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
|