A loop antenna can greatly improve medium wave reception. Loop antennas
are directional and receive signals along the plane of the windings. The
directional quality improves signal to noise ratio of the desired signal
while rejecting signals perpendular to the plane of the windings. Larger
loops are better than smaller ones but good results can be obtained
from moderate sizes of one or two feet on a side. The shape doesn't
make much difference so the loop can be circular, rectangular or
a triangle shape.
The main idea is to cover as much area as possible,
so I would imagine a circular loop would be the best. The loop pictured
here measures 15 inches on a side and is about 1.5 inches wide. It was
wound with 16 turns of #35 copper wire, and has a Q of about 100 at
600 KHz. Larger guage wire might have been better (less resistance)
and therefore higher Q and selectivity, but the arrangment here works
pretty well with a bandwidth of about 6Khz at 600Khz. The loop is tuned
with a 30-365 pF capacitor and covers the standard broadcast band of
550-1700 KHz. The antenna signal is inductively coupled to the radio's
internal ferrite rod antenna so no wire connections are needed.
Simply place the radio near the loop antenna and adjust the position(s)
for best results. You may have to adjust the tuning of both the radio
and antenna several times for optimum results.