Aerials for Low Power AM

Low power AM aerials are harder to build than their FM cousins. They have to be longer, they need to be vertical to work well (which becomes awkward as they get longer), and ideally they need long ground radials.

If you are building a Part 15 station, which limits the aerial to 3 metres, efficiency will be low. Even so, practical LPAM aerials that look awful on paper work well in the real world. Lower frequencies propagate better, which almost makes up for the inefficiency, and there are tricks to make shortened aerials perform better.

Aerial theory

Frequency can be measured as a physical length, and medium wave wavelengths are very long: at 1600 kHz a quarter-wave aerial would be 154 feet. For practical reasons, and for Part 15 broadcasters for legal reasons, most people have to settle for something much shorter.

There is a lot of information about LPAM aerials online. Much of it is about 9–10 foot base-loaded verticals for 100 mW Part 15 use, but those designs still work at 1 to 10 watts.

Recommendations

• A good ground (earth) is absolutely essential.
• Tune the aerial to resonance, with a tapped loading coil or some kind of aerial tuning unit.
• Use a higher operating frequency where you can. 1400–1600 kHz is a good area to try.
• The longer the aerial the better. In theory an aerial for 1485 kHz would be 220 metres long; nobody has that space, so most people use a shortened base-loaded vertical of 9–10 feet with a tapped loading coil to tune it.