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).


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.

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.)
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.


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.


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.
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.


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

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, t

he 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.
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.
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.


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.