[Lowfer] Guided Waves

[Lewis Denton]

     Why does it always look so small when you finish typing ?
     I hope some of you get a better picture of how LF propagates
     from this as I have.
Lewis
                           GUIDED (LOW FREQUENCY)
                                       WAVES


    At frequencies below app. 300 KC, the propagation picture presented
For medium and high-frequency ground and sky waves becomes less and less
Precise.
The effective conductivity of even the poorest soil becomes quite high,
Because a much thicker layer of soil is involved, and as the freq.
approaches
The upper audio range the earth begins to act like a nearly perfect
conductor
As far as absorption and reflection are concerned.
Thus the attenuation of the ground wave due to earth losses is quiet small,
And attenuation of a multiple-hop sky wave due to earth losses likewise is
Quiet small.
    As the freq. is lowered the effective conductivity or coefficient of
 Refraction of the ionosphere is increased to the point where the lower
Edge of the ionosphere (D or lower E region, depending on freq.) acts
Like a reflecting mirror at a height of from 35 to 60 miles, and for this
Reason the wave is considered as being reflected rather than refracted.
The absorption is very low, particularly at freq. below 100 KC.,
Because the wave is reflected before it penetrates very far into the
Region that ordinarily is responsible for most sky wave absorption.
    Another characteristic of low freq. waves is that there is no skip
Distance for sky waves, because it always is returned at all angles of
Incidence, including the normal.
    So we have a sky wave that travels great distances with no skip and
Little absorption, the spreading of the sky wave being confined to one
Plane after it once fills the space between the two concentric conducting
Spheres which confine and guide it.
     This sky wave or guided wave suffers less absorption and therefore
 Is stronger during the night than during the day, during the winter
Than during the summer, and during the less active portion of the sunspot
Or terrestrial magnetic cycle. These normal variations, especially the
Diurnal variations, becomes less pronounced with freq., until at freq. in
The neighborhood of 15 to 25kc. The signal intensity is of the same
Order of magnitude at all times, even during sever ionosphere and magnetic
Storms.
     There is a pronounced dip in the intensity of the sky wave at or
Slightly after sunset, and sometimes another, less pronounced, around
sunrise.
Like other variations, the magnitude of the dip decreases with freq., until
at
20 KC or so it is insignificant.
The reasons for the dip in intensity are not definitely known or fully
Understood, though various theories have been advanced.
     Like high freq. sky waves signals, low freq. sky waves are propagated
Differently in different geographical locations, though in most cases the
Effects are not so pronounced, and tend to become still less at the lowest
Practical radio freq. (about 15kc) is reached.
     At distances close to a Low freq. Xmiter. The surface wave
predominates,
Because the ionosphere reflected wave must travel so much farther to reach
The receiving point. At extreme distances, the sky wave predominates,
Because the ground wave travels in more continuos and intimate contact with
the ground, and while the absorption of the ground wave is low, it
nevertheless
Is greater than that of the sky wave.
     At intermediate distances from 200 to 1000 miles (depending upon
particular
Freq. And the ground constants over the transmission path) the two
components
Will be of the same order of magnitude. However instead of producing severe
inference-type fading as occurs at high freq. Under the same conditions,
Changes in signal strength occur gradually.
This is explained by the fact that at Low freq. The sky wave path is
relatively
Stable in term of wavelength. At a distance where the intensity is low
because
Of cancellation effect, the intensity an hour or a few hours later may be
max.
Because of a gradual shift in the effective height of the reflecting layer
Due to sunrise or sunset. However this cannot be classed as fading.
     The Comparatively stable interference pattern, which is produced, will
Resemble a standing wave, and their will be locations where the resultant
intensity is only a small fraction of the intensity a few miles closer to or
Farther from the xmiter. At a different time the phase different may be such
That the intensity at the same location is greater than existing a few miles
In either direction along the signal path.
     It should be noted that at the extreme lower range of radio ordinary
used
, Or about 15 kc., the height of the reflecting medium is only about four
wave length, Under these conditions it is difficult to make a distinction
between the ground wave and the sky wave, as they began to lose their
identity as such,
And combine to produce a resultant mode of propagation, however,
interference
Patterns and effects still exist.


God Bless America