Better FM Coverage Can Be Yours
by Richard J. Fry
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.
|
