What is in these "couplers" is it just a huge inductor to cancel out
the capacitive reactance?

I discovered this phenomenon the hard way when I replaced the shielded
cable on my car antenna with just some simple coax. Absolutely no AM
signal at the end. Then I figured out why the real cable is a coax
with a very tiny center conductor inside a hollow tube to minimize the
capacitance. If you used something similar to connect any antenna to
your loran it would probably work better.

Have fun.