By Gary Thorburn
<gthorbur@ub.com>

A homebrew 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.

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.