Better FM Coverage Can Be Yours
Consultants Share Ideas to Help You Improve Your
Station's Performance
A station's antenna and its siting can be critical
to its commercial success. Incorrect choices are difficult to evaluate
and expensive to fix.
Without the professional analysis of an experienced
consulting engineer, some managers may have a tendency to choose
and install FM antennas with a certain amount of misplaced belief,
often based on "street lore" and emotion.
To provide some insight into this subject, three
prominent broadcast consulting firms were asked to respond to 15
topics about FM antenna performance. The first of the consultants'
responses are shown below, and the rest will follow in future issues.
The participants are Ben Dawson of Hatfield &
Dawson; Bob Culver of Lohnes & Culver; and Don Markley of D.L.
Markley & Associates.
Note that any consultant would need specific site
and application information to provide an accurate recommendation
for a given situation.
1. Number of antenna bays vs. antenna height AGL
vs. ERP vs. terrain profiles.
Ben Dawson: The proper selection of high-gain
antenna configurations, such as those for UHF TV with typical gains
of 30 or more, requires careful evaluation of the terrain so that
the combination of gain, beam tilt and horizontal plane pattern
provides more or less uniform illumination of the desired service
area.
This is less troublesome at VHF frequencies, where
the antenna gain typically is lower, but it still should be the
basic determining factor in most situations.
Bob Culver: The antenna vertical pattern,
being controlled by the antenna aperture length (hence number of
bays and spacing) is the issue here. Obviously this is a tradeoff
relative to antenna gain, transmitter power output (TPO), necessary
line size (power rating) therefore loss, station ERP, antenna height,
line length (loss), etc.
There are a multitude of interrelated variables that
have to be considered to arrive at the desired end results. Some
variables will be primary in that there is little room for change.
For example a very low antenna height may dictate
1/2-wavelength bay spacing and perhaps a short antenna aperture.
On the other hand, a remote location may supply a limited prime
power capacity and therefore a low transmitter power, requiring
a relatively high antenna gain and low loss system.
Don Markley: For best service, we usually
recommend the maximum power and height for the class of station
with a reasonable number of bays of antenna. Like many FM criteria,
this is a judgment call.
For example, you don't normally go after 100
kW ERP with a two-bay antenna. On the other side, avoid 12 bays
if you can. We usually recommend three or four bays for Class A,
B1 or C3, five or six bays for Class B or C2 and eight bays for
Class C0, C1 or C.
This also requires some judgment regarding the terrain
in the area. In very hilly or mountainous terrain, use fewer bays
and more transmitter power. High-gain antennas seem to work best
for level terrain such as on the prairies out west.
2. Vertical bay spacing (effect on non-ionizing
radiation on the ground near the site and other pattern parameters).
Dawson: Reduced spacing (typically 0.95 lambda
or lower, down to 0.5 lambda) provides substantial benefits in reduction
of nearby NIER levels, reduced potential for site electromagnetic
incompatibility and reduced foreground reflection.
Culver: I have no problem with using reduced-bay
spacing to achieve reduced ground-level RF energy exposure level.
Using 1/2-wavelength spacing is physically easy with some antennas
where a 180-degree phase shift can be arranged by inverting the
antenna elements and will then work with a 180-degree inter-bay
line length.
Reduced spacing, but not 1/2-wavelength will reduce
downward and side lobe signals. This can be used where less spacing
is not physically possible (panel antennas) or electrically desirable.
The resulting vertical pattern is determined by the
full antenna assembly. Some individual elements are inherently suppressed
in their downward signal and the array can be more easily built
to limit downward signal.
Markley: We normally just use one-wavelength
spacing unless there is a problem with close-in radiation.
Richard Fry is a retired FM applications engineer
with almost 35 years of service with major U.S. broadcast transmitter
and antenna manufacturers. He can be reached via e-mail to rfry@adams.net.
|
