Field Report: SWR Illumitron
Mar 1, 2003 12:00 PM, by Dale Harry, CPBE
KKXS-FM, Shingletown, CA, suffered from a problem shared by many small FM stations in rural America: the population in this northern California market is spread over a large area, and rough terrain presents serious obstacles to a Class A FM station. Shingletown is too small to support a station profitably, and most of the market�s listeners live in Redding (27 miles away) or Red Bluff (47 miles away). For KKXS to compete with several Class C stations in the market, something had to be done to improve the KKXS signal. The station needed substantial improvement at an affordable price. It was not practical to increase power or to move closer to Redding.
When I discussed this problem with other broadcast engineers, I learned of an FM antenna that is making a reputation of improving coverage for small FM stations. I was skeptical at first, until I talked to engineers and station owners who are now using single-lobe or reduced-sidelobe antennas. Their reports were encouraging.
I was referred to SWR, which sent literature on the Illumitron FM antenna, and also referred me to a local engineer who could look at the coverage problems and evaluate how much improvement was possible with the Illumitron antenna. The consulting engineer�s recommendation was instrumental in the station�s decision to purchase a four-bay Illumitron, which was installed in January 2002. Much of the following month was devoted to checking the performance of the new antenna and comparing field intensity measurements.
Performance at a glance
- Reduced side lobe and downward radiation
- Reduced multipath effects throughout coverage area
- Better field intensity variation specifications
- Improved coverage area performance
- Exceptional cost-per-performance ratio
While the signal is still weak compared to the Class C stations, there is a significant improvement in coverage. Many shadows and holes in the service area are gone. The signal is noticeably improved in buildings and behind obstructions.
Improving signal strength
Perhaps more significant, multipath and picket fence effects are much reduced. One listener said that �the station is more fun to listen to� with less noise and interference. Field intensity readings showed an average increase of almost 3dB with the new antenna, but the consulting engineer told me that this difference is most likely due to loss of signal from the old antenna, because of the way it was mounted on a tapered tower. The average field intensity is related to distance and effective radiated power (ERP), rather than the type of antenna in use.
The major difference in signal strength (or field intensity) for the Illumitron antenna is a remarkable stability. While most FM signals vary �10dB or more over short distances, this antenna�s signal remains almost constant. Variations of 1dB or 2dB are typical, except near overhead wires and other reflecting objects. For KKXS, the minimum signal strength was almost 5dB higher, and maximum signal strength was only 1.2dB higher. Receivers only care about minimum signal strength, and this explains why the Illumitron improves coverage and signal quality. The station had found subtle, but significant signal improvement.
KKXS� antenna has not been in use long enough to have an effect on ratings or revenue, but the station is encouraged by what it has heard from others using a similar antenna.
We are still a Class A FM station, but we don�t feel quite so small anymore.
Harry is proprietor of Sierra Broadcast Service, providing engineering services to Northern California-area broadcast stations.
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