‘Raise Power or Raise the Antenna?’
|The tool uses Longley-Rice modeling and terrain
mapping data to provide coverage maps of up to 10-meter accuracy in the U.S.
An accurate computer model of a station’s coverage is important, if not
critical, in deciding whether you can make facility changes to help the programming
and sales departments meet their goals.
Perhaps your transmitter site was chosen when
distribution of the local population was different than it is today. Pastures
have evolved into condo complexes; wooded areas have given way to strip malls. Your
audience and revenue sources have shifted while your transmitter location has
remained the same. Should you raise your antenna height? Increase your power?
Move your site?
reason for a good propagation tool is that technologies such as digital
television, HD Radio, RDS and transmitter GUI interfaces have put even more of
a premium on STL bandwidth to and from a transmitter site.
A presentation by Chuck Kelly, director of sales at Nautel Ltd., is
planned for Tuesday during the Broadcast Engineering Conference of the NAB Show.
The title is “A Free Online VHF/UHF Propagation Tool for Technical and
“Our goal in developing this tool was to put
easy-to-use and easy-to-interpret RF coverage and path analysis in the hands of
station personnel for the preliminary evaluation of transmitter sites and tower
height, antenna gain and TPO tradeoffs,” Kelly said.
In addition to selecting an optimum transmitter site, there are
considerations such as antenna height above ground and number of bays vs.
transmitter power. The ability to model these prior to making an investment is
In addition to coverage mapping, the
system can be used to make an STL path analysis including a path profile showing
Fresnel zone clearance and a path fade margin budget.
Further, given the increasing need to transport audio and data to
the transmitter site as well as diminishing availability of analog telephone
lines, STLs are a popular alternative. They offer greater bandwidth, are under
the control of the station and are generally more reliable. The proper
selection of antenna height is critical at both ends of the STL path. Kelly
says the tool allows the user to display a path showing free space and Fresnel
Zone clearances. It also calculates a path budget and fade margin.
The tool employs Longley-Rice ITM terrain method.
“The Longley-Rice ITM is well accepted and covers a wide
frequency range that is ideal for FM/STL/TV use.” One of the most important
advances in terrain data accuracy was the release by NASA and others of Shuttle
Radar Topography Mission data online. This terrain data was gathered during an 11-day
mission aboard the space shuttle Endeavour in February of 2000.
Another valuable feature in the tool is population count that can
be compared against various runs of different transmitter sites and antenna
says the tool — found at rftoolkit.net— is not a substitute
for an experienced engineering consultant, but it can help a station obtain
approximate general answers quickly and at no cost.