Harold A. Zahl

Harold Adelbert Zahl (1904–1973) was an American physicist who had a 35-year career with the U.S. Army Signal Corps Laboratories, making major contributions to radar development.

Career and accomplishments
Harold Zahl was born in Chatsworth, Illinois, the son of an Evangelical minister. While still in high school, he became an amateur radio operator (call letters 6BHI). He graduated in physics and mathematics from North Central College in Naperville, Illinois, in 1927, and then attended the University of Iowa where he earned the M.A. degree in 1929 and the Ph.D. degree in 1931, both in solid-state physics.

Upon completing his doctorate, Zahl joined the staff of the Signal Corps Laboratories (SCL) at Fort Monmouth, New Jersey. At the same time, he was commissioned a First Lieutenant in the U.S. Army Signal Corps.

Zahl’s initial work at the SCL was in research on detecting aircraft using thermal radiation from their engines. In 1934, he filed a patent application on “The Art of Locating Objects by Heat Radiation.” Initially held up because of its classified nature, this was eventually granted in 1946. In 1936, the SCL started research in Radio Position Finding (RPF – later called radar). Zahl participated in the development of the Army’s first fielded RPF system, the SCR-268.

While the SCR-268 was being completed, development of an improved RPF system started and Zahl, now a Major, was assigned to lead the effort. To use a common antenna for both transmitting and receiving, Zahl invented a gas-discharge device, called a duplexer. Two configurations of the RPF emerged: the SCR-270 (mobile) and the SCR-271 (fixed-site). These systems started to be fielded in 1940, and were used throughout the war.

The early systems had large antennas. To reduce their size, a transmitter tube that could produce high-power signals at a much higher frequency was needed (antenna size is inversely proportional to frequency). Zahl developed such a tube in 1939. Called the VT-158, it was capable of 240-kW pulsed power at up to 600 MHz. After the start of World War II, this tube formed the base of the AN/TPS-3, a light-weight, portable. early-warning radar, and a companion the AN/TQS-3, a mortar-detection radar. A total of about 900 of these sets were built and used extensively by the Army, particularly in the Pacific Theater. (The name 'radar' took the place of 'RPF' in 1940.)

The SCL reorganized in 1942, and the radar activities became the Camp Evans Signal Laboratory. For the next several years, Zahl worked closely with the Radiation Laboratory at MIT in their development of microwave radars. He was promoted to Lieutenant Colonel. At the close of the war, he resigned his commission and became a civilian employee at Camp Evans.

In 1948, Zahl was named Director of Research, and remained in this position until retiring in 1966. During these years, he made many personal contributions to advancing electronic technologies.

Zahl was a resident of Holmdel Township, New Jersey, where he owned the Hazienda Evergreen Plantation, and died at Riverview Hospital in Red Bank on October 12, 1973.

Recognition

 * IEEE Harry Diamond Award, 1954 “For his technical contributions, his long service, and his leadership in the U.S. Army Signal Corps research program.”
 * Department of the Army Decoration for Exceptional Civilian Service
 * Legion of Merit