LAN noise filter

Suffolk_guy61 · March 18

Does anyone know if you can get interference/noise down the RJ45 cable from your router on the 0 to 30Mhz spectrum. I have my Kiwi connected to the internet via a TP Link powerline adapter to get the internet from the main router in the front room to the back of the house, (the internet travels via the electrical circuit) then the RJ45 cable goes from the tp link adapter to the Kiwi. I have seen LAN noise filters available but would they do any good I'm wondering. I am just trying to see if I can reduce interference, but I think it's due to living in an urban area surrounded by all our modern day electrics etc.

n6gn · March 19

One absolutely can. Common mode current (not differential between pairs say) can be the most common cause of degraded noise floor.

Current coming down the LAN (or PS), common to all conductors, flows into the 'ground' plane of the Kiwi (PS lines look like they are shorted at RF frequencies, LAN has relatively poor CM rejection through its ferrite even if its a balanced circuit), along the groundplane and out the shield of the antenna SMA on the other end.

That path even if only a few milliohms resistive is several cm long and voltage drop is created due to it's inductance of 32nH/inch from permeability of free space. The difference between the V at the ADC's reference and the preamps 'ground' essentially becomes an input to the Kiwi.

A 50 ohm source driving current through that path produces an unwanted response only ~85 dB down. That may sound like a lot but remember that the input floor of the preamp is down around -155 dBm/Hz so it doesn't take much LAN or PS noise current to cause a problem.

That's why a low inter-winding capacitance isolation transformer at the Kiwi's SMA can often make a big difference in performance.

We live in a noisy world but it is possible to make great reduction in the Kiwi's susceptibility to much of these sources with effort. Better to remove the susceptivility than play whack-a-mole with every new source of QRN - if you even recognize that it's there.

Note that there's another common way noise gets into the Kiwi when there isn't good balance at the antenna. Current on the coax feedline converts to differential voltage to the degree that the 'balun' isn't perfect. Typical ferrite baluns may only provide a few dB of extra balance when compared to the CM impedance of the feedline .

There's a little more detail on some of this in some notes I wrote which are available on the wsprdaemon.org pages.

jks · March 19

Most of your problems would probably disappear if you got rid of that awful Ethernet-over-powerline RFI generator and replaced it with this on the Kiwi end: TP-Link TL-WR802N

Also, reduce the Ethernet speed to 10 mbps on the admin network tab.

jimjackii · March 19

I can confirm that these Powerline Apaters are a major source of interference. I also used them to connect to a garden shed. Unfortunately, there was a lot of interference (horizontal lines) on the Kiwi waterfall, which became stronger with more network traffic.

I now have a WLAN link with 2 outdoor WLAN devices from TP-Link CPE510 in use. There is probably still some minor interference, but not as much as with the powerline adapters.

Ragards, Steffen