Free-space field levels are common in a tropospheric duct environment even at distances well beyond the horizon, when the frequency of propagation is above the minimum trapping frequency of the duct. Thus the likelihood of experiencing such a field enhancement is greater at higher propagation frequencies as long as molecular absorption does not become appreciable. A formulation of the wave-guide duct problem is described which facilitates numerical solution without encountering the numerical difficulties for higher frequencies reported by previous investigators. Field predictions are shown to compare favorably with measurements for 2.201-GHz propagation in an elevated duct environment.