box loop antenna
box loop antenna can be one of the most remarkable and fun
ways to improve your MW/AM reception on almost any radio.
Most old AM radios made in the US (ca 1930-1975) used variable
tuning caps with two sections, one for tuning the incoming RF
signal, the other for tuning the local oscillator. The RF tuning
section is typically about 40pf at the top of the band (1600 kHz)
and about 365pf at the bottom (540 kHz). The local oscillator
section has a smaller max value.
You want one of these, or anything that covers the 50-365 range.
A zero-to-500pf would be nice, because it allows more room for
error in winding your coil. You do not want something that
tunes from 365pf to 500pf, even if available.
So, tear apart a yard-sale radio, and use the "bigger" section
of the variable cap -- its the 365pf part. It will work just fine.
After you build your loop, set the cap to approx where
you would expect a known MW/AM radio station (for example, 900
kHz would be about half way along a typical dial) Then add or
remove turns to the box loop to get the best signal from this
station at approx the right point on the dial.
Adding turns to the loop will increase inductance, which lowers
the resonant freq for a given capactiance. So, if a 900 kHz station
is coming in with the plates of the cap almost fully unmeshed
(lowest capacitance), then you have too much inductance in the
coil. You can reduce the inductance by removing turns (one at
a time), or by spreading the turns apart, if your design permits
This kind of project is about the most fun no-cost project
for a MW radio fan I can think of! Its incredible what a cheap
portable radio can do, mounted inside of a box loop. For a nostalgia
trip, I use my Zenith Royal 500 (ca 1958, but $3 at a yard sale)
American-made 8-transistor portable, and listen to CBC news from
Montreal or Toronto.