Microwavelmillimeter-wave propagation in woods and forests was investigated at 9.6, 28.8, and 57.6 GHz. In order to perform the measurements under well defined, reproducible conditions, a regularly planted, well groomed stand of trees of about the same growth (pecan orchard near Wichita Fails, TX) was chosen as the test site. The experiments were repeated, over the same transmission paths, under both summer and winter conditions, i.e., with trees in leaf and without leaves. Of particular interest were the range dependence, beam broadening, and depolarization of millimeter-wave beams in vegetation and the frequency dependence of these effects. The experiments have shown, in particular, that the range dependence is characterized by a high attenuation rate at short vegetation depths and a reduced attenuation rate at large depth. For trees fully in leaf, the transition between the two regimes can be rather abrupt and the change in attenuation rate substantial. Just after the transition significant beam broadening (and depolarization) occurs. A theory of millimeter-wave propagation in vegetation was derived using transport theory. Theoretical and experimental results are in good qualitative agreement; both show the same trends. The theory explains these trends in terms of the interplay between the coherent (direct path) field component and the incoherent (multiscattered) field component. Achieving good quantitative agreement will require further refinement of the theory.

Measurements on point-to-point transmission at street level were performed in downtown Denver, CO, with RF probes that operated in the upper microwave and lower millimeter-wave bands. All probes were mounted on self-contained vehicles, thus permitting a variety of path scenarios. Information on performance of SHF/EHF channels when propagating in an urban environment on both line-ofsight and non-line-of-sight paths is presented.

Within the past four years, experiments with high power, high frequency radio waves have proved the feasibility of temporarily altering the ionosphere's properties. Many different radio and photometric effects have been observed as a result of the ionospheric modification. This paper provides a survey of some of the effects observed with vertical incidence radio techniques when the US Department of Commerce's Platteville facility, near Boulder, Colorado, is used to illuminate the overhead ionosphere with an equivalent isotropically radiated power of the order of 100 Mw. A variety of different and repeatable phenomena are observed to occur in the F region. A few always occur; for others, apparently, certain ionospheric conditions must exist. E‐region effects observed by these techniques are small, but substantial effects are noted in the D region.

A method is described to accomplish automatic data selection and profile inversion to obtain ionospheric electron‐density profiles from digitized radio soundings. The profile inversion is based upon the well‐established formulation of Paul [1960] by which the optimum radio‐frequency sounding intervals can be specified from an approximate knowledge of the profile. The expected virtual height coordinates (h') at these frequencies (ƒ) are likewise estimated, and procedures are then used to select h'(ƒ) observations nearest the predicted coordinates from a subsequent digital ionogram. From these the next profile is obtained. The process adaptively follows the changing shape and detail of the profile. The procedure requires an average of 20 sec per profile on a standard data‐processing computer and can be adapted, with benefit to on‐line real‐time use in our ‘dynasonde,’ digital ionosonde.

Some early observations of significant ionospheric modification resulting from F‐region heating by a very high power, high frequency transmitter located near Boulder, Colorado, are presented in this paper. A description of the Platteville heating transmitter facility, the locations of ionosonde and photometric diagnostic equipments, and the geometry of the experiment are given in a companion paper by Utlaut [1970]. A single‐plane inverted log periodic antenna, which has a half‐power beam‐width of the order of 90°, was used for transmitting. The ionosonde transmitter, which has a peak power of about 25 kw, was operated by using a 200‐Hz pulse repetition rate with a pulse duration of 64 μsec.