Adjustable antenna coil from scrapsI wound a coil on the pen barrel and inserted the core.

This post gives instructions on winding an atenna coil.

http://radio.radiotrician.org/2018/05/how-to-wind-ferrite-antenna.html

This is good for what it is but what if you want this instead?

It is a bit to expensive for most bottom feeders.

This one was about half price but still out of my reach.Besides it is usually more fun to make your own when you can.

This one offers a starting point.

http://radio.radiotrician.org/2016/12/making-slug-tuned-coil-with-pictures.html

 We need a screw, nut (insert) and tube (pen barrel).

A core , screw and nut

I drilled a hole in the screw and inserted the core lead. A little solder should keep it in place.

I would a coil on the pen barrel and assembled them.

I soldered the first one.soldered and glued glued the second. I think the glue helped.

The first adjust from 30uh - 100 uh.

The second adjusts from 40uh - 200uh.

The core is 3/4" and the screw is 40 tpi so the coil requires 30 turns for a full range adjustment.

If your core doesn't fit a tube you have available you can make one from craft paper or paper tape.

Check the junk box and think about it.

Enjoy the hobby.

Comment:
The connector is a 
strip of sheet metal about 1" long. I wrapped it around a rod and crimped it with needle nose pliers to form a loop. Then holding the loop split the ends to form mounting ears. They had a drop of glue applied then masking tape to hold them in place. a more traditional treatment would be to cut a strip of cardboard such a s a cereal box and split it to allow the tabs to be inserted. then gluing it in place. I think I may make a couple more using that idea but for now the proof of concept is looking good. Well? Not looking good but functional. The store bought ones are prettier but the price is right on these.