TY  - RPRT
TI  - High frequency radio propagation charts for sunspot minimum and sunspot maximum 
PY  - 1947
PB  - National Bureau of Standards
CY  - Gaithersburg, MD
SN  - NBS CRPL 1-2_3-1
DO  - 10.6028/NBS.CRPL.1-2_3-1
ER  -