VHF and Microwave Propagation Characteristics of Ducts

Observations from many years of amateur radio operations together with commercial microwave propagation studies and are used to illustrate the nature of the VHF propagation in ducts. Recently developed formula for characterizing VHF and microwave propagation in ducts are used and modified to reconcile the observations with theory. Measurements from a high resolution SODAR are used to show the complex structure and characteristics of elevated ducts. The ducts are shown to have very strong temperature gradients and to form in substantially the same range over long periods. The ducts are very stable in the vertical plane and dissipate during the day. Equations are used to illustrate how propagation probably occurs in ducts and how signals enter and leave a duct. The nature of VHF propagation via ducts is illustrated using basic ray diagrams.