RFI from 3-8 MHz
On kiwisdr.w1euj.com, I've got this blanket of pulsing noise over these frequencies. I'm looking for the most effective ways to figure out the source.
Location: KiwiSDR setup is located in the barn, which is 100 feet from the house (buried power line to it) and 200 feet from the house and horse barn on the adjacent properties. This is the only thing in the barn that is powered.
Kiwi: The Kiwi is in a metal trashcan, with the power, Ethernet and antenna cables running out the rim. The antenna input has a Mini-Circuits SMA LPF for 30 MHz.
Antenna: A large wire Vertical Delta loop in the trees behind the barn (about 30 feet away) with 50 foot apex, feedpoint at the bottom corner. 4:1 balun at the feedpoint, running to LMR style cable along the ground for 30 feet to the entry point on the barn. Runs through a grounded Polyphaser arrestor, then the wall, then inside for 5 feet to an Array Solutions reciever protector, then 1 foot of cable to the inside of the trashcan.
Power: A linear power supply (2-prong AC connection and inside a plastic box), with the DC lines then wrapped (10 turns) on a large ferrite ring before then running a very short distance into the trashcan.
Ethernet: This is unique. I an running a pair of Ubiquiti Bullet M5 radios to link the barn to the house. They are connected to small yagi antennas mounted on the corner of the barn closest to the house - about 20 feet away from the Kiwi and connected to it via the CAT6 cable. The Bullet in the barn is powered from a linear power supply. Both the power and Ethernet lines are wrapped through several ferrite rings.
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Hi
Remember that the Ubiquiti Bullet M5 have a buildin switchmode psu :-)
/Bjarne
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I'm going to run a long cable out to the barn to eliminate the Bullets and see what changes.
I've definitively eliminated our house as the cause, its down to the neighbors and local sources.
I've also had a listen and it does seem quite bad, especially around 6MHz.
I notice you are using a delta loop antenna. Has it got any sort of Balun attached to it ?
If not I'd suggest trying a decent 1:1 choke balun. Try some thin coax wound 17 times though a FT240-43 or FT240-31 sized ferrite core.
Is the noise still present if you disconnect the antenna but just touch the screen of the antenna lead onto the outer of the KiWi SMA antenna connector ?
If it's still present it could be some sort of ground loop between the antenna and DC & Network cables.
As before try wrapping each cable through a ferrite core using the same type and number of turns as before.
If you still can't find the source. Try making a small 'sniffer' loop by connecting the inner of a length of coax back to the screen of the coax so that it forms a loop with a few inches diameter. The loop should have a continuous screen around its circumference, but it should not be connected at the 'open' end where the inner protrudes and connects to the screen. Connect the other end up to your KiWi and pass the loop over various items of equipment, cables, cases and anything else that's in the vicinity.
You should be able to see the noise and gradually (you may have to reduce the size of the sniffer loop) home in on the exact source.
If this doesn't work build a small tuned loop and DF the source by walking around an taking bearings to triangulate the source of the noise.
Here's the loop I use.
More info here
http://www.g8jnj.net/hfloopantennas.htm
Regards,
Martin - G8JNJ
More chokes on order, but it looks like I'm on a hunt for this summer.
Thanks Martin!
OK, it's quite possible that the 4:1 Balun is a or 'Voltage' balun (check to see how many cores it's wound on. If it's only one, and has DC continuity from primary to secondary, it will be), which will have very poor common mode rejection.
To be honest even if it is a Guanella or 'Current' Balun wound on two (or more) cores, if it has a greater than 1:1 transformation ratio, the chances are that the common mode rejection will still be quite poor. This is because each core is effectively connected in parallel (from a common mode impedance perspective). So the greater the transformation ratio, the more cores end up being paralleled with the subsequent reduction in common mode impedance.
The best option would possibly be to add an extra 1:1 choking Balun (coax wound on a suitable core) on the input of your existing 4:1 Balun.
Use the correct type of core material and number of turns to maximise the choking impedance at the frequencies where the interference is at its worst.
Steve, G3TXQ, has a useful design chart on his website http://www.karinya.net/g3txq/chokes/
Regards,
Martin - G8JNJ
OK that sounds promising :-)
I'dd certainly try a 1:1 Balun on the antenna coax next.
The great advantage of the KiWi is that it's got it's own built in spectrum analyser, so if you add a small directional antenna such as a loop, it's very easy to DF noise sources.
Just use a very long length of coax between antenna and KiWi and a WiFi connected tablet, so that you can wander around and take bearings.
Regards,
Martin - G8JNJ
A couple other tests come to mind - one is that I've not been able to run a test where the Kiwi was COMPLETELY isolated and on some sort of battery power. Anybody have a known method to make clean 5 VDC from battery for the Kiwi?
Regards,
Martin - G8JNJ
Added #43 toroids to all the things, and even one more at the antenna end. Again, it seems like improvement but not the solution. Getting a friend's loop to help diagnose this. In the meantime, there was a development on how the noise is acting - its gone from constant to pulsing. I can see the promised land through the gaps.
It may be worth trying a different power supply in place of the existing dog fence 'wall wart' before trying anything more complicated.
A lot of these supplies have little or no RFI suppression components fitted and may be the source of the noise which is being re-radiated by the 'fence' cables.
Regards,
Martin - G8JNJ
My last post on the subject will be how it turns out with whatever gets fixed/fitted and the resulting waterfall, wrap it all up with a bow for future sufferers.
My neighbor was VERY helpful in working with me to turn this-and-that off (overhead LED lights, pumps, WiFi, etc) but no source was found. Its a barn, ain't much in there - its a mystery what else is tucked away in the rafters that she wouldn't otherwise know/have control of. Whatever it is, digging around further would take more patience that I'd be able to muster myself, and the fix likely more costly than the option of relocating my antenna and equipment.
I left it where I would get back to her if it disappeared and ask what changed. Until then, the noise stays till late fall when I get the change to relocate the antenna. Its a shame - those trees are perfect for the job, and the new setup isn't likely to be as good.
Are there any (battery backup) safety lights there? Freezers?
I had to make the change anyway, ice storms brought down a pine tree, destroying the antenna and damaging the barn where I had the Kiwi.