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#33
December 17th 05, 05:27 PM
posted to rec.radio.amateur.antenna
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Antenna reception theory
Richard Harrison wrote: Reg, G4FGQ wrote: "What do photons have to do with winning a contest?" I don`t know. But, B.Whitfield Griffith, Jr. has some observations in "Rado-Eledtronic Transmission Fundamentals" that may be a useful check on your computations. I expect he checked, rechecked, then checked again before publication. He put a transmitter on an elemental antenna but it would work the same in reverse. From page 325: "An Elemental Antenna Since the length of an antenna is commonly expressed in electrical degrees and since this allows the convenient use of trigonometric tables in computing the ratios of currents in various parts of an antenna, let us choose as our elemental antenna a piece of wire which is 1-degree in length and in which the current is constant from one end to the other.We shall first assume that this elemental antenna is located far out in space, so that its field is not disturbed by reflections from the surface of the earth or from any other object. This, of course, is a most improbable set of conditions, but we can certainly imagine that we have a situation such as this and compute from the field equations its electromagnetic result. These computations will show, first of all, that the maximum field intensity will be produced in the directions which are at right angles to the direction of current flow. This is a reasonable result, since the magnetic field which is produced by the current surrounds the wire in concentric rings and thus gives rise to a radiation field which moves outward at right angles to the wire. As a matter of fact, the field intensity, measured according to our standard procedure at a distance of 1 mile in any direction from the radiating element, will be found to be proportional to the sine of the angle between the direction of the current flow and the direction in which the measurement is taken. If we represent the field intensity at 1 mile in any direction by the length of a vector starting at the center of the element and extending in that direction, the tips of the vectors will mark out the radiation pattern of the antenna element. A cross section of the entire radiation pattern of this element is shown in Fig. 39-2; the entire pattern would be obtained by rotating the figure about the axis of the antenna element. But this pattern tells us only the relative signal strength in various directions; it is a normalized pattern, with the intensity in the direction of maximum radiation being considered simply as unity. We need much more information than this; we must know the relationship between the current and the actual value of the field it produces. Further computation from the field equations gives this relalationship; we find that a current of 1 amp flowing in the antnna element will produce a field intensity of 0.3253 mv/m at a distance of 1 mile iin the direction of maximum radiation. We have said nothing about the frequency, and we do not need to; as long as the wavelength is shorter than 1 mile, so that there are no serious induction-ffield effects to upset our calculations, this figure will be correct at any frequencyfor which the length of the element is 1/360 wavelength. The field intensity of the elemental antenna is directly proportional to the current. Therefore, if the current in the element is 15 amp, the field intensity will be 15 X 0.3253, or 4.8795, mv/m at 1 mile. Similarly, the field intensity is directly proportional to the length of the element; an element which is 2-degrees in length, carrying a current of 1 amp, will thus produce a maximum field intensity of 0.6506 mv/m at 1 mile. Best regards, Richard Harrison, KB5WZI How did you get .3253 mv/m at 1 miles? Dave |
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