ABSTRACTS OF RADIOLOCATION CONVENTION SURVEY PAPERS462

therefore tends to occur in an intense form in tropical (but notequatorial) climates. Inland during fine weather, super-refraction is most noticeable at night and usually disappearsduring the warmest part of the day* Over the sea, super-

refraction is most marked where the warm, dry air of an adjacentland-mass is able to extend out over a comparatively cool sea.Only rather qualitative forecasting of super-refraction is possiblewith present meteorological knowledge.

EXPERIMENTAL STUDIES OF THE PROPAGATION OF VERY SHORT RADIO WAVES

By E. C. S. MEGAW, M.B.E., D.Sc, Associate Member.*

(ABSTRACT of a RADIOLOCATION CONVENTION paper which was published in 1946, in Part III A, No. 1, of the Journal)

The paper presents a survey of the experimental work on thepropagation of very short waves, and especially of centimetrewaves, carried out in this country during the war.

After referring to the main differences in propagation charac-teristics between these waves and longer radio ground-waves, thestate of the art at the beginning of the war is briefly noted andthe development of the war-time experimental work is outlined.Some simple theoretical characteristics are then described toprovide a background for the experimental results which follow.

Preliminary measurements were carried out during 1941-42over a variety of land and sea paths; long-period measurementsto establish the correlation between radio propagation andmeteorological conditions in greater detail were carried out later.This main experimental programme, which commenced in 1943,consisted of measurements of 3-cm, 9-cm, and 3i-m waves oversea paths 57 miles and 200 miles in length; and of 9-cm wavesover a single 38-mile land path. The measurements were con-tinuous over some of these paths for periods between two andthree years; and, particularly as regards detailed correlation withmeteorological measurements, the work is still in progress.

Fig. 1 shows an example from the earlier measurements of theclose agreement of observed field strength with that calculatedfor a well-mixed atmosphere, under bad weather conditions.

Fig. 2 shows an example of the striking changes which can beproduced by increased atmospheric refraction at low levels infine weather. Some general conclusions from the preliminarymeasurements, which have largely been borne out by later work,are given below:

CONCLUSIONS FROM THE PRELIMINARY STUDIES

Optical Paths over Sea.For short paths, up to 10-20 miles, the observed field is usually

within experimental error of that calculated for a standardatmosphere. There is little or no fading well within opticalrange (insensitive to refraction) but variations of 10-15 db occuroccasionally at 10 miles, just optical.

Over longer paths, with correspondingly greater heights,variability increases; at about 60 miles just optical ± 10 db isquite common with occasional brief 20-db fades. The calculatedrange of variation for changes in effective/real earth radius from1 to infinity is only rarely exceeded for this path length.

Increasing the distance to around 100 miles, now with moun-tain-top heights, shows a further marked increase in variability,especially during fine weather; only part of this is readily ex-plicable in terms of sea-reflection interference; 30 db fades lastinga few minutes are not uncommon. The observed occurrence ofreduced general levels in fine weather should probably not beregarded as firmly established, but fades of at least 10 db lastingabout an hour definitely occur. Permanent atmospheric absorp-tion is unimportant at 9 cm over this distance. Short-periodfading on 9-2 and 10 0 cm is quite different though generaltrends are similar.

• Admiralty Signal Establishment. Formerly with Research Laboratories of theGeneral Electric Co., Ltd., Wembley, England.

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Fig. 1.—Measurements between ship and shore on 9 cm; bad weatherconditions.

Mean transmitter height 260 ft; receiver height, 32 ft.Horizontal polarization.Dotted curve calculated for standard atmosphere; points observed.Weather: fresh E. to N.E. wind, occasional sunshine, cool, sea moderate to fairly

rough.

Optical Paths over Land.For paths up to about 50 miles, the results are similar to those

for sea paths, except that unexpectedly low, though quite steady,levels have been observed over some short paths, perhaps owingto destructive interference with reflection points near one ter-minal. Uncertainty about ground reflection makes accuratecomparison with the calculated field at very short distances moredifficult.

For a 30-mile path (just optical) the range, and sometimesthe detail, of fading is similar to 10-1, 9-2 and 3 -4 cm; at othertimes the detail is quite different.

Variations over optical land paths tend to be greatest at night.

Non-Optical Paths over Sea.In most cases where the minimum signal level (apart from

ABSTRACTS OF RADIOLOCATION CONVENTION SURVEY PAPERS 463

1000level at night (radiation inversions) and rather less variability atother times than for roughly similar sea paths. The total rangeof fading also tends to be rather less. Occasional peak signalsreached about 5 db above free-space level, as compared with15 db above for sea.

Overcast skies, or moderate to strong wind, tend to suppressthe diurnal variation.

Polarization and Bearing Changes.Usually change of polarization has no obvious effect except at

interference minima caused by reflection from a calm sea; beyondoptical range during heavy fading there can certainly be a dif-ference in detailed variations between vertical and horizontal,possibly with a small advantage for vertical on some occasions.Apart from the occasional arrival of signals at high sites fromangles much below the horizon, in addition to the normal angle,no change in bearing in excess of the measurement accuracy of alittle better than 1 degree was ever observed.

The full paper should be consulted for an account of the long-period measurements and of additional studies of such mattersas reflection and scattering, and the effect of obstacles.

0 10 20 30 40 50Distance from transmitter, sea miles'

Fig 2.—Measurements between ship and shore on 9 cm; increasedfield strength produced by refraction during change from bad tofine weather.

Mean transmitter height, ft; receiver height, 32 ft.Vertical polarization. . . .Dotted curve calculated for standard atmosphere; points, with trend indicated by

full curve, observed. . • , .Weather: light easterly breeze, sky clearing, warm, sea fairly calm.

short fades possibly due to interference effects) could be measuredit is quite close to that calculated for standard atmosphere;heights ranged from tens to hundreds of feet. On the otherhand, the maximum level rarely exceeds that corresponding tofree-space propagation, especially if brief peaks are ignored,though it approaches this value for considerable periods in fineweather.

For sea paths within a few tens of miles of land such that thesignal level for a standard atmosphere is well below the free-space value, there tends to be a diurnal maximum in the after-noon and evening, and a minimum in the early morning; theseobservations apply to anticyclonic weather with wind mainlyoff-shore.

Non-Optical Paths over Land.General conclusions are similar to those for sea as regards

range of variation. They are less definite because of greateruncertainties as regards geometry for the land paths, althoughgenerally flat country was chosen.

There is a marked tendency to diurnal periodicity with high

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Fig. 3.—Approximate hourly mean signal level for 9-cm and 3-cmtransmission from South Wales to Scotland (200-mile paths), 6th-8th August, 1944.

Fig. 3 shows an example of long-distance transmission oversea on centimetre waves under favourable weather conditions.