RFI issue

[Hedges]

I'm a bit lost to be honest, I'm not so advanced in electronics sadly ...

If I recap, you suggest twisting N + L + G and wind them in the same toroid on AC-output, right ? Same for DC-panels-input or not ?



No I don't, I have a little oscilloscope though, if it can be any usefull.
Do you mean something like this ?
View attachment 208684

I could build one I guess.

Something like that. Maybe that shielded coax is better by rejecting voltage coupling.

I've tried a simple loop of wire, later bought a couple near-field HP probes. But I got better results using a pre-amp as well. Voltage sensitivity of some oscilloscopes is only about 1mV. The conducted EMI on AC limit per FCC is about 1mV.

If your scope has FFT, that can pick up tones well below the noise floor, extending dynamic range.
(surprising, but it works. It can see signals 1/10th of one ADC LSB.)

Just a loop of wire held in parallel with each suspect cable or wire should pick up inductively coupled current. That way you could figure out which cable is the radiator - AC, PV, battery, communications.

Or feed into your receiver, which has no trouble seeing the even smaller signal at a distance.

 

[Cagire]

With a loop on a BNC on the receiver, I'm not able to pick up any signal from the cables right now. I think the suppression is quite efficient, to be honest. From the very short tests performed on the radio and my normal antenna, I would say 80% of RFI has been suppressed compared to before the work.


Nevertheless, I'm picking up a strong signal when sniffing on the top and bottom of the inverter itself, very wide peaks every 20 kHz, with smaller ones in between.

 

[AntronX]

You may be picking up magnetic fields from currents inside the inverter or inverter enclosure is radiating. There is another way to test common mode current radiating over wires connected to inverter. Take clamp on ferrite core or split core and clamp it on a pair of wires, for example AC out and feed one turn of your BNC loop through that ferrite core also. This will form a transformer with AC wires as primary and BNC loop as secondary. Common mode current will get coupled into BNC loop and will be received by your radio. Simply placing BNC loop next to wire to be tested provided too little coupling to be sensitive.

 

[Hedges]

How big are slots in the enclosure? Including distance between screw, unless there is gasketing.

Shielding ("Faraday Cage") only works far-field. If there is a magnetic component inside, it will reach right through the sheet metal and induce current in another wire, so that sniffing is not conclusive.

(Antron typed faster than me.)

 

[AntronX]

Here is my 705 picking up S9+20 RFI from my Victron inverter with one loop of wire through clamp on core on AC wires. Clamping on additional choke on that AC wire produced 2 dB reduction of receiver signal. This method is very sensitive. Also it has to be battery powered receiver isolated from mains power as to not create interfering path for RFI. This RFI is undetectable on my backyard dipole with constant S8 RFI on 80m band from neighbors.

 

[Hedges]

A whole 2 dB!

If you make multiple turns through that additional choke, does it attenuate much more?

Where I did this successfully, problem had been improperly grounded RJ45 emitting and coupling to power line.
I did 3 turns through a ferrite with shielded patch cord, then put into shielded female/female bulkhead RJ-45 connector, and grounded that connector. This let us pass FCC testing at EMI lab. 3 turn choke by itself hadn't, and grounded bulkhead connector hadn't, or so I'm told.

For this power cord mitigation, it could mean grounding the ground wire after the additional choke. And attaching sniffer pick-up after the ground.

 

[AntronX]

f you make multiple turns through that additional choke, does it attenuate much more?

Yes it does. 3 turns will create 13 dB attenuation on 3.8 Mhz.

For this power cord mitigation, it could mean grounding the ground wire after the additional choke. And attaching sniffer pick-up after the ground.

There is no ground, that green wire is floating. Solar panels, MPPT, battery, inverter and load (midea air con I am running on inverter at the moment) are all floating disconnected from earth ground. RFI common mode current is created by long ac power cord antenna effect. With this configuration Victron low frequency inverter (Phoenix 800VA) is undetectable under suburban RFI environment. Their Smart Solar MPPT on other hand is a broadband spike generator on HF mostly on 40m band.

 

[AntronX]

Out of curiosity I calculated common mode current flowing over that AC power cord assuming perfect 1 to 1 transformer coupling. So the reading on the radio is S9+20 = 53 dBm = 0.000000005 watts. Converting to volts at 50 ohms = 0.5 mV / 50 ohm = 0.01 mA or 10 micro amps. To get exact common mode current I would have to measure coupling loss of that split core transformer at frequency of interest. That result would only measure current in receiver bandwidth of around 3 kHz. Total common mode current will have to be measured with broadband RF detector of 0.1 - 30 Mhz BW. But this method using HF receiver is good enough for testing attenuation effect of ferrite chokes.

 

[Hedges]

No ground, so whatever EMI filter Midea has, it's shunt caps don't have anything to dump current into.

Come to think of it, that's the case with a 3 prong to 2 prong adapter when no grounded screw is available, or GFCI outlet on 2-wire circuit. Like all the PCs in my "new" house.

0.5 mV? I think that's just under 1 mV FCC limit. For conducted EMI frequencies up to 30 MHz.
What frequency do you see?
Higher frequencies would be radiated EMI limit, V/m at some distance.

Nothing grounded is an interesting case, no leverage to drive currents in a wire (e.g. short whip antenna for low frequencies). Higher frequencies, various ways to make standing waves.

 

[AntronX]

This was at 2718 kHz where RFI was strongest. Everywhere else its much less. PV wire is ~100 feet and AC power cord out of inverter is 50ft both ran in same direction separated by 10 feet, creating sort of J-pole antenna on the ground effect.

 

[Cagire]

Used a clamp 31 + loop on receiver :



I got nothing in AC-out and nothing in earth ground (both <S1 + a few micro spikes))

 

[AntronX]

Looks like your PV wire may still be the source of your remaining RFI. This could be due to PV- wire hard switching -450V at 50Hz in relation to ground with lots of ringing as well. That creates strong RFI due to dV/dt and halfbridge IGBT ringing. So we have to figure out how to filter that without excessive capacitive loading on those IGBTs.

 

[JoeHam]

This was at 2718 kHz where RFI was strongest.

I was wondering how you picked that frequency.

 

[wpns]

I was wondering how you picked that frequency.

I didn't think he _picked_ that frequency, I read it that he _measured_ that frequency

 

[AntronX]

Pretty much, lol. Just spun the dial for highest S meter reading.

 

[AntronX]

Found couple FT240-31 chokes and measured their attenuation. Here are results in dB for attenuation for 20 turns, 26 turns and both of those chokes connected in series. Right table side shows difference between two chokes in positive attenuation dB. For example at 28.3MHz going from 20 to 26 turns produced 3.2 dB less attenuation. At 7.1MHz 26 turn choke had 1.5 dB extra attenuation over 20 turn while both of them connected in series produced 9.3 dB extra attenuation compared to 26 turn alone and 10.8 dB extra attenuation compared to 20 turn choke alone.

MHz	20 turns	26 turns	20 + 26		20 to 26	26 to S	20 to S
0.6​	19.6​	25.2​	29.8​		5.6​	4.6​	10.2​
1.8​	29.9​	36.1​	39.6​		6.2​	3.5​	9.7​
3.7​	34.8​	39.8​	43.7​		5.0​	3.9​	8.9​
7.1​	37.1​	38.6​	47.9​		1.5​	9.3​	10.8​
14.2​	39.7​	36.4​	47.1​		-3.3​	10.7​	7.4​
21.1​	35.0​	31.7​	41.7​		-3.3​	10.0​	6.7​
28.3​	33.1​	29.9​	39.3​		-3.2​	9.4​	6.2​

 

[mactech]

The choke on the right with white wire looks identical to the ones I am using and it works quite well to suppress the upper HF 30m and up was S9 noise and made it unusable. I am fairly certain it is 20 turns.

I need an oscilloscope. It's been years since I've had a functional one.

thanks for the details.