: ,_*:_:_:: é {A . \H 45 o! \\ I . ‘b ”Lyle q.“ a .. 4:50., .ik . I . NIlb .. f ”R.“ 1.“.- W4C . I . L k . .1 ... 94- .N v . g \- . ton. .. ‘1. . .., . 7- «(m H-.. . v ., . , A n.-.‘ .. .‘é ... . \t .... a T. 3 a 0‘ . w .. ‘ “Ml.“ .ml‘m In?“ 1.. NJ ~ .. A» a. M - h H‘ .r \v hi” . .. l J“ ‘ ’V‘ .{ a "A. ‘ . . f .....\ u s, e . m. _. .... PU f. . . .L . . .. a , O rut. \, u U‘ G ...‘ l‘.~“ m... SIC-ll“ ‘1” X. In - _. .n. . .2“ ... . .k. .: : ... u r.“ .5..'0 A. .K ‘ 'o ' C v. ‘ I; - J uiflllfilvun mmcn a.“ t . “-‘J' III ICU-'IUUUI"II’III-I'IIIII"IIII'IIUI.II-I lillllv III‘IIOIII C... Illa”... -.!.-I.‘I llhl'él“..|‘ll "U‘.IIIICII.1UI.'I! U‘l‘llo 3.0. II 1 I w... w ._. A H. T. . it! I}! I A RADIO FIfiLD STRfiEGTH SURVEY OF WEAR A.Theaia Submitted to Th6 Faculty of The MICHIGAH STATE COLLEGE 3?. Robert‘D. Martin Gilbert Burrcll N.’ Candidates for The Degree of Bachelor of Science June, 1928 THESKS TABLE OF COHTENTS vStatement of Problem Theory of lave Propagation Directional Effects of Antennas Description of Station WkAR Field Apparatus Calibration of Field Set Preooanre Data cenelneiene Acknowledgment Bibliography 1( :3“? Page 23 26 55 49 51 55 78 82 83 1. Statement 01' Problem The art of taking field strength measurements of radio station IKAR was undertaken primarily to ascertain the directional effects of the radiating system. the shielding effects of the antenna supporting structures and surrounding buildirgs, and to determine if there were an erratic conditions on or about the college campus which might effect the transmission of radio signals. Letters and reports received at the station from radio listeners throughout the country indicate very clearly that the signals are not being received with equal efficiency in all directions. The taste perfumed, and reported in this thesis. were expected among other things to definitely decide if local con- ditions were responsible for the variation in transmission efficiency in different directions from the station. Other problems or special side issues were to be solved and dealt with as they arose in the natural course of taking manuremmts as were the subjects suggested by advisers as the work Ill-grossed. 2. Reports of this nature naturally include a descrip- tion of the station, of field apparatus. methods of calibration and procedure, and a discussion of difficul- ties encountered in.the field and.results obtained. In addition to all this. it is thought advisable to include somewhat of the theory involved in such a test. For instance, the theory of wave propagation and directional characteristics of different types of antennas. A discussion of this theory follows. '5. theory of have Prepagatien The space about an antenna carrying alternating current is occupied by two fields each.comprising an electromagnetic and electrostatic component. One of those. known as the induction field. is surging back and forth from the antenna. no energy is lost to this field from the antenna circuit. The induction field is familiar to any electrical engineering student as the phenomenon of induction is produced‘by it. The induc- tion field ie‘comprised of electric and magnetic compon- ents uhich.are in time and space quadrature with each other. There is another field present knosn as the radiation field. This field.repreeents energpwwhich.is transferred to the medium surrounding the antenna and never returns to it. The radiation field is also»divided into»eleetric and magnetic components, but unlike the‘components of the induction field these two components are in time phase and at any point rise and.fall simultaneously. figs. 1 and 2 show the phase relation'between the electric and magnetic field. The reason given for the presence of the radiation field is that in rapidly changing fields ell the energy does not have time to return to the antenna and travels away from the antenna with.the speed of lights. The space surrounding any conductor carrying alter- 5156 ”Ha L/Né-J MAGHErIC— L/Nts olna‘crton of thIrt' WIHTIO/Y SHOWING RELATION earn-SEN ELe‘chIc ””0 ”‘0"57’4 C°”‘P°”£'I1rb OF RAD/A170; Y FIELD As way: ADVANCES AWAy Fflorva AHTE/Y/YA E AND [‘1 ARE/N SPO<£ OUADKAT‘UAE BUT IN TIME phase“ 5. hating current of any frequency will be occupied by both induction and radiation fields. Hanover, in the case of the lower frequencies such as the commercial frequencies used for power transmission most of the energy has time to return to the conductor and for this reason the radiation field will be negligible. On the other hand, when higher frequencies are used such as radio frequencies the field is changing so rapidly that a great portion of the energy does not return to the antenna and the radiation field become of importance. fhe induction field sill still be present but inasmuch as this field varies inversely as the square of the distance and the radiation field varies as the first power, the induction field will become negligible at a short distance from the antenna. fhe above theory is according to Liorecroft's in chapter 9 of Principles of Radio Communication. The induction field becomes negligible at distances greater than one wave length. At distances equal to 2?; the fields are equal. For points closer to the antenna than this the induction field predominates. For points farther away the radiation field predominates and the induction field falls off rapidly with the distance and becomes negligible. This theory is according to H. H. Dellinger in Principles of Radio Transmission and Reception lith Antennas and coil Aerials, which.appeared in the October 1919 issue of the A.I.E.E. Journal. In the same article hr. Dellinger presented a mathematical proof that the induction field varies as the square of the distance while the radiation field varies as the first power. Moreoroft evidently accepts this proof an authoritative, and it is repeated here. DERIVAEHONS or THERETIOAL 1'03th 1. Radiation From An Antenna "Formula (8) below, giving the radiated magnetic field at a distance from an antenna, is a'wellvknewn formula. It has been given by various writers, and is the only one peeented in this paper that requires any deep consideration of fundamental electromagnetic theory. The result is in fact implicit in.Maxwell‘s classical treatise, 'Electricity and Magnetism'. The_derivation given here is much more direct and brief than the others the author has seen, and is given only fbr that reason. The derivations of formula (10) and following ones are still simpler, and will be of more interest to most readers. "The units used in this paper are international electric units, the ordinary electric units based on the ohm, ampere, centimeter, and second. (See paper by the author on 'International System of Electric and magnetic Units', Scientific Paper of the Bureau of Standards No. 292) 7. 1 The unit of magnetic field intensity is the gilbert per cm., often called the cgs. unit. The only exception to the use of units of the international system is in cer- tain.of the practical formmlas where lengths are expressed in meters or miles where so stated. “In the following discussion is calculated the magnetic field intensity produced by'a flat-top antenna, having electric current of uniform'value throughout the length of the vertical portion. Most antennas in practise approximate closely this condition. The symbols used are: i . instantaneous current I a maximum value of current 0 3 effective value of current H . instantaneous value of magnetic field intensity . maximum'value of magnetic field intensity , effective value of magnetic field intensity 1 height of aerial .‘ distance from sending aerial - 2 1 times frequency of the current - time i - wave length °>*E°-s'=a=r I . velocity of electric waves - 3.2 1010 cm. per second Subscripts e - sending, r . receiving, a . antenna, 0 I 0011e 8. "In Fig. A the upper heavy line represents the flat top of the antenna, and the lower heavy line the grounding area. Suppose a current is flowing, having the instantaneous value 1 in the vertical portion. The magnetic field intensity at any point due to a varying current is different from that due to a steady current. Consequently the field cannot be calculated in the same way tint the magnetic field intensity of a straight wire is ordinarily calculated. When the current is varying, the magnetic field intensity is calculated by the aid of a quantity called the vector potential in such a way that the variation with time is taken into account. The in- stantaneous value of the vector potential of current in the vertical conductor at a distance d in a plane per- pendicular to the conductor, is All glib. (1) where (1) indicates that for any time t the value of i is taken for the instant (t - d/c). I r.‘ _h___¢.___ii P Pig. A - Calculation of Magnetic Field at a Distance Fr om An Antenna. Suppose the current in the antenna is a sine-wave alternating current, 1 I I0 sin 00‘: (2) 9. Therefore (1) .- Io sinuo (t - d/c) A. hill :- h? sinw(t-d/o) (3) The magnetic field intensity is calculated from the vector potential by the general relation.Ht - 0.1 curl A, which for this simpde case of a straight conductor becomes Ht' 1 Ease (4) the direction of Ht being perpendicular to the plane of h and d. From equation (3) It . - h cools cos th-d/o) «- h 13 sinth—d/c) (6) "This equation gives the magnetic field intensity at any point P at a distance d from the antenna. The second term represents the ordinary induction field associated with the current, while the first term is the radiation field. At a considerable distance the second term is negligible because the second power of d occurs in the denominator. The first term then repre- sents the magnetic field radiated from an antenna at the distance d from the antenna. The distance d is measured along the earth's surface, because the waves follow the curvature of the earth's surface instead of proceeding straight out into space. For a considerable distance from the antenna, the maximum value of the magnetic field intensity during a cycle is therefore no . nwcgs 10 . hpressing in terms of effective values, a. an): (6) W 'Henceforth H means the radiated field unless it is specifically stated to be the total field. The last equation may be expressed in terms of wave length instead of by the relation A)__ u 2' (7) c "R Therefore H . 21 h I 7x3 Using the subscript s to indicate that it is the sending rather than the receiving antenna which is considered, Hu2 I 8 [Enhi () "This derivation follows the conceptions presented in the early pages of Lorentz, ’The Theory of Eleotrons'. It is equivalent to Herts's intricate proof, but is more direct. The way in which the result is exp-eased here accords more closely with the physical ideas and with actual practice, being upressed in terms of current rather than electric charge, since it is current that is actually measured in an antenna and tin current further- more is generally uniform in the vertical portion of the antenna. "formula (8) gives the radiated magnetic field from as sending antenna at a distance d along the earth's sur- face. The units are the gilbert per cm. for H, the ll. ampere for I, and the centimeter for all lengths, as previously*statod. “Undamped alternating current in the antenna was assumed. The Gaza result, however, is obtained if ‘he current is damped. At very great distances from the sending aerial, the magnetic field is less than that calculated by formula (8), because of absarption of the user of the wave as it travels along. ’zhis may be taken into account by multiplying the right-hand member of (8) by a.correction factor 31. The value of this factcrvfcr daytime'trnnemiaeion ever the ocean, derived from the experiments of L. ”if. Austin, Scientific Payer of the Bureau of Standards no. 159; 1911, is for d and both in meters. Ehis correction ordinarily seeds to be applied only when the distance is greater than 100 kilometers." This roof that the radiation field varies inversely as the first paer of the di etame does not agree with experimental data ,resentod by Lloyd Begonschiod in an article entitled "Radio Broadcast Coverage of City areas" 12. thich appeared in the A.I.E.E. Journal for November, 1926. Parts of Espcnschied!s conclusions and curves are shown bech. The parts concerning fading is not pertinent to the above proof, but it has not been out from the article because it contains some interesting conclusions upon wave prOpagation and because the curves concerning fading are necessary to show the decrease in field intensity. -------'The Character of Radio Broadcast Transmission. 'The ideal law for broadcast distribution would be one whereby the transmitted waves are propagated at constant strength over the zone to be served and then fall abruptly to zero at the outside boundary. All receivers'aithin the area.would be treated to signals of equal strength and no interference would'be caused in territories beyond. The kind of law which nature has actually given us involves a rapid decadence in the strength of the waves as they are propagated over the service area and then instead of a rapid cutoff a per- sistence to great distances at field strengths which although often too low to be generally useful is suffic- ient to cause interference in other service areas. This situation is illustrated in Fig. 3. The upper curve shows the relation between intensity and distance, the lower portion the interpretation of this curve in terms of areas of reception. The attenuation traced'by the l3. hoary line of the curve is that of the components of radiation which is propagated directly along the earth's surface. It is this radiation which is ordinarily ‘ utilised for reliable broadcast receiption. ‘The shaded portions near the outer ends of this curve are intended to indicate the appearance of variations in the signal intensity which occur at greater distances, particularly at night and Ihich are known as fading. fig. 8 - The Attenuation of Broadcast Waves in Reference to the Areas Served. The evidence of recent researches, particularly those made at short wave lengths,indioates that these fading variations are due to radiated energy'which has left the earth's surface at the radio transmitter and 14 has been reflected back to the earth's surface from a conducting stratum in the upper atmosphere. At broad cast frequencies the reflected wave component is observed at night, but has not been noticed during the day. At locations close to the transmitting station the effect of the reflected component is negligible as compared with the strength of the directly transmitted waves. At increasing distances the directly transmitted waves die away to very low values and the indirectly transmitted waves begin to show up and become controlling at the longer distances. The fluctuations themselves appear to be due in part if not entirely to variations in the reflected saves themselves resulting perhaps from fluc- tuations in the conditions of the upper atmosphere. Thus it seems clear that radio transmission involves save components of two types, one which delivers directly to the receiving area immediately surrounding the broad cast station a field capable of giving a reliable high grade reception, and another transmitted through the higher altitudes which permits distant reception but not with the reliability and freedoms from interference re- quired of high.grade reproduction." .flr. Espenschied shows at this point the results of field measurements upon WEAF in New York and WGAP in Washington. The shaded portions show the variation due to fading. The daylight curve will be of interest to this thesis in that it shows that the decrease of field 15 intensity with distance is not a straight line function. . egg/at c/omc WCAP GvOK.C, .._.,l ‘ - _, I . ..__,- _4/oocc 6". 1 Paid she/7‘9» w \F )_ b 5 D~ C E’ v“: ‘9 \ Q! Q ~ ; ,- 4—; l_ L , ;_ L. _. IO 0 50 00° "0 100150 500 O ‘0 I” :50 10. {so 300 UlS/ANCE In MILF5 fig. 4 ~ Results of a Few Measurements Upon the Reduc- tion in Field Strengths With Distance Including Distances at Which Fading Occurs. Hr. Espenschied's paper continues "A fact which is of importance to- our understanding is that fading which ordinarily is noticed at distances of the order of 100 miles; may under some conditions become prominent at distances as short as 20 miles from the transmitting station. Such short distance fading has been experienced in receiving an in certain parts of lestchester County, le- York. (Bee "Some Studies in Radio Broadcast Trans- mission" by Brown. nartin and Potter Proceedings 1.3.3.. February. 1926) It appears to be a case where unusually high attenuation caused by the tall building area of mttan Island has so greatly weakened the directly transmitted wave as to enable the effect of the indirect wave component to become pronounced at night. 16. 'In general the attenuation suffered by the normal surface-transmitted.wave varies over wide limits depend- ing upon the terrain which is traversed. This is dis- closed by the curves of Fig. 5, which shows the drop in field strength with distance for a 5 KW station for each of the following conditions: a. No absorption, the inverse distance curve, a = 0 _ b. Sea water, for which absorption is relatively small (a - 0.0015) c. Open country and suburban areas (a s 0.02 to 0.05) as measured in the vicinity of New York and Washing- ton, D.0. d. Congested urban areas (a = 0.04 to 0.08 as measured for Manhattan Island. ' "The factor a will be recognized to be the absorp- tion factor of the familiar Austen-Cohen empirecal formula. e - 0.009 T‘". e a a ' I7\ ' P - radiated power in watts d . distance in kilometers . wave length in kilometers a . absorption factor e .- volt's per meter. ' I'The first term represents the decrease due merely to the spreading out of the waves, the second term the decrease due to the absorption of the wave energy by the imperfect conductivity of the earth's surface." 17. ICHO VOL 75', riffs/1’ i l‘ K J I '0' 1 k ‘6 Q ‘0 ‘0. lg. Pig. 5" - Effect of the Terrain in Reducing the Field ‘ Strength of a Broad cast Transmitting Station The curve in this print which is of particular value to this thesis is the inverse distance curve. It is undoubtedly true that absorption had a great deal to do with reducing field intensity at a point some distance from the transmitter. The inverse distance curve takes into consideration no ahsorption, hence any decrease in field strength is caused by the natural dying out of the wave due to increasing distance. It is not clear how.nr. Espenschied obtained such a curve. Probably by adding the absorption factor to the ordinate of one of the other curves. The important thing about this curve as compared with Dellinger's mathematical proof is that the field strength varies inversely as the distance to some power greater than “it’s 18. Another excerpt from Mr. depenschied's paper follows: "------~A question which naturally arises is that of how strong a field as measured in this way is required for satisfactory reception. It is too early in the art to answer this question very definitely, for it depends first upon the standard of reception which is assumed with reapect to quality of reproduction and freedom from interference and second upon the level of interference. The intefferenoe, both static and man-made varies widely with time and location. It is, therefore, obviously impossible to give anything more than a very general interpretation of the absolute merit of field values. Observations made by a number of engineers over a period of several years in the New York City area, having in mind a high standard of quality and freedom from inter- ference, indicate the following: 1. Field strengths of the order of 50,000 or 100,000 microvolts per meter appear to be about as strong as one should ordinarily desire. Fields much stronger than this impose a handicap upon those wishing to receive some other station. 2. Fields between 50,000 and 10,000 [u v/m represent a very desirable operating level on which is ordinarily free from interference and which may be expected to give reliable year round reception except for occasional inter- ference from nearby thunderstorms. 19. 3. From 3.0.000 to 1,000 /u v/m the results may be said to run from fair to good and even poor at times. 4. Below 1,000 In v/m reception becomes distinctly unreliable and is generally poor in sumer. 6. Fields as low as 100 f v/m appear to be prac- tically out of the picture as far as reliable, high quality entertainment is concerned. 010. . Q Jar 7+ ,: “t + \ A C 4‘“ v ' ' J / 2 , as 7,4 + ,/ r 3 C J ‘V \ . ¢l 4LIL5 5 v f ' I Naatutc‘ 'o-vcr rig. 6 - Showing the imrease in Radiated Power Required to Increase the Range at Which a Field of 10,000 [u v/m is Delivered. Curve A - lithout absorption. Curve B - With absorption. "It is seen from the three proceeding figures that a 5 I! station may be expected to deliver a field of 10,000 micro-volts per meter some 10 to 20 miles away and a 1,000 micro-volt field not more than 50 miles. 20. l'rom this it will be evident that the reliable high quality program range of a 5 KW station is measured in tons of miles rather than in hundreds. "Rough though this interpretation of field strength is, it indicates clearly the need.which.exists for the employment of higher transmitting powers. The range goes up*with increase of power disappointingly slowly. Even were no absorption present in the transmitting medium, the range in respect to over-coming interference would increase only as the square root of the increase in pgwer. This is shown in curve A of Pig. 6. "It shows that a station which actually radiates five as of power would deliver a 10,000 [u v]. field at about 40 miles, a 20 XI station the same field at dis- tance 80 miles. Actually with absorption present the distances are less. This is shown by curve B which gives the corresponding relations for the absorption observed for suburban and country terrain. To extend the 10,000 lu.v/m field from some 15 out to 30 miles would necessitate an increase in the radiated power from about 5 to 100 KW.“ It can be seen upon comparing the theory and proofs of the foregoing authors that Espenschied differs rad- ically from Morecroft and Dellinger. The field which Espenschied measures is most assuredly the radiation field spoken of by both Morecroft and Dellingcr, because 21. the measurements are made at a considerable distance from the transmitter. It is not easy to understand Just why the reflected wave spoken of by Espenschied should be so much stronger than the direct wave. Assuming spherical prepagation of waves and neglecting absorption of the so-oalled direct wave it must necessarily follow that the intensity of the fields at the point of reflec- tion and at a corresponding point along the earth's surface would be the same because thus far the two are of the same nature, namely that of the radiation field and any decrease in intensity will be due to the spread- ing out of the spherical wave. The area of a sphere increases with the square of the radius, therefore, the intensity of any charge on the surface should decrease as the square of the radius. From the point of reflec- tion it is hard to say just what the nature of the reflected wave is, but at any rate in order to reach any spot on the earth's surface the reflected wave must travel much farther than the corresponding direct wave. It may be that the difference in the absorption factors I of the lower and higher strata is large enough to account for the greater strength of the reflected.wave. Zenneok and Seelig handle this subject in a slightly different manner. They say that the electrostatic and electromagnetic components of the radiation field or space waves, as they call this field, each vary inversely 22. as the first power of the distance. All authors on the subject seem to agree that the electric and magnetic components of the radiation field are in time phase though in space quadrature, and that the flow of energy is perpendicular to each of these component directions. If we assume that these components are equal and that each varies inversely as the first power of the distance then it followw that the radiation field varies as the inverse square of the distance. This theory agrees with ESpenschied's experiments. 23. Directional Effects of Antennas Inasmuch as the main object of this field strength survey was the determination of the directional effects of the radiating system of WEAR, it will be well to in- vestigate the directional effects of various forms of antennae. The folloWing extract was taken from pages 90 and 91 of "Radio Engineering Principles” by Laser and Brown. "The energy radiating qualities of an antenna depend on the shape of the fields of the antenna, that is, on the strength of these fields in the various directions around the antenna.' It is known that the shape of the circuit directly and fundamentally affects the shape of the field. This will be studied here in somewhat greater detail in the case of antenna circuits. 'Consider a vertical wire antenna, shown in plan view by point A in Pig. 7?, and assume that a number of observers equipped with receiving circuits, all identical. are scattered about the antenna. Assume also that each one of the receiving circuits has some device which per- mits of measuring the current induced in it when the antenna circuit "A" is oscillating. Now if the observers move toward or away from the antenna "A? until they all obtain the same current reading in their receiving cir- cuit, they will finally find themselves on a circle having ”A" as its center. This shows that a vertical 24. wire antenna radiates with equal strength in all direc- tions. . ‘1 similar test repeated'with an inverted ”L" antenna results in a radiation curve illustrated in Pig. .‘7 -B, where ”G" is the grounded and and "E" the free end of the antenna. A 'T" shaped antenna, being esentially made up of two inverted 'L' having a common vertical portion, has a curve similar to that shown in Pig. .‘1-0. A V-shaped antenna, consisting of a double 'L' antenna with the horizontal portions in the form of a “V" with a common vertical portion, has a radiation curve as shown in Pig. .‘7 4). ”The directional properties of receiving antennas are the same as those of transmitting antennae. As a general rule it may be said that the maximum directional effect is in the plane containing the antenna serial and lead-in wires, and in the direction of the lead-in end of the antenna. If, as in the case of a “V" antenna, this plane is not well defined, then the directional effect is in the plane containing the lead-in wire and the geometrical center of gravity of the aerial." The plan view of the antenna and oounterpoise of IKLB is shown in Fig. 8. According to the precesding data on directional preperties of antennae, the maximum directional effect would be expected to lie in a vertical plane that passes lengthwise through the antenna. The -._._.-—. effect of the oounterpoise design, antenna supporting structures, nearby buildixgs, etc. on the directional properties of the antenna were setters of conjecture and were left to be decided by the, results of the survey. //’——\ / I; e \\\—/f ,w"-?l/ES SHOW/I76 DIRECTIOHAL :_,.x},.’;;eAcremsr/cs or DIFFERENT egg or ANTE/YHAS 26. Description of Radio Station WEIR The transmitting room of WEAR is located on the second floor of the hflchigan State College power house. The transmitter has been constructed and put into cper- ation by the technical staff of the station. The transmitter has a rated output of 1000 watts. Four W.E. 212 D tubes rated at 250 watts each are used in parallel for the oscillators. Eight W.E. 212 D tubes are used in parallel for the modulators, while two W.E. 211 D tubes rated at 50 watts each are connected in a push pull stage for speech amplifiers. The Heising system of modulation is used. In this particular test we are mainly interested in the oscillating circuit, so this will be the only one taken up in detail. The modulator and speech amplifier tubes were removed during the field test to prevent a waste of power. The oscillating circuit is a closely coupded'meisner employing a tank circuit for the elimination of harmonics. From the set a feeder tube goes through the roof to a small pent house where the antenna and counterpoise tuning condensers and inductances are located. The tank, plate, grid, feeder snd.antenna circuits are all metered. Radiating System The radiating system consists of a T-type antenna and a fan-shaped counterpoise. The antenna has four 27. .339... pm Eoom scapuscao lo. 9 phosphor bronze enameled wires held in place by 24 foot wooden spreaders. The whole is supported between a triangular steel tower and a steel water tower by a galvanized steel cable connected to the spreadcrs by larger pwrex strain insulators. The antenna hangs at an average height of 176 feet above ground. The counter- poise consists of 25 No. 9 enameled copper wires supported at the center by a copper ring connected to and insulated from a large soil pipe near the pent house. The wires are supported at the outer edge by no. 6 galvanized wire strung bet-eon the triangular tower to a building back of the generator room and from the building to the water tower. The counterpoise wires are of equal length. PENT HOUSE I \ \ pLA/Y l/lé:W 0’: 9017-5? urycs :counT£RP015£ l RA 01" 7/”6 5 YSTEM Hrs/4 V’:_‘;/_'.7_E_sh;_fq_" TEN/VA SCAI. 5 / " f: 25 ‘ “x I qxxvts WKKDOQFU btfhnxddxomo Nw~0tumkt§ou \A .‘QQDW Wk ‘VQ 0k j'llu'.» —+n- ..__“‘,,___ kl X . htok (exact OK: - (tenure. \A NlQ§h that Q ask Gk _ Tulll'... 1 + (3.3. 29. They are insulated from the galvanized supporting wire by pyrex insulators. The copper ring at the center is connected to the tuning inductance by a heavy capper strap. The counterpoise is grounded at a node to stab- ilise the system and.facilitate tuning. The radiation resistance of the system is approx- imated to be 14 ohms. Station Surroundings The station is surrounded by many buildings, as one can see from the maps used in the field test. Some of the buildings have a great deal of steel in their struc- ture. Consequently they might be expected to absorb considerable energy. An interesting example of this was noted in the building upon Which.the triangular tower rests. This particular instance happened before the present counter- poise was installed. It may be added here that the phenomenon has not been observed since. This building has steel window frames with.steel rods attached to hold the windms open if desired. Ghostly voices and music were reported to be heard in one room of this building at certain times. Investigation showed that these times coincided with.the broad casting periods of the station. Further investigation brought to light the fact that if one of the window rods happened to touch a radiator under the window during the broad casting period the rod would Home of WKAR. 51. talk and sing. It was this discovery that prompted the construction of the present counterpoise. Station Operation During the Test In a test of this kind it is imperative that con- ditions at the station be kept constant in order that a constant field may be laid down. as one of the authors of this thesis is an operator at the station, it was decided that he should Operate the station while the other took the field readings. at times the other station operators were good enough to take the station watch. This greatly facilitated the test for two could work much faster making field tests than one. Although all HBtGTB were kept as constant as possible especial attention‘sas paid to keeping the frequency and the antenna current constant. The reason that these two readings were considered the most important is because the radiated power of an antenna is computed as the square of the antenna current times the radiation resistance and because the radiation resistance varies with the square of the frequency. Radiation Resistance Radiation resistance is defined by the formula P'- 123, where I is the antenna current and P is the radiated power. The total antenna resistance is affected by several factors 1. The resistance of the conductor. 2. The resistance of neighboring closed circuits; 3. Engnetio material close enough to the antenna to be magnetized. 4. Losses in the dielectric of any condenser in the circuit. 5. Corona losses from parts of the circuit. 6. Radiation of electromagnetic energy. snore simply stated antenna resistance is divided into two components. A. Loss resistance (including losses caused by the first five shove rectors). B. Radiation resistance. The radiation loss is the useful part of antenna losses and makes it possible to deliver signals at a distance from the antenna. For this reason the radiation resistance is the more important component of the antenna resistance and should comgriee the larger pert. It is important then to be able to castrate the radiation resistance from the total antenna resistance in order to arrive at the power transmitted. There are many methods of measurement and many formulas for arriving at the radiation resistance. The formulas are all more or less approximate inasmuch as they are all greatly affected by the type of antenna and local conditions. 55. Dellinger gives the follOWing formula for a flat top antenna at wave lengths considerably greater than Ra . (39.7 %)2 Moreoroft is reaponsible for the following formula the fundamental. for a simple antenna 2 R an 5 1‘2 a 'l 0 73"” It is not necessary to go further into the theory oi'radiation resistance for purposes of this thesis. The point we wish to show is that all formulas for radiation resistance show that the resistance varies as the inverse square of the wave length or as the square of the frequency. Hence by keeping the frequency constant the radia- tion resistance is kept constant. As the formula for radiated power is P. . 123 if'the current and radiation.resistance are kept con- stant, the radiated power must also be constant. In other words, there is a constant field laid down in the space surrounding the antenna. Unavoidable variations of antenna current of .05 amperes were noted during the test, the antenna current varying from 10.55 to 10.65, but for the most part the current remained steady at 10.6 amperes. The frequency was mintained comtmzt to within a few cycles or 1080 3.0 by means of comparison with a General Radio Company's Standard 21030 Crystal Oscillator, approved and calibrated by the ‘Z‘sureau of Standards. Apparatue The apparatus necessary for taking field strength measurements of a radio transmitting station consists of e.conplete1y equipped specially designed radio re- ceiver and apparatus for calibrating the receiver to some standard. The calibrating apparatus necessary can be decided upon only after the final design of the re- ceiver has been completed. The receiver for this sort is e device for determining the relative intensity or strength of ”field” set up about a radio transmitting antenna and consists of the following essential parts: (1) an antenna, (2) a tuning device. (5) some term of detector. (4) an indicating device. in antenna is necessary for the interception of the trmsmitted signals and may be any or the existing types. A tuning device or scheme is necessary to bring the antenna and the associated.circuits to resonance with the transmitter frequency. A detector that will respond to the received.high frequency currents is necessary to actuate the indicating device. The indicator should be e device that will permit the comparison of intensity of signals received at different places and at different times. The detector might be of crystal or vacuum tube type. Because of its greater sensitivity and more nearly constant Operating characteristics, the vacuum tube detector is the only kind that will be considered. rectors Affecting Design of Receiver The various factors affecting the design of such a receiver are: (l) the wane available for transportation of the device. (2) the character and condition of the land were usesurements are to be taken. (5) the relative poser of the transmitter under consideration. (4) the distance from the transmitter of the points where treasure- .ments are to be taken. Obviously. it must be built sturdy enough to withstand the :olte and Jars that it is likely to receive in being carried about. and also be duly pro- tected if it is to be subjected to adverse weather con- ditions. The effect of the greeedine factors upon the design of such a test set sill be taken up in the order named. The menu available for transportation of the re- ceiver and the character and condition of the land where measurements are to be taken are factors closely related to each other. It is plainly evident that the character of the land my seriously affect or restrict the use of some desirable and available arena of transportation. If readings are to be taken ever a large area in the streets of a city. an automobile is generally used. Again. if readings are to be taken about the countryside. an automobile can be used to get near the areas under question. but the receiver will have to be carried by hand in order to reach points in the fields. To take 37‘. readings in city streets within a small area, a hand cert would be useful. If the land approximates the conditions prevailing on the carbons at Michigan state College, the only practical method for moving the receiver about is to carry it by hand. The small area of the campus, the comparatively short distances between points where readings are to be taken. and the character of the land prohibits the use of an automobile. The campus is dotted with trees. buildings, gardens. fences. etc.. so carrying the receiver by band was the only logical scans of transportation. If the outfit is to be carried ex- clusively in an automobile, a bnity receiver using the conventional large and heavy stm'age batteries would not be objectionable. Some thought must be given to limiting weight and size of the set it it is to be moved around on a hand cart. Finally. it the set is to be carried about by hand. its weight and bulk must be the emllect possible that is consistent with reliable ognration and sturdy design. The relative pmer of the transmitter under consid- eration and the distances from the transmitter of the points shore readings are to be taken are factors which affect the necessary sensitivity of the receiver. If the transmitter is of low poser or if measurements are to be taken at comparatively long distances from it. the receiver must. of course. be extremely sensitive and 38. polorml in trder that the weak signals received my be and. to actuate an electric meter of some sort. Conversely. if the transmitter is of high power or the distances be- tween resentment points and the transmitter are small. 3 less sensitive receiver mm be used. is stated before, the receiving device must be strong enough to withstand the Jolte and Jars tint it ie likely to receive in be mg; carried about. In View of this fact, any vacuum tubes used should be preferably mounted in faring cushion socmts and mounted so as to be protected from any direct blue. Readings are not usually taken in windy or stormy weather, so the 11‘s- caution of rotating, the eggeretue from the elem etc can usually be disregarded. As the radio frequency field is being investigated it is satisfactory and preferable to transmit an umed- ulstei carrier wave. This eliminates one source of error in the rewdinge due to wasible aim-ages in the percentage of module ti on. and also reduces the interference caused by the transmitter to a minimum). This fact will influence the denign of the receiver considerably. It would be mzdecirehle to determine the relative strength of the received signal by me me of an audibility meter on the receiver because the ear is a rather poor judge of var- istion in sound intensities. It is generally conceded that measurements taken by this method would be accurate 39. no closer than teonty-five per cent. This then elimin- ates entirely the desirability of modulating the carrier wave and the use of on audio frequency amplifier to in- crease the sensitivity of the set. 33.13 usual method of noting the change in strength oi" the received signals is to notice the chums in pl‘E-to current through the last tu‘oe by mans- of s milliemetcr. S‘he mine of plots current through the tubes of s properly balanced audio fro-\‘gusncy smolifior is pry-.oticflly unaffected by changes in the strength of the received signal, so on amplifier of this typo would be useless also on a receiver that was eguipped with u. millinmetor for the msasm‘ouont of modulated signals. It follows that the only remining. mans for in- creasing the sensitivity of c aim-ole recc ivor used for this work is the nddit ion of radio frequency emolii'iors. The kind of receiver generally used for field strength Ltoosuroment work is the suwr-heterodyne or c multi-stsge tuned redio frequency type. Both woos were (rigimflly contidercd tcr use in the sort: covered by this thesis. It was suggested that a one tube receiver might be sufficient for our use in view of the purer of the college station (loco watts) and the $131.11]. area that was to be covered. Preliminary tests moved the suggestion correct and the final design sort: was concentrated. on a one tube 61871000 Deteile of Receiver Used The type of antenna to be need was the first point to be decided open. It was cuggoeted that e arm-.11 rero- ticel antenna supported by a ”fish pale” be used, but thie idea can finally dieoarded in favor of the loop antenna. The loop antenna hes the advantage of be 1:15; 1083 untieldy and easier to more about among, the many trees on various» pirte of the compute. Alec, the loop 908868363 well known directional effecte which would make it poceible to determine the direction of wove promotion at the varioue points. It wee proposed that it erratic 100p positions were noted during the testing, 90:30 interesting points might be brought up. The tuning of the circuit was simple. being done by a variable “air“ condenser connected in oerellel across the loop. L’reliminnry tests on an experimental set-up ehowed that this condenser (with a maximum capo- city of 250 mi.) was difficult to adjust to obtain resonance in the loop circuit. 80 a ”midget” vernier condenser (with e maximmn capacity of about 25 emf.) one connected in pfzrellel With the main tuning condeneer. Final taxing was accomplished with the emll condenser. is eteted before, a vacuum tube nee need for the detector. ”Grid biaa method of detection" was used as it pen-outed the variation in ecneitivity of the detector byvarying the amount of grid bice. This feature is valuable when taking reuoinga extremely close to the transmitting atation.aa will be shown later. Eho infli- oating device was in tho form of a milli-umater in the plate circuit of the vacuum tube. The connaotiona of the receiver :38 it was finally mood (are ohm-*3 L $15.10 The list 1 1 Pi )4 h’ to Id #0 ha :4 Fl r4 C)! . of parts used is as follows: 1009 or coll antenna 250 mmf. variable air condenser 5" dial for above oondonoor 25 mat. variable air conoonoer equipped :3 11.21 mo 1) UV-199 tyye vacuum tube UV-199 typo vacuum tube Bockat S.P.S.T. filament switch 25 ohm rhoootot eguippod.with knob O to 4 V. voltmeter O to 1 3A milliamoter 4.5 V. "C" batteries 22.5 V. '3" battery (amall size) small shelf bracket tripod and small plane tools small "0" clamgu xisoellanoous - mounting board, scrawa, comec ting, wire, etc . The 100pvantonna.was nude up of eleven turns of no. 14 bare etranéed oogpor‘uire'wound pancake style in < 1;; - m 0 .+ _ _ _ Ft 9 + _ _ l :: _ 7 g 1 Ix. II F - moQICtkWL lul a - otiitmia‘u § , - v on \ SS m _ 4 : iIJ . 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Espenschied, in his report on "Broadcast Coverage of City Areas", states that field survey readings in New York:City were taken at one mile intervals on concentric circles, whose radii differed by five miles. The rela- tive distances between points where readings were taken and the area covered in this survey compare favorably with the above mentioned test and the authors believe the results obtained are as accurate as the results of similar commercial tests. The curves shown in Fig. 15 contain many small irregularities, but the general projection to the west is what would be expected from the Toshaped antenna. The projection that would be expected from the east end of the antenna is apparently destroyed due to shielding and absorption effects of the water tower and chemistry 79. building. In fact, a slight shadow effect persists between the station and chemistry building caused prob- ably by the water tower which is used as a support for this end of the antenna. The steel tower supporting the other end of the antenna exhibits a slight shadow effect near the antenna also. Assuming that the shape of the counterpoise has the same effect on directional characteristics of a radiating system as the shape of the antenna, Pig. 8 would indicate that a directional effect would be obtained in a north and slightly easterly direction. The curves show this to be true except for slight shadows caused by the agricultural and old veterinary buildings. A strong shadow or shielding effect is noticed on ' the far side of the library building. This is the only shadow effect that persisted.for any great distance. Although the outer curve seems to show that this shadow is starting to heal, it is a noteworthy fact that this shadow is in the general direction of Grand Rapids and reception in that direction is reported to be very poor. Reception of IKAR in a southwest direction is also very poor, but the results and curves obtained in this survey indicate that the poor results are due to condin tions outside the immediate vicinity of the transmitter. It is noted that the strength of signals on the far 80. side of the river are much less than the signal strengths obtained on the near side. That such a large drop should be obtained in such a short distance is peculiar and inconsistent with the results obtained on other parts of the campus. A satisfactory explanation of this phenomenon has not been obtained, inasmuch as the absorption over water is much less than that over land and trees. Varying Poser Test In the discussion on "Theory of Wave Propagation" it was shown that various authors differed as to the manner in which the radiation field varied with the distance from the antenna. A test was made to discover, if possible, whether the radiation field varies inversely as the first power of the distance or as the inverse square of the distance. In Fig. 6 it will be noticed that Espenschied varied the radiated power in order to beep a constant field at points of varying distance from the antenna. In the varying power test made in this survey the distance was kept constant and the variation in radiated and received ' power noted. 3 According to Dellinger the induction field becomes negligible at a distance of one wave length. The wave length of am is 277.6 meters or 907 feet. For this reason the receiver was set up near points I-17 somewhat further away than one wave length. The watches of the I r. . ‘ .. .. . .. .. . . V . . .. o . . A . . . _ r . . . . . n _ A .H a .. n4. _ . . H . . . , . . . . . . _ _ . u . _ . . ... .. its. . o . ._ v . i . . y .i.n. .i .. . vi... . .i .\ ... . . . v . i . . a . . . . L _ . . . u . u _ a y . _ — _ 81. Station Operator and field operator were synchronized before the test and certain times arranged for decreasing power and making readings. The power was left constant at each point for 3-1/2 minutes to give the field aper- ator time to observe readings. Inability to reduce the plate voltage of the oscillators below 420 volts prevented readings lower than 3.5 amperes antenna current. For this reason the lower part of the curve in Fig. 16 is theoretical. However, it is evident that such a curve must pass through the origin so the lower part must be approximately correct. The resulting data was plotted.with antenna current squared as abscissa and receiver loop voltage as ordinates. As radiated power equals 123 and the radiation resistance is constant with constant frequency it is correct to use I: as the abscissa as it is proportional to the power output. As the loop voltage is preportional to the field intensity it may be used as the ordinate. From this curve it can be seen that the effect at the receiver varies about as the square root of the radiated power. Varying the received power with constant distance is the same as varying the distance with con- stant received power, so it would appear correct to state that according to this eXperiment the radiation field varies approximately as the inverse Square of the dis- tance. 82. ACKNOWLEDGMENT We are indebted to the Service Department of the Michigan State College for the use of the College Radio Station, WKAR, and for maps of the campus, to the Electrical Engineering Department and capacially MI. B. Osborn of this Department for many helpful suggestions. We are also thankful to the Technical Staff of WEAR for their cooperation and help in standing station watches during the tests. K. 83 BIBLIOGRAPHY Principles of Radio Communication - Second Edition Morecroft. Radio Frequency Measurements E. B. Moullin. Principles Underlying Radio Communication U. S. Signal Corps. Radio Engineering Principles Lauer and Brown. Wireless Telegraphy Zenneck and Seelig. Manual of Radio Telegraphy and Telephony Robinson. Radio Instruments and Measurements U. 8. Bureau of Standards Circular No. 74. Experimental Electrical Engineering - Vol. II Karapetoff. A Radio Field Strength Survey of Philadelphia McElwain & Thompson - I.R.E. Proceedings, Feb., 1928. Radio Field Strength Measuring System for Frequencies Up To Forty Megacycles Friis and Bruce — I.R.E. Proceedings, August, 1926. 84. BIBLIOGRAPHY (Continued) Portable Receiving Sets for Measuring Field Strengths At Broadcasting Frequencies Axel G. Jensen - I.R.E. Proceedings, June, 1926. Principles of Radio Transmission and Reception With Antenna and Coil Aerials J. H. Dellinger - A.I.E.E. Journal, October, 1919. Radio Broadcast Coverage of City Areas Lloyd ESpenschied - A.I.E.E. Journal, November, 1926. o «3 a. I _-" l ‘r. 3* 1804 3 1293 03145 III II III All. 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