A little while ago I purchase a Xeigu 6100. One thing I did notice however was an issue where my local AM station transmission bled over onto the radio amateur bands. Very odd.
Anyway, after some googling I found the Xeigu transmitters do not have any form of built in broadcast ineferance filter (or high pass filter with a cut off around the bottom of the 160m band) and therefore the front end of my radio was getting swamped by a large local signals, specifically AM broadcast signals. Whilst googling I found a design for a "Popcorn BCI" filter which is essencially a high pass filter with signal attenuation below 1.8mhz.
The desing was very simple so I thought lets build one and see what happens. I designed and had made some PCB's. Bought some toroids and built a BCI filter.
I measured the cutoff and attenuation of the filter using my NanoVNA, this matched the design of the filter I put into rf-tools.com designer. It turned out the Popcorn filter was in fact a Butterworth Series First 5th Order filter using standard part values. The filter responce desing on RF-tools.com matched very closely the VNA reading I took after I made my first filter.
The filter cut off starts at the bottom of top band and then attenuation builds up quickly to -10db at about 1.3mhz and down to -68db at around 50khz.
With 9 PCB and parts left, I decided to sell the BCI filter to Xeigu users with the same issues I had. One of our customers video's the effect using one of the filters had on his Xeigu 109.
All in all, it worked out pretty well.
One thing I will play with will be to change the values of the capacitors to around 2x1.5nf and 1x470pf to start the cut off at the bottom of the 80m band. It may be a good option to have as well.