TY  - RPRT
TI  - Letter Circular 645: methods of using standard frequencies broadcast by radio (supersedes Letter Circular 567)
PY  - 1941
PB  - National Institute of Standards and Technology
CY  - Gaithersburg, MD
SN  - LCIRC 645
DO  - 10.6028/NBS.LCIRC.645
ER  -