Rob's web

How high is an inversion?

Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - QST - How high is an inversion?

The idea still persists that great height is important in long-distance v.h.f. work, despite the fact that all our v.h.f. records beyond 200 miles have been made by home stations,-most of them close to sea level. From W1KXP we have an interesting report of tests made during routine flights by planes of Northeast Airlines, giving evidence that it is entirely possible to go too high in search of that "ideal location" for v.h.f. record-breaking attempts.

As ground-station radio operator at Boston, W1KXP has ample opportunity to observe changes in v.h.f. propagation, as planes and ground stations use a frequency of 130.3 Mc. whenever conditions permit communication on that frequency. Southbound flights from Boston usually remain on v.h.f. until they reach the vicinity of Middletown, Conn., about 100 miles distant, and midway along the route to New York. On the night of June 5th-6th the New York flight was SO-plus at 2500 feet near Middletown, so they remained on v.h.f. for check purposes. Over Bridgeport, 40 miles farther on, the signal was still very strong, and the temperature aloft was three degrees above the ground reading. A climb to 3500 feet with continuous altitude readings provided a check on signal levels, as follows: 2700 feet - S8, 3000 feet - S6, 3200 feet - S4, 3500 feet - nil! A return to 2500 feet restored the signal to S9-plus. The temperature at 3500 feet was 75 degrees, 10 degrees above the ground reading. The last v.h.f. check was made at 1500 feet over Port Chester, just north of New York, at which time the signal was still S7 at Boston.

Shortly after midnight a northbound flight was worked by the Boston ground station as low as 1000 feet near Bangor, far beyond the customary v.h.f. range. On a climb out of Bangor, signals were recorded as follows: 1000 feet - S2, 1500 feet - S7, 2000 feet - S8, 2500 feet - S9, 3000 feet - S7, 3500 feet - S2, 4000 feet - nil! These two flights worked one another that night while more than 300 miles apart.

Support for the proponents of early-morning v.h.f activity is given by W1KXP with reports on the morning of July 23rd, when ground stations heard strongly at Boston at 8:00 A.m. included Worcester, New Bedford, Hyannis, Nantucket, Martha's Vineyard, and Portsmouth, N. H. At 8:00 A.M. the ground temperature at Bangor was 64 degrees, but at 1800 feet it was up to 80! Conditions remained good until shortly after 9:00 A.M. when things returned rapidly to normal, with only Worcester and Portsmouth remaining audible, and these were well down. Flights then also faded out at the normal points, about 100 miles distant.

The distances mentioned are not DX, in terms of amateur work on 144 Mc., for all this work is done with nondireetional ground plane antennas, but the figures given do : how something of the tremendous difference in signal levels caused by sharply increasing temperature gradients in the first few thousand feet above the earth's surface. They also show us why it is foolhardy to head for Mt. Washington, a 6300-foot elevation, when the aim is to set a new world's record on a v.h.f. band. Present-day v.h.f. records are made by alert operators who take advantage of fortunate atmospheric conditions. The line-of-sight idea belongs to a bygone era. - E. P. T.