INTRODUCTION: When we talk
about noise, we are talking about unwanted
signal. We 'break out' noise into three basic categories;
that is background noise, modulated noise and interference noise.
Background noise is the noise we always want to stay above, it
is sometimes called the 'noise floor'. Signal can always be amplified
above the noise floor but once it gets buried in background noise
(falls into the signal floor) it can not be retrieved; this is
why the LNB amplifies the weak satellite signals as soon as they
are received before passing them on into the cable and to your
receiver. Modulated noise is undesireable signal that enters
into a system and rides on a signal, using your system power,
producing undesireable side effects in video quality. Interference
is noise that comes in on the same frequency(ies) as signal and
masks (overwhelms) parts or all of the desired signal (see
diagram).
(top of page)
BACKGROUND and SYSTEM NOISE:We
live in a world composed of atoms and molecules and although
an object, such as a rock or a circuitboard, may appear motionless
in reality inside it is 'jumping around' with molecular motion.
This ceaseless molecular motion emits (gives off) energy, i.e.
generates electromagnetic fields, and portions of this energy
are in the microwave range. So in a microwave system (which is
what a satellite system is) there will always be a level of detectable
noise (background noise) inherent to the system and this noise
is the internal noise of that system and we call that base level
of noise the noise floor - it is impossible to go below a noise
floor except by lowering the temperature of the unit to absolute
zero which is the point where all molecular motion ceases. (Absolue
zero is minus 459.69F or minus 273.16C.) However, we try to lower
the noise floor of electronics through better circuitry design.
For instance, an LNB is rated in 'noise temperature' (in degrees
Kelvin) and 
this is a value indicative of the noise floor of that
unit - this is the noise the LNB contributes to the system. It
basically means that any signal that passes through the LNB will
have to be above that rated value or the LNB will not detect
it and if the signal can not be detected then the LNB can not
operate on it.. Old timers in the satellite industry will remember
the first C-band LNBs had ratings of 100+ degrees and now we
are using LNBs with ratings of 25 degrees or less - this change
in LNB noise floor is due to advances in circuitry design, component
isolation, semicondutor efficiency, crystal growth, external
coatings, etc. Internal heat in power supplies, amplifiers, etc.,
also generate noise. All units that a signal passes through -
amplifiers, splitters, frequency converters, etc, - add noise
to the signal. Detectable noise can also be generated through
electric motors (I have seen a broken motor mount generate a
spurious signal that modulated onto the desired signal), neon
lights and even defective automotive ignition systems. The bottom
line to a satellite owner is that when the noise floor of a system
is lowered then weaker signals can be efficiently processed through
the system and the smaller satellite dish is required.
(top of page)
EARTH THERMAL NOISE: Another
contributor (component) to background noise is random noise -
naatural noise that is not related to system equipment. One thing
to remember, the satellite dish itself receives random 
noise from the earth in addition to signals from space,
this random noise is often called earth thermal noise.
Earth noise is something we can not control and is generated
by the same internal molecular motion of all matter as is the
case in system electronics. Therefore, when the dish is at its
peak (i.e. not looking at the ground - over the horizon) it is
receiving less earth thermal noise than when it is positioned
looking out on the horizon. Thus, lower end satellites will always
show a weaker signal than higher arc satellites - all things
being equal - because more earth noise is being received at the
same time low end arc satellite signal is being received than
when top of the arc satellites are being received. If your satellites
of interest are on the low end of the arc and those satellites
are delivering weaker signals to your system after your best
efforts at tuning the dish, then you will require a larger diameter
dish though installing the best rated LNB you can afford might
overcome this. Note, a larger diameter dish will take in more
thermal noise, of course, but the increased satellite signals
it will gather are more significant than the increased thermal
noise it will pick up. (Side lobes
of a larger dish are smaller in comparison to its main lobe so
a larger dish receives less per cent noise per signal as compared
to a smaller dish and, as the chart indicates, consequently shows
to receive less noise than a smaller dish so that a larger diameter
satellite dish is the clue to overcoming weak signals from low
end of the arc satellites.)
(top of page)
FREE SPACE LOSS: Not all
low end signal loss is due to increased earth thermal noise reception;
low end satellites pass through more atmosphere to reach your
dish than high end satellites and that reduces low end satellite
signal strength; this is called free space signal loss. Note
in the free space path loss chart, the higher the 
frequency, i.e. Ku-band over C-band, the greater the
signal attenuation. Because the satellite at the top of the arc
is the one you are closest to, there is more distance from your
dish to the end of the arc satellite than to the top of the arc
satellite so end of arc signals travel through more free space
and have more free space signal loss. The distance to a satellite
from a receiving location is called the slant path distance (slant
range, slant path) and the greater the slant path distance the
greater the free space loss. In addition to horizontal differential
slant path losses there are latitude losses which mean that locations
at the higher latitudes have a greater slant path to the satellite
belt than do satellites at the equator and therefore have more
free space signal loss than equatorial receiving locations. In
summary, the more a signal passes through earth atmosphere (the
greater the slant path), whether in clear weather or through
rain or whether it be C-band or Ku-band, the greater will be
signal attenuation and the greatest attenuation will always be
at the higher frequencies. Free space loss does not cause noise
to enter your system but it does prevent signal from being as
strong as you may require. The combination of free space loss
and earth thermal noise are the reason
that after all the best efforts at tuning your dish the low end
satellites are still being received at less signal quality -
if this is the case with your system, and the low end satellites
are very important to you then you will need a larger satellite
dish.
(top of page)
RAINFADE:Rain fade is all
about signal absorption and scattering of 
incoming signal. By far the greatest single event
reduction in power of signal is caused by rain, not so much water
vapor, i.e. humidity and fog. Since rain only forms in the troposphere,
which extends seven miles above the earth, and satellites in
geostationary orbit are 35,800 km above the earth, a signal travelling
through a rain cell will experience attenuation during only a
small portion of its transmission path. In fact, terrestrial
microwave transmissions are more susceptible to the effects of
rain
attenuation because their signal paths are entirely in the troposphere,
and the signal may pass through an entire rain cell. In general,
C-band signals to be affected would require rain storms approaching
hurricane conditions. I have watched C-band in tremendous thunderstorms
in Houston, i.e. in the prime footprint, with no change in reception
whatsoever; however, here in Mexico on the same satellite in
same intensity of rain the signal was degraded a letter grade
in quality. And in both locations, when the system was tuned,
the video quality rating on the descrambler said reception is100%
. The point being, obviously, the more marginal you are in the
EIRP footprint, the more effect rain will have on C-band regardless
of what your 
receiver rating indicates. Otherwise, consider C-band
not to be affected by rain - now that I have said that, if you
are an SMATV operator, get a
good link budget (this is a real easy program to use and
an example output of its TV screen display
is seen here) analysis that includes rain intensity (rain volume)
as one of its parameters from your commercial equipment supplier
before purchasing the dish as your clients will not tolerate
any outage. 
Professionals use rain zonal maps (the USA map is
produced by NASA) and rainfall-time intensity maps to calculate
what 99.9% availability would be for a dish system to receive
distribution quality signals. They take the 0.1% of the time
rain rate and insert that value into link budget equations in
their dish size calculations. For C-band it is not as critical,
in regards to Ku-band, however, the diameter of a rain drop is
definitely detrimental to passage of Ku/Ka-band signals. In the
above referred to Houston storm, Ku was down a grade in video
quality whereas here in Mexico it was wiped out completely -
and I have top grade Ku video reception on clear days here in
Mexico and top grade Ku equipment. However on ultrafoggy days,
here in Mexico, I have noted no degradation of either C or Ku
band reception so the conclusion is that signal loss through
fog is a minimal practical concern. In regards to diameter of
the raindrop, signal attenuation is proportional to the wavelength
of signal frequency and the size of the raindrop through which
the signal has to pass. Transmissions at C-band have a longer
wavelength than transmissions at Ku band, and are therefore less
susceptible to rain attenuation. For example, a C-band frequency
has a wave-length of approximately 7 cm, and a Ku-band frequency
has a wavelength of approximately 2 cm. Any raindrop in the path
of either signal which approached half the wavelength in diameter,
will cause attenuation. It is to be noted, Ku-band attenuation
in rain is approximately nine times that of C-band or 9:1dB -
for each one dB loss in C-band expect nines times that on Ku
(remember that each 3dB signal loss is a halving in power). Note
rain attenuation effect on Ku-band with change in dish look angle
- the conclusion being that there is less loss at greater look
angles. As you know, this angle is dependent on the latitude
and longitude of the earth station. The lower the latitude of
the earth station the higher the elevation angle, and the less
atmosphere through which signals travel. The higher the latitude,
the lower the angle, and, therefore, the more atmosphere through
which a signal must travel and the greater the probability of
it having to travel through rain. Rain fade does not cause noise
to enter your system but it does prevent signal from being as
strong as you may require.
(top of page)
TERRESTRIAL INTERFERENCE:Terrestrial
interference (TI) occurs when a home satellite system receives
unwanted microwave signals from a nearby microwave source operating
in the same band of frequencies as the received satellite signal.
The most common source of TI comes from microwave relays (towers)
operated 
by telephone companies in the C-band range, although
airport navigation systems also can disrupt satellite reception
at all frequencies - especially near miliary installations in
countries without a clear frequency coordinating authority in
their central government. TI is a land based phenomena generated
by land based microwave relays and broadcasts transmitting in
the same frequency range as satellite downlinks. Because land
based transmissions are much more powerful (and nearby to your
equipment) than space based satellite transmissions, land microwave
signals will dominate into your receiving equipment and this
unwanted signal reception by a satellite antenna system is termed
TI. Telephone company relays, i.e. microwave tower to microwave
tower single carrier transmissions, are the most common source
of TI because they were allocated the C-band frequency range
before satellites were in existence and the first satellites
built were to accomodate (and built by) telephone companies so
as to provide a means to transmit telephone traffic over great
distances. The irony and benefit of the situation is that most
telephone traffic has moved from satellite carriers for long
distance transmission services to fiber optic carriers. In 1965,
when Intelsat 1, Early Bird (satellite
history), was launched, the satellite provided almost 10
times the capacity of the then submarine telephone cables for
almost 1/10th the price. This price-differential was maintained
until the laying of TAT-8 in the late 1980s. (TAT-8 was the first
fiber-optic cable laid across the Atlantic.) Satellites are still
competitive with cable for point-to-point communications, but
the advantages have been shifting to fiber-optic cable for telephony
traffic in more and more point-to-point applications. Satellites
still maintain two advantages over cable in that they are more
reliable and they can be used in point-to-multi-point (broadcasting)
applicatons. The benefit of the shift in telephony traffic to
fiber optic cable situation is that microwave land towers have
drastically been decommissioned, thus numerous TI sources have
been eliminated. But by no means are telephone microwave transmission
the only potential TI source. TI can be at the frequencies of
direct detection through the LNB or be at lower frequencies which
enter your system at points other than direct detection by the
dish.
Detection of potential direct entrance TI can be as simple
as performing visual inspection at a possible installation site for nearby (within site) microwave
or broadcast towers 
or
can be as detailed as connecting an LNB (the same frequency to
be installed) to a spectrum
analyzer (or through your receiver to the TV) and pointing
the LNB at suspect TI sources while watching the analyzer for
signal reception. Remember to check both polarities on the analyzer.
For a single carrier microwave
tower you will see a 
'spike'
on the analyzer screen, and for multiple carrier towers you will
of course see multiple spikes. In the case of general interference,
of airport or military frequency patterns, and reflected interference
(scattered from nearby buildings or walls, etc.) you will typically
see a series of spikes and often in jagged patterns not dissimilar
from that as seen when looking at normal data carriers or compressed
digital signals from a satellite transponder.
On the TV screen you will see anything from annoying sparkles
(which a Chapparal receiver will clear through its filters) to
the beginning of picture fadeout (a blizzard of sparklies) to
blank screen - these are the symptoms of direct carrier, dish
detected (in-band), TI interference. Noise interference other
than in-band noise, i.e. not in the direct satellite reception
frequency range, will appear as diagonal interference lines across
the TV and sometimes can be the cause of ghost images. Any output
port in an audio/video distribution system must end in a device,
example being a TV or 75ohm terminator. Terminators trick the
cable into 
thinking
it is connected to a device otherwise the open end can allow
spurious signals to enter the system and/or the signal will 'hit'
the end and be reflected back along the line as a ghost signal.
The later two problems are not usually dish related reception
problems rather are leakage problems at frequencies out of satellite
frequency range; such leakage is most likely to occur in a distribution
system (rather than a home system) where there are more cable
connections carrying lower frequency signal to allow outside
signal to enter the system - and we forget to terminate each
open and we get in a hurry making connections (see proper
procedure to make connections). The more connections existing
in a system the more chance for out of band interference,. i.e.
unwanted signal ingress.
(top of page)
TI below about 300MHz enters a system through poorly grounded
or improperly connected 
equipment. For frequencies
above 300MHz, wavelengths are sufficiently short that they can
enter a system through poorly shielded electronic cases or directly
into openings, i.e. non terminated connections. (NOTE: The wavelength
is 2/10 of an inch at 300MHz.) If 'loose' signals are out there
wandering around, and they find an entrance into your system,
they will take it and then use their power to travel with your
signals in a modulated fashion or corrupt your signal by adding
to the noise floor, a very simple fact.
Some LNB received TI will be distorted from its original frequency and muddled in its amplitude due to reflections and scattering, or because it is generated at non-standard frequencies, and will appear in-band to the desired signal but sufficiently off center from signal (see adjacent diagram) to avoid major signal interference.
In general, TI can enter through unterminated cable ends, poorly grounded connectors, open equipment cases, impedance mismatches and directly through the LNB and can either contribute to the noise floor or modulate their pattern onto your signal or interfere directly (replace) with received signal. It is the latter TI, direct in-band TI that replaces signal, which is the killer and which we are primarily looking for in a choosing an installation location.
It is good practice, in addition to specifically pointing
the LNB directly to a visible tower, to 'sweep', i.e. scan, the
LNB 360 degrees around the installation site to check for unwanted
in-band signal such as reflected from nearby buildings/walls,
etc. (Be sure to note the directions of all noise.sources.) Make
two sweeps each one with the LNB held orthogonal (ninety degrees)
from the other to be sure you are checking both polarities -
maximum TI might be coming in at a skew. 
If you detect TI, then
rotate the LNB (while aiming it at the TI source) to see what
the maximum signal interference level will be and to see exactly
which direction the maximum interference is originating. You
can check that the source is really TI by placing your hand over
the LNB at which time the offending signal should disappear from
detection; if not, the test equipment is malfunctioning (low
battery or whatnot). TI often enters from an off-axis angle,
through a dish side lobe (see lobe
discussion below) and can be most annoying and detrimental
to signal quality even when the dish is not aimed directly at
the TI source. If you can not locate a site free from noise then
note the frequency of the noise (if using a spectrum analyzer)
which will tell you which satellite channel(s) will be affected
and note from which direction the noise is located which will
tell you the satellite(s) which will be affected. If you are
using a test dish for this test and a full transponder display
on the analyzer, dial the screen needle to the satellite channel
nearest to the TI spike(s) then flip to image display and see
which channel is affected and of course you already know what
satellite you are on by comparing the received channels to a
channel chart. NOTE: As stated previously, TI is a big concern
in countries where frequencies are not regulated and wattage
happy users are abundant and a site survey that omits TI detection
analysis, especially in urban areas, can result in a 'messy'
situation after installation. Any customer that is counting on
their MTV and finds it blanked out by TI will not be happy. Remember
that microwave traffic is not usually continuous in its transmission
and may be off during certain periods of the day so it does not
hurt to check your potential site locations at several times
during the day/night.
Direct detection TI can be avoided by selecting a location where
its signals can not reach the satellite dish - either 
directly or via reflections. And the choice of satellite
dish can assist in rejecting TI. The deeper the satellite dish,
i.e. the less the F/D ratio of the dish, the more narrow will
be its acceptance of satellite signals and the less chance unwanted
signals will enter the feed assembly. Simply stated, the deeper
the satellite dish, the closer the feed assembly is to the center
of the dish and this physical attribute of dish design lessens
the amount of unwanted signal which can enter the LNB. You can
see from the chart, at F/D=0.25 how difficult it is for signals
from the outer edges of the dish to enter the feedhorn; a deeper
dish gathers less signal thereby has less gain than a shallower
dish (the greater the value of F/D then the shallower the dish
and the greater the gain) but has greater off-axis signal rejection.
A dish is considered deep with F/D ratios of 0.25 to 0.32 and
is considered shallow with F/D ratios of 0.33 to 0.45. The actual
parabolic design (from one dish to the other) actually determines
the reception pattern of a dish and how much signal rejection
it will have in addition to how much gain it will have.
(top of page)
SIDE LOBE DISCUSSION: Quantitative
reception patterns of a dish are made by measuring the physical
dish signal response characteristics (i.e., the dish is outfitted
with a feed and LNB) to a known test signal. The test signal
is aimed at the stationary dish and at regular intervals, as
the test microwave source is physically rotated around the dish
a full three hundred sixty degrees, its per cent reception by
the dish, as compared to the known test signal generated by the
test source, is monitored and recorded. The recorded data is
then plotted in intensity, as a ratio of received signal compared
to the strength of the test signal, or in units of decibels (dB)
down from the maximum main beam amplitude, and such a map is
called an Antenna Signal Response Map or more commonly an Antenna
Lobe Map. It is the main beamwidth on a lobe
map that determines how much signal enters the throat
of a dish. The lobes on each side of the main lobe, called sidelobes,
are how off-axis noise enters a dish; and, of course, direct
line of sight TI comes straight in through the main beam. Directional
noise, coming into a dish in a side-ways fashion (see diagram
above), can be controlled by using a dish with smaller side
lobes, i.e. the narrower a side lobe, in width, the less opening
does noise have to enter a dish. It is important to look at both
the main lobe and side lobe maps when considering dish selection.
Looking at a lobe map, you can see a broad beam brings in more
signal (because it is more 'open') which means the higher gain
a dish has the broader its main beam is as well as the broader
its side lobes are. In summary, shallower dishes have broader
beams than deeper dishes, shallower dishes have slightly more
gain than deeper dishes, and shallower dishes let more noise
enter into the system than do deeper dishes. It is a fact of
life and physics that dishes have multiple lobe patterns so consider
them when choosing your dish. Also, it is important to note that
the higher the frequency of reception, i.e. Ku-band over C-band,
the narrow the beam width (this is why Ku satellites are more
difficult to track than are C-band satellites). The beamwidth
of a Ku pattern will be one-third that of C-band because Ku frequency
is three times that of C-band frequency; beamwidth decreases
as signal frequency increases. Beamwidth also decreases as the
size of the dish increases which is why larger dishes are more difficult to track than are smaller dishes; the
side lobes of a larger dish are smaller in comparison to its
main lobe than they are to the main lobe of a smaller dish, so
a larger dish receives less per cent noise per signal as compared
to a smaller dish (see adjacent diagram on dish diameter vs.
noise). In general, the more narrow the main beamwidth the more
narrow the 'window' satellite signals have to enter the dish.
Remember, although a deeper dish will provide less gain to
received signal than a shallower dish of the same diameter, the
side lobes of a deep dish are suppressed and more narrow, and
it is through side lobes that much reflected TI and unwanted
noise (such as thermal noise) enters
a dish, therefore a deeper dish also receives less background
noise than a shallower dish. And if all else fails, get a larger
diameter deeper dish
(top of page)
AVOIDING TI: Avoidance
is the best method to deal with TI. Positioning the dish using
natural shields to block incoming TI is an excellent method of
combating TI. In general, the higher you place a dish the more
chance there will be of encounters of noise - noise of all types;
and the more you shield a dish, i.e. install next to structures,
the greater the chance you will be protected from noise (exception
being metal structures that reflect heat and structures that
absorb heat then radiate it out at night - concrete; because
heat begats thermal noise). I always like to install the dish
up against something - a 
fence, grove of trees (shrubs), wall, etc - to avoid
back plane iincursion of noise into the dish. Anytime you can
put a tree or wooden object (fence) between the TI source and
the dish you will block TI. Microwave transmissions will not
pass through wood products, trees, etc. (Remember, microwaves
will pass through a tree if it looses it leaves and trees will
grow.) Positioning the dish 
behind
houses, buildings, etc., will also block TI however note that
such structures will also reflect TI and could be the source
themselves.
(top of page)
UNAVOIDABLE TI: Unavoidable
TI can often by countered by applying internal notch filters
(in the satellite receiver, see diagram
above) and/or by offseting the center reception frequency
of the affected channel, especially on C-band, with minimal resulting
picture degradation. This is possible because the FCC (Federal
Communications Commission, USA) has allocated telephonic C-band
carrier center frequencies to be 10MHz above and below that allocated
for satellite video useage. Broader band single TI carriers are
more difficult to notch completely but you will be amazed at
what a quality receiver can do with TI which is why I had rather
have a used Chaparral receiver than anything new. Chaparral's
internal filters are (were) the best. (NOTE: Wideband (videoconferencing,
data, and digital carriers) TI carriers usually cannot be filtered
with internal satellite receiver filters alone though you can
have some success using such filters in combination with a per
channel frequency tuneable capable receiver where you can offset
the satellite tuning range then apply filters.) For in-band TI,
not on top of the desired signal, internal receiver bandpass
filters are very effective in cleaning up the picture (see diagram above). For much more information
on custom TI rejection and filter units to be used in 'hardcore'
unavoidable TI, follow this link, Microwave Filter Company. I
have never used them other than for information purposes but
they have been in business the length of time of the home satellite
industry and their webpage shows a good effort although the TVRO
section is currently under construction.
(top of page)
SUMMARY: The
primary condition in any satellite system, of course, is receiving
sufficient signal to be above the noise floor. Despite meeting
that criteria, if terrestrial interference finds its way into
the system it can negate all the best efforts of installing quality
equipment. In-band TI, coming into the system through the LNB,
can appear directly on top of the satellite signal and also can
be adjacent to the desired signal. Out-of-band TI can invade
the system through open connections and poor grounding and can
contribute to the noise floor and/or modulate itself on the back
of the received satellite or distribution signal. In-band TI
is best dealt with through appropriate choice of dish installation
site. If it is in the system, it.can be effectively filtered
using the filters internal to a quality satellite receiver in
cases where it is an isolated narrowband carrier; otherwise you
can offset the center frequency of the affected satellite channel
and then apply filters. In a worst case scenario, expensive filters,
tuned to the exact frequency of the offending TI signal, will
need to be placed at the dish. For out-of-band TI, recheck all
system connections and be sure all open connector ends are terminated
and be sure all equipment is meant to be matched with each other
as stated by equipment manufacturers.
(top of page)
MISCELLANEOUS:
SOLAR OUTAGE (SUN TRANSIT, SOLAR INTERFERENCE): The
ultimate
noise, solar outage. This is one of the items no one ever pays
any attention to except satellite service providers and owners
of commercial installations and then when it happens to you,
as a homeowner, and your TV image goes from excellent to sparkly
to all snow you get a quick lesson in what it is all about; it
is a phenomenon unique to the satellite world.
Twice a year, in the spring and the fall, coincident with the
spring and fall equinoxes (March 20/21 and September 20/21, i.e.
when the seasons change), the sun passes across the equator where
it crosses (passes) directly behind each satellite in the orbital
belt. Remembering that the directional beam of a ground station,
i.e. satellite dish, is always set to face a geostationary satellite,
on the day when the sun is directly behind an orbiting satellite
it will cast a perfect shadow of the dish feed assembly into
the very center of the dish, i.e.when the main beam of an earth
station receiving antenna is in direct line of sight with the
sun it is when sun outage occurs. As the sun passes the antenna
beam's field of view, the ground station's receiver picks up
the sun interference (as transmitted through the feed assembly)
and that causes a drastic deterioration of the receive C/N (carrier
to noise ratio); in other words, it is all noise and no carrier.
We also use S/N (signal to noise ratio) equivalently with C/N.
Basically, the satellite signal is overwhelmed by the unwanted
signal from the sun; the signal from the sun is what we call
noise. This phenomenon behaves as if the noise temperature rises
in the ground station's receiver.
At this time, until the sun moves, it will cause about ten minutes
loss of signal from that satellite. On the days before and after
when the sun is directly aligned with that satellite there will
be less outage - as it approaches alignment with a satellite,
each day there is more outage than the preceeding; and as it
moves away from alignment with a satellite, each day there is
less outage. Of course, the actual days and times when your dish
will be affected depends on your latitude and longitude, longitudinal
position of the satellite, and diameter of the dish. For the
March equinox, solar outage begins in February, affecting the
northernmost latitudes, moving to affect the equator on the day
of the equinox, and ends in April, affecting the southernmost
latitudes. For the September equinox, the travel pattern of the
sun is reversed so the pattern of solar outage begins in the
southernmost latitudes, in August, and ends at the northernmost
latitudes in October. Note that sun outage effects are vice versa
between the southern and northern hemispheres. For cases where
a ground station is located west of the affected satellite, sun
interference occurs in the morning; and for a ground station
east of the affected satellite, sun interference is in the afternoon.
If you have a bright, shiny (or light colored), solid satellite
dish, then the sun is 'cooking' your dish electronics so during
periods of solar outage it is best to move the dish to another
location. Mesh antennas rarely have a problem with overheated
feed electronics during periods of solar outages. The actual
mechanics of solar outage are that the solar flux, which is radiated
in the 4 and 12 GHz frequency bands, introduces additional noise
into the antenna, forcing system noise temperature to rise. This
occurs because the satellite's weak coherent microwave streams
become overwhelmed in the microwave noise from the sun. In turn,
this noise causes receivers to operate near or below their threshold.
Solar outage affects only the downlink, not uplinks.
How adverse will be sun outages can be determined with diameter
of an antenna, signal margin of the ground station, noise temperature
of the receiver and noise inherent to the total communication
system. But beyond the esoteric calculations, we know that diameter
of a receive dish is related to quality of received signal; specifically,
dish gain increases proportionally to diameter of the dish so
the intensity of the effects of sun outages increases (is greater)
in larger dishes.
However, the larger diameter a receive antenna is, the shorter
time and the fewer days sun interferences last. We prove this
by the following formula: 'B = 0.5 + Bo' where 'B' is 
the
angle of degrees at which the sun crosses the half power beamwidth
of a receive antenna and where the half power beamwidth of the
antenna is 'Bo'. (Note the apparent diameter of the sun as seen
from the Earth is about '0.5' degree.) The sun rotates one degree
in 4 minutes therefore the longest time length the sun moves
by the receive antenna's half power beamwidth is calculated as
follows: 'B
degrees x 4 minutes/degree';
and noting the sun's declination changes 0.4 degree a
day, the maximum number of days the sun stays within the receive
antenna's half power beamwidth is: 'B degrees / 0.4 degrees/days'.
A value of the receive antenna's half power beamwidth, 'Bo',
for typical parabolic antennas, where wave length of receive
frequency is 'L' and diameter of antenna is 'D', is predicted
as follows: 'Bo = 70 x L / D' so that 'Bo' increases with smaller
dish diameters. (If you have ever tuned a large diameter dish
then you know that it is more difficult to tune than a smaller
diameter dish and that is because it is more focused in its center
beam, i.e. its half power beamwidth is smaller.) Therefore, the
larger diameter a receive antenna is, the shorter time and the
fewer days sun interferences last.
SOLAR
OUTAGE and REFERENCES:
Go to Size Main
page - Solar Outage - to see detailed links to specific
satellite operators' web sites for their solar outage charts
and tables and information sets.
(top of page)