Signal expansion is on the mind of many FM broadcasters. For some, boosters are one way to achieve this goal.
FM boosters are similar to translators but rebroadcast the signal on the original frequency. They usually are used to fill in areas within the contour of a station that are blocked by terrain.
Stanley Salek, senior engineer at Hammett & Edison Inc., of San Francisco, will speak on this topic at the NAB Radio Show in Austin. He also delivered papers to NAB audiences about this topic in 1992 and 1996.
Boosters began to appear in the early 1980s but they were limited in power and had to be fed with an off-air signal.
In 1987, the FCC changed its translator rules, allowing higher power and alternative feed methods. In the early 1990s, TFT introduced the Reciter, providing synchronization and time delay adjustment using a combination of analog and digital technology.
As digital techniques evolved, GPS time synchronization was adopted to improve these functions in later booster system products.
System designers need to plan and execute a booster installation properly for it to work well.
“One of the most common problems is not having adequate terrain shielding,” Salek said. “If you have line-of-sight from the transmitting tower to both the primary coverage area and the area you want to fill in, booster technology may not be the best choice.”
(click thumbnail)Representative combined coverage of a San Francisco Class B FM station and booster, part of the multi-station booster system at Mt. Diablo. Shown are FCC contours and projected 54 dBu TIREM terrain- sensitive coverage for combined main and booster stations. The booster typically operates at 185 watts ERP and uses a custom multi-element transmitting antenna. Note the break in coverage over mountainous terrain separating booster communities Concord, Walnut Creek and Pleasanton from the rest of the Bay Area to the west. Courtesy Stan Salek
Another mistake, according to Salek, is use of a single-polarization antenna on the booster.
“This is usually done as an economy measure, but if your main antenna is circularly polarized, so should your booster antenna.”
It is also vital to determine the proper feed and synchronization scheme and implement it in a stable way.
“The main and booster signals cannot be synchronized in all locations, and there will always be a zone of self-interference,” adds Salek. “The trick is to place those zones away from populous areas.”
In the desired coverage area, there should be a seamless transition so that the main signal trails off as the booster signal increases proportionally.
Salek said geography is key in determining whether a booster will be successful.
“In the West and in parts of New England, there are mountains that clearly shadow areas within a station’s contour. In the Middle Atlantic states, there tend to be more rolling hills, and these coverage gaps are not as well defined.”
Not surprisingly, there are more boosters in the western states. Although the terrain is suitable for boosters in parts of New England, the technology does not appear to be as widely adopted there.
The question of IBOC boosters is being raised more often these days.
Salek notes this is a developing area, with a number of opportunities for research. The concept of single-frequency networks is also being developed.
“Digital techniques offer definite advantages over analog in terms of synchronization,” he said.
Salek added that many signal problems are more solvable with digital transmission techniques than analog.
“The time interval between frames of a digital signal allows for multipath before it interferes with symbols of digital signals. That might be exploited in implementing digital signals over a wider range than analog.”
A good rule of thumb is that the ratio of the main transmitter signal to the booster signal should be at least 10 dB at locations greater than 14 miles from the booster and in the direction of the main antenna.
He adds that booster coverage or multiple booster use can be tweaked by the use of directional antennas to protect the main signal.
“Booster coverage in the direction away from the main antenna can be as great as desired, provided the signal conforms to the FCC coverage allocation for the main signal.”
As digital synchronization techniques are integrated into booster technology, are analog boosters a thing of the past? Not necessarily.
“System designers with relatively simple, straightforward installations can still use analog boosters, which often offer a significant cost savings over digital.”
Once a booster is installed, its performance should be validated. Salek adds that the best means to do this is with a mobile spectrum measurement system. This usually consists of a spectrum analyzer, GPS receiver, DSP module and associated antennas.
“It is important to verify the measured synchronization point with the calculated point to ensure best performance in the transition area.”
To do this, separate measurements of main and booster transmitting facilities are made, one at a time, along identical measurement routes. Calculated data is then synchronized for position so that facilities can be compared and D/U (desired-to-undesired) ratios can be determined.
An installation where the technology has been successful is Mt. Diablo in the East Bay portion of the San Francisco Bay Area.
Briones Ridge and other area terrain isolate these East Bay communities from direct reception of Bay Area stations that have their transmitter sites located close to San Francisco. Boosters on Mt. Diablo, one of which combines nine stations on a single transmitting antenna system, are able to rebroadcast signals with minimized self-interference.
Salek’s session at the NAB Radio Show is “FM Boosters: Opportunities and Challenges” on Wednesday Sept. 17.