I hope) injured my left eye after my eyeball was stuck by a small burning ember, I'm out of action for the mean time, apologies.

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Re: Mains noise

Post by g1hbe » Sat Aug 18, 2018 7:37 am

Wow Phil, commiserations on the eye injury. I hope it recovers soon, but I know how slowly eye injuries go and how painful and irritating they can be. Good luck with it.

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