Simplest one you can make is by taking the main tuning capacitor and ferrite
rod antenna out of an old AM radio, wiring them in parallel, then putting that
circuit in series with your antenna. That will allow you to peak the frequency
you're tuned to, reducing all others. If you wire that circuit (an LC circuit)
from your antenna to ground, it will notch the frequency it's tuned to ...
kinda handy if you've got a local pest that splatters across the bands.

jw
K9RZZ