Low power AM (LPAM) Transmitter Antenna

LPAM antennas are more difficult to construct than their FM cousins.

  • They have to be longer
  • And they have to be vertical to work well (which becomes an issue if the antenna is longer).
  • Idealy they need to have long ground radials for the same reason.
  • If you plan on building a Part-15 station, which limits the antenna size to 3 meters, the efficiency will be low.

Practical LPAM antennas, while seemingly awful on paper, work quite well in the real world. Lower frequency signals propagate better than higher frequency, which almost makes up for antenna inefficiency. Also, there are tricks to make shortened antennas perform better.

Antenna Theory

First, it is important to understand that frequency can be measured in terms of physical length. And Medium Wave AM antenna are very long. For example, for 1600kHz the length would be 154 feet. As you can see, these are very long antennas! For reasons of practicality (and for Part-15 broadcasters legality) most people will have to settle for much shorter antennas.

  • There is a huge amount information on LPAM antenna's on the Internet, bear in mind a lot of these are for 9-10 foot base loaded vertical antenna (for Part 15 100mw) they still will work at 1 to10 Watts!

As for recommendations

  • A good Ground (Earth) is absolutely essential...!!!
  • The antenna must be tuned to resonance (by either a tapped loading coil or some type of ATU)
  • The higher the operating frequency the better (1400-1600 is good area to try)
  • The longer the antenna the better, ideal an antenna for 1485kHz would be 220 meter's long. No one has that kind of space so a lot of people use Shortened Base Loaded Vertical Antennas (usually 9-10 feet), with a loading tapped coil to tune the antenna.

There is a huge amount of information on Google just search for "PART 15 AM Antenna"

Here are a couple of links to get you started with loads of information, its up to you to decide what suits your needs

AM Transmitting Antenna Articles

AM Transmitter Ground Systems