AM broadcast band radio signals travel great distances at night. You can hear medium wave signals from all around your continent. However, you can’t normally hear stations from across the Pacific or Atlantic. That’s not possible unless you use a Beverage antenna (shown above.)
Two things prevent Intercontinental reception of AM radio signals. These are absorption and interference. To begin with, absorption is the reduction in strength as the signal bounces towards you. Signals become weaker each time they bounce off the ionosphere. Boosting antenna gain can overcome some absorption. Furthermore, interference arises when other signals drown out the desired signal. Interference comes from closer transmitters and noise. Interference is reduced with a highly directional antenna that rejects signals and noise. The Beverage antenna is high gain and very directional. Hence, it is great for long distance reception.
A radio hobbyist in England recently demonstrated listening to a whole bunch of transatlantic signals on his Beverage antenna. It is 200 meters long and aimed at the United States.
Beverage Antenna – How it Works
Beverage antennas were invented by Harold Beverage around 1920. He discovered that a straight, long wire is very directive. Mounted just a few feet above ground, it does a great job of receiving signals at long distance. The wire is normally 6-10 feet above ground, and at least a wavelength long. For the AM broadcast band, this means at least 600 feet or 200 meters long. The wire is terminated with resistance at the far end.
For technical reasons, signals from the favored direction travel down the wire towards the receiver and build up in strength. Signals from the other direction get terminated in the far end resistor. These effects are most pronounced when the antenna is at least a half-wavelength long.
If you have the space, beverage antenna is simple to build because it is just a length of wire and a resistor. The main challenge is figuring out how to mount the antenna at the proper height. Many users just attach the wires to trees. Another challenge is that you need several beverage antennas if you need several directions. This works as long as there are enough trees aligned with the desired direction. Here is a demonstration of how to build these antennas.