This paper discu sses the cum ulative probability distributions of fi eld strength for four 200 km VHF p at hs in Illin ois in t erm s of a classification of r cfractiv e ind ex profiles. It is shown that exten ded elevated layers produce signal enhance ments of 10 to 25 db a bove the level observed in unstratified conditions. Assuming the layer characteristics given b y rad iosonde data, the bes t agreement between calculated and m easured valu es of fi eld strength is obtained us in g a layer model with a lin ear n-profi le. The possible influ ence of smaller layers is also discussed in relation to the observed r esults for cond itions judged to be unstratified or 'Nell mixed on t he basis of sonde data.
An experiment was designed to look for scattering planes from plasma waves traveling parallel to the geomagnetic field lines produced by ionospheric heating. Signals were observed that appear to be due to forward scattering from irregularities associated with ion‐acoustic waves having wave numbers 1.5 to 2.0 m−1. The data, however, are ambiguous as to the orientation of the strongest scatterers. VHF scattering from this effect is very small (30 dbsm) compared to scattering from field‐aligned irregularities (80 dbsm).
_High-frequency radio ~ignals transmitted over two separate 10,000-km temperate-zone paths were rece1ved. over 1-month penods at sea level and at elevated sites. Five frequencies were transmitted from Okm~w~ to Malta an_d one frequen_cy from ~uerto Rico to Hawaii. A comparison of the signal s~rengths mdJCat~s t~at h1gh antenna s1tmg prov1des definite advantages over low siting for longdistance commumcatwn.Ant~nnas at the high sites, which were located an average of 700 ft above the low sites, received greater Signal strengths than those at the low sites, the amount of signal strength advantage (as much as 10 d~) decreasing with in~reasing frequ~ncy. The higher sites were also less susceptible to temporary signal-strength reductions. When signal strengths were in the process of rising and falling as a result of the change of the MUF in the morning and evening, respectively, high antennas provided up to l 1 /2 hr more receiving time for a specified signal threshold.
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