Mini Whip at a noisy urban area - tips for grounding and the de-noising?
I plan to buy a second Kiwi for at home.
Noise levels are increased dramatically at the last 2 years. My home is in a 10-story soviet style block. The MiniWhip is attached to the elevator house on the roof - no other way here. Pic: https://postimg.cc/qzjfLnXp
New motors for the ventilation system, new elevator control system, etc.
The MiniWhip worked with acceptable SNR with grounding 3 points of the feedline - no any common mode choke. Worked good with my Kiwi which is at the country house now (with the dipole)
First grounding point: PCB of the MiniWhip - connected to the lightning rod. Second GND: around the halfway of the feedline to the another lightning rod and finally inside the flat next to the shack connected to the water pipe of the radiator.
So I added a common mode choke ~3.6m far from the Whip, made with FT240-31 toroid with 22 turns of an RG174 thin coax (pic: https://postimg.cc/xq9n30Dy), noises are decreased BUT a "gain peak" appeared around 6 mhz. The 49MB stations overloaded the receiver. First GND remained at the PCPB GND point. The 3.6m coax resonates on 6 mhz? Okay, I put the first GND point BEFORE the CMC (not at the antenna side) and the "gain peak" was gone, but the LW band became noisy. Let's try some thing. I put back the GND to the GND point of the Whip and I move the choke ~ 10m away from the whip - the gain peak appeared again, but on 9 MHz. So the 31 MB stations overloaded AGAIN. I moved ~20m from the whip, next to the 2nd GND, (the 2nd GND is on the TX side of the Choke). Gain peak moved to ~12 mhz. I added the 2nd GND point to the antenna side of the CMC - gain peak gone but LW+MW became noisy! Argh! So the lightning rod became noisy too. Disconnecting the 2nd GND - more noise! CMC removed - loot of noise!
So what is the best solution for me? I must filter the GND too. The whole feed line is ~40 meters long.
Maybe this can work? Whip - 3.5m coax - CMC - 1st GND (connected to the RX side of the CMC) - Coax - CMC - 2nd GND (Connected to the RX side of the 2nd CMC too) - Coax - PSU with GND - Coax - Choke - Shack?
I plan to add FT140-77 with 19-20 turns of thin cable to the whip and to the shack and keep the FT240-31 but with 13 turns of RG58/59 instead of 22 turns of RG174. (Combining them to cover the 100 khz - 30 mhz band...)
What do you think?