[URL unfurl="true"]https://blog.kn4vhm.com/some-sdr-resources/[/URL]
[URL unfurl="true"]https://janilab.hu/index.php?route=product/product&language=en-gb&product_id=52[/URL]
[URL unfurl="true"]https://www.ebay.com/itm/151894217160?mkcid=1&mkrid=711-53200-19255-0&siteid=0&campid=5336136228&customid=&toolid=10001&mkevt=1[/URL]
and more detail here
[URL unfurl="true"]https://www.sv1afn.com/en/product-category-4/HF-Amateur-Bands-Preselector.html[/URL]
In any case your first line of defense is an antenna resonant at the desired frequency. The next step is impedance matching. Finally a band pass filter or switchable network (preselector.)
Analog HF preselector simple
[URL unfurl="true"]https://aa7ee.wordpress.com/2021/03/23/a-passive-tunable-hf-preselector/[/URL]
Analog HF preselector not so simple
[URL unfurl="true"]https://www.herostechnology.co.uk/pages/hf_preselector.html[/URL]
Analog HF preselector explained
[URL unfurl="true"]https://www.hamradiosecrets.com/hf-preselector.html[/URL]
Yet another
[URL unfurl="true"]https://www.hamradiosecrets.com/hf-preselector.html[/URL]
'bout covers it ;-)
