A cheap and easy passive MW antenna helper for pocket radios can be made using an old loopstick antenna and a mica tuning capacitor.
It can be used to give a boost or also to null out an offending station in the wrong direction. I built a bunch of these decades ago. It's not a huge boost but often enough to pull the station out of the noise, and it is pocket friendly.
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It can be made from any old transistor radio parts. The cases have an interesting back story. This was about 1991. I had to make a trip to the dump one day, and noticed several sealed boxes freshly dumped right by the side of the road to the dump site. Stopped, got out and kicked one open and it was full of these "CB Rings", all brand new, packaged with instructions and a strip of double sided foam tape. Turns out these were some failed product from the waning days of the CB craze. (poetic, no?) I grabbed a box full.
It was a FM transmitter with a circuit board that appeared to be along the lines of Mr. Microphone circuitry, with a condenser mic. It came with a little plastic tuning tool. You would put a 9v battery in, turn it on, tune to empty FM channel, then stick the CB Ring to the bottom of your mobile CB with the mic hole over the radio speaker. Now you could listen to CB over the car stereo. Woo Hoo!
Where the heck did they come from? Oddly enough, a small world. I worked at the local radio station at the time, and asked the Gm/owner if he knew anything about that product. His face turned red and out came a string of what for him, a Baptist deacon, were the equivalent of curse words and aspersions. After he calmed down, he told me that he had bankrolled that endeavor and the guy who talked him into it had been the station "engineer" at the time, and had a bunch of the things manufactured, then quit his radio job, and skipped town with the rest of the money. They ended up in a storage building for years, then to the dump.
Great project cases though. Wish I had some today.
BTW.. Did you know that Mr. Microphone lives still today?