A Few Aspects of the Screening of Aerials

Radio Record, Volume II, Issue 3, 3 August 1928, Page 61

A Few Aspects of the Screening of Aerials

By

Megohm

The technical and constructional contributor "Megohm" gives weekly ‘in’ the: "Radio Record" a valuable series of articles of current radio -interest. He is a highly qualified and experienced constructor. and all — ‘his articles are based on practical experience. . "Many ‘readers express high appreciation of his. valuable advice.-Ed.

# S an opening to the subeu Ject the matter of absorption of wireless waves at the transnitting aerial will be briefly . dealt with in order to show thé éver-present tenc.ency to loss of signal strength by absorption. We have all heard of the highpowered broadcast’ station 5XX at Daventry, England. This station has two massive steel musts to support the aerial. Theye also existed at Birmingham a low-powered station, 5IT, which was to be closed and replaced by a high-powered station on normal wave-length, known as Daventry Junior, or 5GB. ‘The latter . station was duly erected; 5IT closed, and 56GB opened. Now something was discovered that the engineers who planned the scheme had missed. Signal Strength at Birmingham from 5GBR wa:s found to be much below what was rightly expected according tc the power radiated from the aerial, and disappointment was general. The cause of this unexpected weakness was found to be absorption of signal Strength by the steel masts of 5SXX, which were in the direc. path from OGB’s aerial to Birmingham. It is understood that the temporary aerials 100ft. high have been increased to. 300ft. and power augmented, ‘with presumably improved results. . Experiments at Rugby have shown that the steel masts there, . insulated at the foot on porcelain’ bases, decrease aerial radiation by about 20 \ per cent. when the masts are temporarily earthed. 4 bent ibes ii 5 he hte ty . iy RAM y CER HC bai hh Awe OTE L STEEL mast in close proximity to ~*~ an aerial acts as an untuned aerial directly connected to earth, and provided that the ransmitting wavelength.is considerably greater than the natural wave-length of the screening mast, absorption will be considerable. For aerial masts of 100ft. or so in height and broadcast wave-lengths from 800 to 500 metres, this condition is fulfilled. ; Receiving Aerials, | made to determine the screening effect of wires adjaeent to a vertical receiving aerial show that only when screening wires are parallel to and less than two feet away, and are earthed. that there is any. very great scteening effect,. and that when. such screening wire or wires are tuned by.a coil and variable condenser between the wire and earth, that the absorbing influence is reduced to a negligible amount. An increase

in the number of untuned_ screening Wires increases the absorption. A German experimenter has shown. that the electric field of a local breadcasting stutiol was reduced:to 10 per cent. of its normal value at a distance of two or three feet from the -base of « iron lamp-post. | ft..in height, and that a "shadoiv" of weakened. signal. strength is evident upon the ground to. a distance from the mast approxi-: ‘mately equal to its height. Field experiments in England -have shown that when using the loop aerial for directional work, the presence of ‘numerous overhead telegraph wires has a weakenin,; effect upon the reception of a uearby received. The screening action of trees is fairly well known, and in this as Well as other cases the position.of: the trees is an important factor. I£ any screening object is near to the receiving aerial and between it and the transmitter, then its full effect will be felt. — The Inverted L: Aerial. "TE popular inverted L aerial may be affected by métal work projecting skywards if in any quantity, such as rows of metal ventilators, pipes, metal staircases, ete, if connected to earth. The gas and water pipes in a house are in most cases not sufficiently numerous or in close: proximity to the aerial to have much effect, but high steel-frame buildings usually have a very definite absorption effect when in the track of the incoming waves, especially if such buildings are higher than the aerial. Although care should be taken to select the best available position for the aerial and lead-in, provided that the flat portion is not appreciably screened there need not: be any greag concern over the proximity of the lead-in to walls-or buildings if the greatest -ayailable separation is secured, . : Experiments made by sereening the lead-in with parallel wires show that the screening is only effective whilst the flat portion of the aerial is shorter than the height of the screening wires.

N describing tests thade to ascertain the signal strength at different distances from 2LO (London), "Wire-. less World" says: It is difficult to allow for all the different kinds of objects which may extract energy from the waves, but a casual view of the countryside will give most people the impression that trees must play a very important. part, as there are few districts in the. area surveyed which are not fairly thickly wooded. F* ° . significant that there are on the whole many more trees in the ccunties south of London than in those just north. This must have been noticed by most people fnrmiliar with the home counties. It is also made convincing by a comparison of almost any two large-scale ordnance survey maps of the north and south of London. Thus, if the absorption by the trees is of importance, we should expect a greater attenuation of the waves in the south than in the north, and it therefore appears that we are perhaps approaching the common solution to both the above problems. Testing the Solution. lt was fortunately found possible. to provide a practical’ means: of testing the tree hypothesis in a fairly simple manner. The trees over which the Waves pass may be considered as upright receiving aerials. Although, of course, they are not tuned to the proper wavelength, they will nevertheless have cur-

rents induced in them.’. me we. ‘consider . the cage of a single tree; we find that this current produces ‘round the tree’ _ a local distortion of the electromagnetic field of the waves. . Now the magnitude of this is a measure of the amount of energy ab-. sorbed by the tree. To measure this effeet, a small portable: direction-finder . was placed at the foot of thetrée under . examination, and since this instrument determines the direetion of the mag- | netic field at any point, it was an easy’. Inatter ta map out the field, and thug determine the extent of the. distortion. The actual way in which this exper! ie ment was carried out is shown in illustrations. In the first, a close-up view of the base of the treé is shown with the megsuring set in the foresround ; in the second, & moye distant view of the carrying out of the test is shown. By making these experiments with many ‘different trees, a general idea of -their absorbing pro--perties could be obtained. Thus it was found that a large tree may absorb as much energy as a quarter-acre | -O£ the _ earth’s surface. . Rough estimates were then made of the density of the trees in. various ‘parts of the country shown’ on the con- ~ tour map. From thejr numbers an estimate of the energy absorbed in -the © "different "directions could be made. It was found that by adding the energy ‘absorbed by the’ earth to the energy absorbed by the trees in a given direction, the predicted value of signal strength came much closer than be.fore to’ the, valyes. found by experiment; im fact,the agreement now was ~ | remarkably close. ". It thus appears that. we need | not. look further than to the effect of trees to explain both why the attenuation. varies with direction, and why. the sig- . nal strength is weaker everywhere than theory predicts."

The theory. mentioned was’ formulated by Sommertield, but only ‘takes: into acount absorption by the earth’s surface, it being impossible to include the effect of trees or buildings,.so that the effect of these is ascertained separately, and. added to the calculated deusity. _ Adjacent Power Lines. _ TOT much data-is to hand regarding possible: screening of receiving aerials by oyerhead power lines.: From the foregoing examples it does not appear possible for such lines to haye any appreciable screening effect. upon, an aerial,. unless the conditions are exXceptionally hard, such as a position very close to the lines, the latter being considerably higher than the aerial. And in sueh a case, the interference from hum would: quite likely. be a much more important:-matter for concern. And the writer recalls an interesting. report sent. in during the "fading"’test last year. This came from the: owner of a five-valve set. residing close to a large mining plant

employing probably 2000 horse-power, all frum electric motors. -High-tension wires supplying the district entered a sub-station near at hand, and amongst nlL this, the receiver functions. without interference, which is surprising, when the small things that will upset most reecivers are considered.

Sereen-Grid. Valve for. Short-wave. NX his notes last week Mr. Sellens mentioned that he had not been yery successful with the screen-grid stage. It is only fair to mention that this stage, so Mr. Sellens informed the writer, was. pot added according to specifications published, in the "Kecord." It is not expected that every‘body will successfully work any new circuit immediately, but in cases where trouble is experienced, a little time spent in ‘experimenting with ‘different plate and screen voltages, choFes, ete., will soon put matters right.