Wireless direction finding loop is a science and art.
One of my favorite old books is Wireless Direction Finding and Directional Reception, by Ronald Keen, published in 1927. I recall buying this used book at the Children’s Hospital Book Market in Winnipeg during the 1970’s. (At least four editions of this classic were published between 1922-1947. You can find my second edition online at the University of Hong Kong. Google has the 1922 edition available online, as well.) This book was all about designing, building and using loop antennas for direction finding.
My first real hobby was Medium Wave DX, or listening to distant stations on the normal AM broadcast band. During junior high school I managed to hear stations from all Canadian provinces and American states. The best antenna for this type of radio listening was the small wire loop antenna, based on the “frame aerials” (see pictures) developed for wireless direction finding nearly a century ago.
The direction finding loop is basically a square coil of wire, usually 2-3 feet on each side, tuned to resonance by a large capacitor. This forms a tuned circuit which is very sensitive to radio signals on one particular frequency. When you change channels, you have to retune the antenna. You can also build loops that are broadband. You have one inside your household AM radio. Loops that fit inside radios are coils wound on cores made of ferrite material, which increases the performance of the small loop.
Direction finding loop – how it works
This loop is very directional. It picks up signals off the ends, but very little off the sides. When a radio signal arrives off the ends of a loop, it induces a voltage on the vertical wires. These voltages are slightly out of phase and that causes radio signal current to flow in the loop. This is good for reception. On the other hand, when a radio signal arrives broadside, it is in phase (the same) across the wires. Little current flows and little signal transformed through the antenna.
The direction finding loop has a figure 8 pattern – strong off each end, and nulled in the middle. Radio listeners will turn the antenna, either to increase the signal off the ends, or null the signals arriving broadside. The effect is amazing. By turning the loop broadside to a strong local station, you can almost make it disappear. This lets you hear distant stations from different directions.
You can try this out for yourself with your AM radio. If you tune in a strong local signal and slowly turn the radio, you can almost make the strong local station disappear at the broadside point.