Just not seeing the attenuation promised from Nooelect Flamingo AM BC Filter ...

[KA1GJU]

OK, spent some time this AM doing A/B tests using the Nooelect Flamingo AM BC Filter.
The setup is:
Inverted L antenna, active 4 port multicoupler, two Airspy units with same firmware, two copies of SDRC running side by side. The copy on the left has the AM BC filter inline and the right copy does not.
Screenshots consist of center frequencies of 590, 1270, 1540, 3080, 4620kHz using the same scale, contrast, zoom, etc.

Per the mfgs test data:

distill_am_dc_to_5mhz_sweep (3).png (49.84 KiB) Viewed 8627 times

PLEASE CLICK ON IMAGES TO SEE THE FULL IMAGE!!!

I should see a whopping 40dB attenuation, but I only get 23dB on 1160kHz:

1160kHz_Comparison.png (413.62 KiB) Viewed 8627 times

On 1480kHz, per their specs, I should see 34dB attenuation, but I only get 8dB:

1480kHz_Comparison.png (283.22 KiB) Viewed 8627 times

On 1000kHz, per their specs, I should see 24dB attenuation, but I only get 2.4dB:

1000kHz_comparison.png (506.29 KiB) Viewed 8627 times

Am I doing this wrong, or is the AB BC filter a POS? I swapped it out with another (I bought 3) and found same results.

[G4ZFQ]

Try it with the filter on the radio.
Run a signal generator over the frequency range.
Should give an indication something like you see - or not.

73 Alan G4ZFQ

[KA1GJU]

I'm not sure what you mean, I did have the filter on one SDR, and the other exact same model and firmware did not have the filter on in the above images. That was a side by side comparison running simultaneously.

Now here's the same filter, with out the active coupler inline.

Just the antenna, filter, Airspy, which according to their specs, I should see ~40dB difference:

Screenshot-2025-03-04-094454.jpeg (178.05 KiB) Viewed 8617 times

Here's just antenna and Airspy (no AM BC filter) only, just a few minutes later:

Screenshot-2025-03-04-094600.jpeg (277.83 KiB) Viewed 8617 times

I see a 23.4dB drop (-82.1dB vs -105.5dB) in what little signal is there on 1160kHz, which per their specs said to have the peak attenuation of ~-40dB.

[KA1GJU]

I tried daisy chaining the two filters to see what attenuation I would get. This time I used an different Airspy on a different antenna (Super Station #1's antenna). The whole reason I'm pulling my hair out is whether the AM station 5 miles away is horked up. I am getting strong signals on the second and third harmonic, according to ALL my SDR's in two physical locations at 5 miles away. I want to know if it's a product of having a -23.5dBm signal on 1540kHz, so I am attempting to knock it down and see what happens to second harmonic on 3080kHz and third on 4620kHz.
Here's some screenshots of me attempting to suppress 1540kHz:

No attenuator:

Screenshot-2025-03-04-102635.jpeg (320.17 KiB) Viewed 8603 times

One attenuator:

Screenshot-2025-03-04-102500.jpeg (328.25 KiB) Viewed 8603 times

Two attenuators:

Screenshot-2025-03-04-102432.jpeg (247.24 KiB) Viewed 8603 times

As you can see, the 1540kHz signal goes from -25.5dBm to -31.2dBm with one filter, and down to -58.2dBm with both inline. That's a 5.7dB drop with one filter, and 32.7dB drop with two filters inline. Since I am approaching the limit for image size per post, the next post I will show what happens to the 3rd harmonic on 4620kHz during the addition of AM BC filters.

[KA1GJU]

So now onto the third harmonic of 1540, which is 4620kHz.

No filter inline:

Screenshot-2025-03-04-102605.jpeg (245.45 KiB) Viewed 8603 times

One filter inline:

Screenshot-2025-03-04-102538.jpeg (234.52 KiB) Viewed 8603 times

Both filters inline:

Screenshot-2025-03-04-102331.jpeg (242.79 KiB) Viewed 8603 times

I don't really see too much attenuation, since there shouldn't be with an AM BC filter if there really is a signal on 4620kHz. There was a 6.4dB drop with one and 7.5 drop with the two filters inline.
I'm not posting the second harmonic images on 3080kHz, but the signals were -85.2dBm with no filter, -91.8dBm with one, and -94.0dBm with both filters inline. Fuzzy math says there's a 6.6dBm drop with one filter and 8.8dBm drop with two filters inline. Which one might expect since I'm tuned closer to the AM BC band versus 4620kHz.

So does this tell me the 5KW daytime station on 1540kHz has issues, or are all my SDR's still overloaded despite knocking down the nearby TXR by ~35dBm. This wasn't an issue this summer, it all started this fall or winter at some point in time.

What say you?

[KA1GJU]

I found my DIY W1VLF AM BC Filter! It blows the doors off the Nooelect Flamingo AM BC Filter.
Paul's measurements are here:

w1vlf_AMBC_plot.png (223.71 KiB) Viewed 8547 times

Here's a before and after on Super Station #1's Airspy HF+

No filter:

Screenshot-2025-03-05-143605.jpeg (352.67 KiB) Viewed 8547 times

With the W1VLF AM BC Filter:

Screenshot-2025-03-05-143824.jpeg (223.55 KiB) Viewed 8547 times

I get a 31.4dB drop on the 1540KHz station, 66dB drop on 1030kHz, and a whopping 87.6dB drop on 800kHz!