Hi Dave,

Well, you can couple the signal from the antenna into the radio in
different ways, and though multiple ways may work, one may be more
practical than another, and may be more efficient with available parts.
The feedpoint of an antenna--the place you couple signals in or
out--may look like a low impedance or a high impedance; it may have
quite a bit of reactance in it (what look like capacitance or
inductance). Getting the most signal out depends on matching to that
impedance. Fortunately, for most medium-wave signals (standard
broadcast), you can have inefficient coupling and still have a very
useable signal, because the signals have to be pretty strong to be
useable. They have to be larger than the atmospheric noise, which is
rather high at those frequencies.

A disadvantage of simply connecting an antenna wire to the "hot" side
of the LC at the receiver input is that a random wire will likely have
quite a bit of effective capacitance, causing the LC to be detuned from
what you expect. And if the wire is fairly long, it may represent a
relatively low resistance, which will make the tuning much less
sharp--it will lower the LC (tank circuit) Q--and you'll be listening
to more than one signal at a time with your simple receiver design.

It can be an advantage to make the coupling variable. You can do that
by changing the number of turns, or by moving the ferrite rod into or
out of those turns, with the inductive coupling. Or you can connect
the wire to the "hot" side of the LC tuning tank through a small
"trimmer" capacitor.

There's a lot more to the story, which you'll learn by studying,
experimenting, and keeping an open mind and high level of curiosity
about things. Joel in your other thread told you some about bandwidth
of LC tanks...you can determine the Q by measuring the 3dB bandwidth of
the tank. Then the Q is center frequency divided by bandwidth. If you
have a good coil and capacitor, and no circuit loading the LC, the Q
might be, say, 200, limited by losses in the coil and capacitor
themselves. If you add a resistor--either a part designed to be a
resistor or a circuit that has an effective resistance--it will lower
the Q and broaden the bandwidth. If the power lost in the resistor is
equal to the power lost in the coil and capacitor, you will have a Q
that's half as large--100 in this case. A receiver designer will pick
a Q that doesn't limit the bandwidth too much, but gets rid of
interfering signals. And if the receiver relies on two or more LC
tanks to track together, the Q will be picked a bit lower to allow
errors in tracking without hurting the performance.

Again...this is only scratching the surface. You should probably look
for some books like "Radio Amateur's Handbook" for a lot more
discussion of things like tuned circuits.

Cheers,
Tom