Build a co-ax filter.

You can build a simple coax stub notch filter.

These are the rules.

From the T connecting point, a 1/4 wave transmission line shorted at the far end is, viewed as open.
From the T connecting point, a 1/4 wave transmission line open at the far end is, viewed as shorted.
The speed of light in free space is about 300,000 km/sec.
To find wave length divide frequency in MHz/300.
The speed of light in solid polyethylene coax is 66%, a value known a velocity factor.
The speed of light in foamed polyethylene coax is 78%.
Find the wavelength in coax take the velocity factor % of free space wavelength.
Use 1/4 of this length for an open 1/4 stub notch.

For example, If we want to filter an FM station at 100 MHz = 3 meters.

We are using polyethylene, so 66% =1.98 meters.

1/4 of that is .495 meter.

I cut a length of coax .495 meter long, and leave it open at the far end.

I make a T connection, with the stub dangeling and I now have a notch at 100 MHz.

Since the notch repeats at 3/4 wave length, I also have a notch at 300 MHz, 500 MHz, and so on.