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FM
broadcast antennas - common misconceptions
By Lee Granlund
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of the most significant factors determining the success or failure
of any FM broadcast station is the quality and coverage of its
signal. A station that does not provide a clean, interference-free
signal throughout its market area cannot expect high numbers
in audience share or revenue, regardless of programming. Many
FM stations have limitations regarding available transmitter
site or power; however, an even greater number of stations suffer
from inadequate coverage due to poor antenna performance. The
result of poor antenna performance is simple, but the cause
is often complex and subject to many common misconceptions: |
1.
All FM broadcast antennas are pretty much alike. Although
they may look alike and have similar performance specifications,
various makes and models may behave in a very different way
in any specific environment, due to subtle design differences.
2. A competent engineer can select the best antenna
for my station by comparing published specifications.
In almost every case, the tower and other factors affect the
antenna pattern and performance as much as the antenna itself.
Overall results of any specific combination of antenna, tower,
and site environment can be determined only by actual measurement
or by an accurate computer model of the entire installation.
3. Replacing an older antenna with a newer model will
correct most signal quality and coverage problems. It
is possible; however, this may only replace old problems with
newer ones. To be safe, the replacement antenna should be selected
and its installation supervised by a well qualified broadcast
engineer, preferably using Site Specific Engineering.
4. If the antenna manufacturer provides measured patterns
of his antenna on a tower similar to mine, I am assured of good
results. Many manufacturers provide "generic pattern
data" taken from measurements using a single bay on typical
tower faces. Only the azimuth pattern is provided, and the elevation
pattern of a multi-bay antenna on your tower is not considered
or tested. At your station, most of the signal may go down toward
the ground or up to the sky rather than reaching your market
area or your audience.
5. The coverage area of any FM station depends only
on the ERP (Power) and HAAT (Height) of its antenna, per the
FCC license. This is true ONLY if all of the authorized
power reaches the intended market area. Experience shows that
only a small portion of the actual radiated power can reach
the audience using "conventional" antenna designs
and construction. Often, due to multiple lobes and nulls, part
of this energy takes a different route to the audience, causing
multipath interference to the main signal.
6. Best coverage for an FM station can be achieved by
locating the antenna at an existing multiple site or "antenna
farm". Such common sites and "Antenna Farms"
are typically located at a place that allows good coverage of
the entire market area, although there may be reasons more political
than technical for determining the specific location. The primary
disadvantage of a common site is the close proximity of other
antennas that may affect the pattern and coverage of your antenna.
It is very important to use "Site Specific Engineering"
for your station, including a detailed computer model of the
site and all nearby towers and antennas. With this in mind,
good coverage can be obtained for all stations.
7. At least one manufacturer claims that his latest
antenna design may double the coverage area and/or population
served by competing antennas used by most FM stations.
How can this be possible? This huge improvement has been documented
for the SWR ILLUMITRON, using proprietary technology that eliminates
sidelobes and puts over 99 percent of the station's signal on
the horizon, toward the market area and the audience. While
the ILLUMITRON produces better signal quality and coverage in
virtually every case, the actual amount of improvement depends
on the performance of the previous antenna.
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Systems With Reliability, Inc.
619 Industrial Park Road
Ebensburg, PA 15931 USA |
toll free: 800-762-7743
tel: 814-472-5436
fax: 814-472-5552 |
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