Copyright ©1996, RF Specialties Group.
When station
engineers for the Blanton Road FM antenna farm in Eugene, Ore., couldn't decide
which way to go on their antenna designs, the RF Specialties group in Washington
state suggested they consider a Shively pattern study.
For Chris Murray, engineering director for McKenzie River Broadcasting, a pattern study proved his theory that an elliptically polarized horizontal/vertical ratio would yield better results than using a conventional circularly polarized pattern for a 5-bay array upgraded from 3kW to 100kW. By modeling his theory to quarter scale at the Shively test range, he proved the pattern would increase coverage while reducing RF density at the site and substantially reduce multipath.
The final system has a ratio of 3.78 horizontal power gain and 1.62 vertical power gain, giving the station a signal that is circularly polarized enough for good radio antenna reception, yet linear enough to boost the signal level for wider reception. As a result, the station was able to use a 35kW transmitter to make its 100kW ERP.
Other custom patterns on Blanton Road soon followed, including one for a class "C3" station sandwiched between two class C powerhouses that uses Shively antennas with interbay spacing of one-half wavelengths (at the output of the 95.3MHz transmitter, signals from 94.5MHz and 96.1MHz were almost 90dB down!). There are now 11 stations out of 13 in the Eugene-Springfield market that use Shively antennas: four use custom H/V polarization, five use special wavelength spacing and two use both. To get going in the right direction on your antenna patterns, call, fax or e-mail RF Specialties.