Radio Kansas Rebuilds for its RF Future
Space combining was one option. This would have required a new antenna and considerable tower strengthening. The system at KHCC, for example, included an ERI SHP-12AC rototiller antenna. One approach to space combining would replace the existing rototiller with a new dual-input antenna. That was far too heavy for the cantilevered lambda section at the top of the tower, which is about 135' long and designed specifically for the 12-bay SHP.
Dual-mode antennas are far heavier than standard FM antennas with much higher wind loads. The cantilever section of the tower, added two years prior, would require replacement - an expensive and inefficient endeavor. For example, the existing 12-bay antenna weighed 2,340 pounds with a wind load of 85 square feet; an ERI Lynx Series II dual-input antenna weighs 5,122 pounds with a wind load of about 217 square feet. (Both measurements estimate 1/2" of ice on top.)
Another space combining option was a second antenna at a lower tower position. This would have also required significant tower strengthening and additional costs to purchase and install the antenna and feedline - far exceeding the price of a new transmitter. The radiation patterns of the two antennas, at different places on the tower, would also not have matched due to differing tower cross-sectional dimensions (4' lambda versus 7'). This would result in inconsistent analog/digital ratios and cause reception problems for listeners.
A third option would have added a Myat combiner to each system. This approach was about as expensive as a new transmitter, but the bigger concern was that the combiner would require its own climate-controlled room at each site. This meant expanding the transmitter buildings, as the combiner was about 4.5' tall, 5' wide and 9' long.
Beyond all this, the existing transmitter could not accommodate the increased power levels with the current combining configuration. At KHCC, to use the legacy FM transmitter with the existing Harris Z16HD+ transmitter, the Z would need to provide 2kW of analog and 6kW of digital with the current 4.7dB combiner to make -10dBc. This is beyond its capacity.
With these options exhausted, a decision was made to re-engineer the split-level combining system and purchase HPX models to operate with the existing Z-Series models - a unique variation on the split-level method that essentially uses the same theory and connection patterns. The objective was to have one transmitter operate in the highly efficient Class C mode. The resulting architecture assigned the Z-Series models to this FM-only duty and assigned the new HPX transmitters to FM+HD in the Class AB mode.
- continued on page 4
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