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National Public Radio
Distribution Division
Satellite Facilities
Revised
March 1998
©1998 National Public Radio Redistributed by
Permission.
In 1986, primarily at the behest of commercial video entities, the FCC
adopted rulemaking which permitted two-degree spacing between satellites.
At the same time, it placed the burden of interference avoidance in the
2 degree environment on the licensee or registrant operating a receive-only
earth terminal. The FCC's terminology states that "... the actual
level of any protection desired by an applicant from inter-satellite interference
for small receiving earth station antennas will be achieved by the choice
of receiving antenna performance selected by parties installing new receive-only
earth stations." FCC 86-133 36552
To
illustrate, prior to 1983, public radio's satellite at 99 degrees West
longitude enjoyed six degree spacing from its nearest neighbor. In
1983, Telstar 301 was placed at three degree spacing at 96 degrees West
Longitude (WL). Today, the final implementation of the 2-degree rulemaking
has been completed. We have had two-degree neighbors in Spacenet
4 at 101 degrees WL beginning in 1991 and Telstar 401 (now Telstar 5) at
97 degrees WL in 1994. 1996 saw the addition of the four-degree spaced
Galaxy 3R at 95 degrees WL and the GE-1 satellite at 103 degrees WL, completing
the two-degree-ization of this portion of the satellite arc.
At the same time that the spacing between satellites has decreased, the power level differences between fully saturated video carriers and the SCPC carriers used for radio broadcasting have increased. Analog video transmissions are commonly run at saturated transponder power levels. The amplifiers used for the transponders on Westar I in 1979 were rated at 5 Watts output; today's Telstar 5 and GE-1 transponders, for instance, are rated at 20 Watts output; an increase of 6 dB.
To obtain the greatest benefit from the limited bandwidth available, each satellite utilizes cross-polarized transponders which allows frequency reuse. This technique permits the utilization of 1000 MHz of bandwidth, even though only 500 MHz is allocated per satellite. This mode of operation, however, requires users to carefully polarize their receive antennas to null out the unwanted co-frequency energy. Operating guidelines of the satellite operators require uplinks to maintain cross-polarization isolation of at least 30 dB.
Each satellite is assigned a frequency plan that is cross-polarized from its two-degree spaced neighbors. For instance, transponder 3 on Galaxy 4 is horizontally polarized for downlink, while transponder 3 (using the same downlink center frequency) on two-degree-spaced Telstar 5 and Spacenet 4 is vertically polarized. Four-degree spaced satellites, however, have co-polarized frequency plans; transponder 3 on both Galaxy 3R (at 95 degrees) and GE-1 (at 103 degrees) are horizontally polarized, which is the same as transponder 3 on Galaxy 4.
A concise chronology of events affecting inter- and intra-satellite
interference to satellites at 99 degrees WL follows:
| 1979 | Audio distribution begins on single-polarity Westar I. |
| 1981 | Operations transition to dual-polarity Westar IV which replaces Westar I. |
| 1983 | Telstar 301 begins operation at 96 degrees WL beginning a "3 degree spacing" environment. |
| 1991 | Operations transition to Galaxy 6 which replaces Westar IV.
Spacenet 4 begins operations at 101 degrees WL. beginning a "2 degree spacing" environment. |
| 1993 | Operations transition to Galaxy 4 which replaces Galaxy 6. |
| 1994 | Telstar 401 begins operations at 97 degrees WL replacing Telstar 301, completing the "2 degree spacing" environment. This results in the loss of usable bandwidth due to excess adjacent satellite interference. |
| 1996 | Feb - Galaxy 3R is placed in service at 95 degrees WL, adding a copolarized
"4 degree spacing" dimension to the interference scenario.
Oct - GE-1 is placed in service at 103 degrees WL, providing additional copolarized energy from 4 degree spacing. Nov - Hughes initiates video operations on Galaxy 4, transponder 2, adding potential cross-polarized interference to earth terminals. |
| 1997 | Telstar 401 fails in January and is replaced by Telstar 5 in July at 97 degrees WL. |
We've witnessed the changing of a once quiet satellite locale into a
raucous, noisy, interference-filled environment. Much like the difficulty
some of us may have in following conversations at a noisy party, our antennas
are being asked to perform a similar task in the new crowded-sky conditions.
Some antennas are not up to that task. As mentioned above, compromises
made in the three basic criteria used for antenna selection; gain, sidelobe
rejection, and durability, come back to haunt us.
The design goal for carrier power levels utilizing ComStream digital equipment is a minimum operating energy-per-bit above noise (Eb/No) of 8.0 dB at the worst downlink in the worst part of the footprint. This provides a minimum operating margin of 4 dB above the Eb/No of 4.0 dB at which the audio encoding falls apart. Other manufacturer’s equipment may have different minimum Eb/No requirements.
Using antenna gain as the sole selection criterion, we can predict performance
in a "going downhill with a tailwind" environment. In the real world
though, we're dealing not only with carrier to noise ratios, we're dealing
with carrier to noise-plus-interference; and the interference may be considerably
greater than the noise. This is where the sidelobe performance of
the antenna becomes so important in discriminating against the undesired
signals from adjacent satellites.
As
an example, the advertised on-axis gain of a 3.8 Meter (12 foot)
antenna used by many stations is 42.9 dBi. Satellites with two-degree
spacing (which is measured from the center of the earth) appear to be about
2.2 degrees apart to a person standing on the surface of the earth at 40
degrees latitude. The gain of an antenna meeting the FCC's "2 degree
envelope" 2.2 degrees off-axis is 20.4 dBi, which is 20.5 dB less than
the advertised on-axis gain of this particular antenna. At 4.4 degrees
off-axis, the "2 degree envelope" gain is 12.9 dBi, which is 30 dB less
than the on-axis gain.
The saturated downlink EIRP of a transponder on Telstar 5 (at two degrees orbital separation) as measured in Washington, DC, for example, is about 39 dBW. As a paper exercise only, and assuming that this antenna follows the 2-degree gain envelope exactly, the power received from 2.2 degrees off-axis would be the equivalent of 16.5 dBW (39-20.5). The downlink EIRP of a nominal 16 dBW channel received on-axis by this antenna from Galaxy 4 is about 18 dBW. On paper, there's nearly equal energy being received by this antenna from both off- and on-axis. In the case of satellites spaced four degrees from the desired satellite, the off-axis energy received would be the equivalent of 9 dBW (39-30). Any additional losses to the on-axis performance due to lower power SCPC carriers, the use of smaller antennas with lower gain, mis-pointing, mis-polarization, or the antenna not meeting its advertised specifications for one reason or another will only worsen the discrepancy.
Transponders 1 through 4 on the 2-degree spaced Telstar 5 are presently
utilized for digital video service, which is characterized by relatively
low spectral density evenly spread over a wide bandwidth. Transponders
1 through 4 on the 4-degree spaced Galaxy 3R are utilized for encrypted
analog video, which results in very high spectral energy near the transponder
center frequency. The ingress of high powered analog video signals
into downlink antennas has created a hostile environment for SCPC service
due to the inability of these antennas to discriminate against the unwanted
energy. One conclusion that can be drawn is that any "protection"
afforded by "2 degree compliant" antennas is worthless. Fortunately,
most antennas in good condition display better performance than the examples
above.
Improper
antenna polarization is the most common problem from which downlink antennas
suffer. While a certain degree of difficulty is created by many stations
not having a spectrum analyzer readily available, more sites are mis-polarized
because of a misunderstanding of the principles of antenna polarization.
Successful antenna polarization is not achieved by peaking the received level of the desired transponder’s signals, but rather by nulling the undesired signals from the cross-polarized transponders. As the graph demonstrates, when peaking for maximum signal strength, a fair amount of rotation of the LNA or LNB is required to achieve a relatively small increase in the desired signal level, while a very small amount of rotation is required to achieve a very sharp null of the undesired signal.
Due to the spacecraft polarization frequency plans, the essentially co-polar interference from satellites two degrees adjacent, Telstar 5 and Spacenet 4, is centered at the edges of the transponders at IF frequencies near 50 and 90 MHZ. Since there are satellites on both sides of Galaxy 4, their energy combines. Because the polarization frequency plans for satellites separated by four degrees are the same, the combined energy received from Galaxy 3R and GE-1 will be centered near 70 MHZ IF, right in the middle of the transponders.
Adjusting polarity of a receive antenna to null out the signals from
one of the satellites at two or four degree spacing on one side of Galaxy
4 can create a worse cross-pole interference situation with signals from
the satellites on the other side. With cross polarized video on transponder
2 of Galaxy 4, optimizing for the best cross-pole isolation on Galaxy 4
itself is highly recommended.
As observed at on-site visits, the ingress of inter- and intra-satellite interference has been obvious, and has been the immediate cause of the impaired service. Repointing and polarization has improved the performance, but not cured the problems with these antennas. Visual observation and measurement of the reflectors revealed that they were warped, squinting and/or out-of-round. Movement of the edges of the reflector in some cases increased the gain, even though the pointing and polarization had been previously optimized. In short, it appears as though a "relaxation" of the antenna reflector's original shape has occurred. As the reflector's focus changes, the antenna's on-axis gain and sidelobe rejection both decrease as its directivity deteriorates.
This relaxation of the reflector shape appears to be more a function of the number of years in service than anything else. Antennas that have been protected from wind and blown debris seem to do better than those which are totally exposed to the elements, such as those in a roof-mount situation. We have also received reports of fiberglass antennas that display delamination and chipping of the reflective surface. Fiberglass technology has improved significantly since the early 80s when many of these antennas were installed. There are several different methods used in the manufacture of reflectors relating to the internal framing and bracing. Clearly some manufacturers do a better job than others.
Light metal and mesh dishes seem to be particularly prone to "dent tuning", displaying large and small dimples (both convex and concave) that seem to be caused by weather (hail and heavy snow loads) and human contact. Some of the lighter weight mesh reflectors are quite malleable, and appear to have lawn furniture origins.
If you have doubts about your reflector, eye-balling is the easiest
check of its condition. Sight along the plane of the reflector lip
closest to you and make sure the plane of the opposite side is parallel
to it. Perform this check from at least two different angles.
Another check which requires a bit more work, but gives a better indication
of the reflector’s shape, is to stretch two strings across diameters of
the reflector perpendicular to each other. The strings should just
touch each other at the point where they cross. If there’s more than
about an inch of space between them, your reflector may be beginning to
sag.
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The "problem" antennas that have been brought to our attention have been in service for ten or more years and have been installed in environments that expose the antennas to the full force of the weather. While there's a lot of manufacturer's hype implying that their antennas will last forever, experience is proving this wrong.
Ten years (or so) may, after all, be a reasonable lifetime for this type of antenna. As for cost-effectiveness, a station would still be money ahead if it bought two $3000 antennas over a twenty year period rather than one $12,000 "battleship." Station's priorities regarding engineering practices, quality concerns, the nuisance factor of changing antennas periodically, possible facilities moves, and perhaps most importantly, the station's budget must be considered. Tradeoffs may be required in making each station's decision.
Experience has proven that either retiring or attributing a finite "lifetime" to a piece of equipment in radio is difficult at best. First things have traditionally come first, and the available capital many times is required to put out the hottest (shortest fuse) fire. Capital funds are hard to come by, and there's plenty of other equipment in a station that needs to be replaced and/or upgraded. However, by taking a long range view, a $3000 antenna that's used for ten years costs a station about a buck and a quarter a day, which seems to be a reasonable investment.
The bottom line question is: "how important to your station’s sound is the quality of the program material received and rebroadcast from the satellite?" If your antenna’s performance cannot be optimized through pointing and polarization to provide that quality, it may be time to think about replacing it. If it must be replaced, don't shortchange yourself by specifying an antenna solely on size or financial criteria. Pick an antenna that will give you the best adjacent satellite isolation and durability for the dollar.
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