Re: RF 2.4Ghz Signals and Weather Conditions

Greetings:

References:
“Transmission Systems for Communications”, Fourth
Edition, Members of the Technical Staff, Bell Telephone
Laboratories, Western Electric Company Technical
Publications, December 1971.

“Engineering Considerations for Microwave
Communications Systems”,
GTE Lenkurt Inc, San Carlos, CA, 1972.

Microwave links are called line-of-sight links
because they follow a straight path and then continue
out into space. However, their are modifications
to this that I will discuss below.
For short distance microwave communication links over
distances less than 8 km (5 miles), weather and atmospheric
effects have only a slight impact on the microwave
signal, except for heavy rain cells at frequencies above
10GHz. The maximum link distance used by the
telecommunications carriers averages to about 48 km (30
miles). At this distance , over flat terrain, the
curvature of the earth would be a hump 46 meters (150
feet) high at the midpoint between the antennas, this
is the horizon. Thus to have line-of-sight
communications, each antenna would have to be on a 46
meter (150 ft) tower to have line-of-sight over the
earth’s curvature hump. In mountainous or hilly terrain
the links can have slightly longer distances by placing
antennas, often on towers, on the high peaks or ridge lines.

In reality it turns out that microwave refraction in
the standard atmosphere slightly bends microwave links
downward so that they can curve somewhat over the horizon and
the earth’s curvature. However, other atmospheric
effects can also bend the beam upward before reaching
the top of the hump breaking the line-of-sight. These
atmospheric effects reduce the signal level in what are
called fades. A fade can last a few seconds to a few
hours depending on the cause.
The amount of time in a year that a microwave link can
be out of service because of fades is an important
parameter in the link design and it impacts the cost of
the system and the antenna spacing. For example a
microwave link with 99% reliability will be out of
service 88 hours per year. A link with 99.99%
reliability will be out of service 53 minutes per year
and a link with 99.9999% reliability will be out 32
seconds per year. Clearly applications such as air
traffic control require great reliability while other
applications may be able to wait for fades to clear
(perhaps with a reduced service fee).

Weather and Atmospheric Effects

Weather and atmospheric effects on microwave
communications and radar systems may be categorized as
follows:

(1) Refraction of the microwave beam by the
troposphere. Refraction effects are frequency
independent below 100 GHz, thus affecting the
performance of most microwave systems.

(2) Effects in the clear atmosphere. Variation in the
vertical thermal and pressure profile including
inversion layers, sudden changes in temperature from
weather fronts, particularly during the hot summer
months and the drop in barometric pressure preceding a
storm front.

(3) Attenuation by atmospheric gasses. Attenuation by
atmospheric gasses begins to effect the performance of
microwave systems at frequencies above 10 GHz. The
problem is very severe at frequencies near the water-
vapor absorption line at 24 GHz and the oxygen
absorption line at 60 GHz.

(4) Attenuation from scattering by both particulate
matter and hydrometers (rain, fog, snow). Particulate
and hydrometer scattering and attenuation effects begin
to appear at frequencies above 3 GHz.
All of these effects must be considered in the design
of microwave communications and radar systems.

Part of the answer to your question is that effects (3)
and (4) above do not impact microwave systems operating
at 2.4 GHz. Because the wavelength is so much larger
than particulate matter and rain drops, even in heavy
rain, these effects are minimal. This makes the
frequency band between 1 GHz and 4 GHz very useful for
radio astronomy and highly reliable communications and
navigation systems such as the Global Positioning
System (GPS) satellite navigation system and tactical
air communications and navigation (TACAN) systems.
Below 1 GHz man-made interference, lightening and solar
storms cause greater interference, the lower the
operating frequency the greater the problem becomes.

Effects (3) and (4) greatly impact
systems above about 4 GHz. Thus the 1-4GHz band is
considered a clear radio frequency window in the atmosphere
which is even superior to the visible light window in
the atmosphere that can be seriously degraded by
effects (3) and (4).

Effects (1) and (2) are of importance at all microwave
frequencies including 2.4 GHz. The radio index of
refraction of the atmosphere, N (as opposed to the
optical index of refraction n), is a number on the
order of 1.0003, varying between 1.0 in the free space
above atmosphere to about 1.00045 at a maximum. In the
normal “standard” atmosphere, a plot of N versus
altitude (as shown in figure 1) has a constant slope
which bends microwave beams slightly downward from a
line-of-sight-path so that they can be received
slightly beyond the optical horizon. Note: These
figures are not to scale because the change in N is a
very small but important number.

Altutude
I--*
I--* <- Free Space
I----*
I------*
I--------*
I----------*
I------------*
0_ 1___________________
RF INDEX OF REFRACTION-->
Figure 1 Altitude versus index for a Standard Atmosphere

In Figure 2 the effect of abnormally high surface
temperature, or increasing water vapor content is
shown. Such a surface condition will result in curving
the microwave beam upward away from the earth.

Altitude
I--*
I--*
I-----*
I--------*
I------------*
I----------------*
0__1___________________*__
RF INDEX OF REFRACTION-->
Figure 2: Abnormally high surface temperature

A rise in temperature with increasing height or a
decrease in water vapor content, or both, will cause
the beam to follow the earth’s surface well beyond the
horizon.

Altitude
I--*
I--*
I----*
I------*
I--------*
I---------*
0__________*_____________
RF INDEX OF REFRACTION-->
Figure 3: A rise in temperature with increasing height

When the changes in N are most severe near the surface
the profile shown in Figure 4 occurs. This condition is
known as a surface duct because the beam will stay near
the surface in the duct for very long distances over
the horizon. The top of this duct can be very sharp (a
few hundred meters (yards) thick and causes an
inversion layer which often traps smog close to the
surface here in Los Angeles. This layer also can have
an effect on polarization which will be discussed
later.

Altitude
I--*
I--*
I-----*
I--------*
I-------* Surface
I-----*Duct
I_1__*____________________
RF INDEX OF REFRACTION-->
Figure 4: A sharp rise in temperature creating a
surface duct.

Figure 5 shows severe surface changes creating an
elevated duct. Concentration of microwave energy in
this duct will cause a great increase in received
energy. However, both the transmitting and receiving
antennas must by within the duct. Obviously this effect
cannot be relied upon for any reliable communications
system because conditions causing the duct are
transient in nature.

Altitude
I--*
I--*
I-------*
I---------*
I-------*<- Elevated
I-------*<- Duct
I----------*
0__1____________*________
RF INDEX OF REFRACTION-->
Figure 5: Severe surface changes can temporarily create
an elevated duct.
Polarization Effects

Relative to the earth’s surface, the polarization of a
typical microwave beam can range from linear vertical
polarization to elliptical (with the ellipse vertically
elongated) to circular (right hand or left hand) to
elliptical (with the ellipse horizontally elongated) to
horizontal linear. These are the polarizations
typically encountered by microwave links operating on
the earth’s surface. However, signals from satellites
aircraft and spacecraft can have linear, elliptical or
circular polarization at any angle relative to the
earth’s surface (depending on the orientation of the
vehicle) so that in reality there are an infinite
number of polarizations possible. For this reason most
antennas used for tracking and receiving microwave
signals from space are circularly polarized because
they can detect all of the above polarization
combinations, although they are not as efficient as a
communications link using antennas with matched
polarizations.

The DirecTV television broadcast satellites use both
right hand and left hand circular polarization
simultaneously. In this way they can double the
capacity of programming by reusing the same frequency
band for two different sets of TV channels. Other space
communications systems use crossed linear polarizations
for this same purpose; however, the earth bound antenna
must be able to rotate the polarization of the antenna
to match the orientation of the two crossed linear
polarizations, a problem not encountered in the DirecTV
system.

When we operate a microwave link with a given
polarization (linear , elliptical, circular etc.) over
a path in which rays of the beam that are not
line-of-sight can reflect off of objects and reach the
receiving antenna at a later time than the
line-of-sight beam, serious interference and
changes in the state of polarizationcan occur.
This interference can also be related to the cause of
fades as discussed above. This type of interfere caused
by the microwave signal itself is a coherent
interference and it can drastically change the received
polarization and or cancel a major portion of the
received signal causing a deep fade. Large, flat
surfaces near the line-of-sight path, such as lakes,
bays, large buildings etc. are the major source of
these problems. Thus a major part of a microwave
engineer’s job when laying out links is not only to
find high terrain and towers for a line -of site path,
but to determine the location of potential sources of
reflections along the path that could also reach the
receiving antenna. By using mountains, and clefts etc.
to block the sources of these reflections is a
difficult and time consuming task and design handbooks
of the type in the references go into great detail
about this engineering process.

Best regards, Your Mad Scientist
Adrian Popa