Radio Kansas Rebuilds for its RF Future

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KHCC's building includes spare 950MHz antennas and an 80kW diesel generator.

KHCC's building includes spare 950MHz antennas and an 80kW diesel generator.

Overall, the FlexStar and BoostPro are very intelligent in their interactions with the transmitters, able to accommodate complicated, multi-mode switching on the fly and maintain power levels through integrated automatic power control (APC). Harris Real-Time Adaptive Correction technology, in correlation with the exciters, has been instrumental in flattening out sidebands to very low levels and keeping the signals within mask compliance. We experimented with upsetting the transmitter linearity by adjusting plate tuning/loading and adjusting bias and screen voltages. On queue, the system kicked in to smooth out the sidebands and minimize residual distortion products.

Transmitter choice

The choice to go with Harris really started in 2006 upon purchasing the FlexStar system and the Z-Series transmitters. Switching to another manufacturer would have been difficult as two of the three legacy FM transmitters we needed to use were from Harris. Our experience with Harris transmitters and Harris customer support was great, and it was important that this complex system be single-sourced.

So, why the HPX? Frankly, it was a gamble to consider a tube transmitter in the world of HD Radio, but the cost of putting two high-power solid-state transmitters side-by-side to supply the elevated digital signals would have been off the charts.

We also had reservations about whether a solid-state transmitter would be cheaper in the long run. Our final opinion was that a high-power tube might actually be less expensive for total cost of ownership. Harris explained that unheard-of tube linearity was achievable through careful bias and screen voltage selection. Factory tests proved they could achieve our digital injection values while maintaining HD Radio mask compliance.

It was also important to provide enough headroom in the FM+HD transmitter to move to other digital modes. The HPX40 at KHCC, for example, was effective as it had enough headroom to accommodate the extended service modes if and when we move beyond the MP1 service mode.

Reliability of operation was also key. Radio Kansas goes dark from midnight to 5 a.m. each day. Davicom MacPlus remote control systems shut down the transmitters each night at midnight with the filaments turning off 15 minutes later. Each morning, the filaments begin warming up at 4:45, and the transmitter plates kick on about 10 minutes later. This all happens automatically and dependably, even during the coldest winter nights.

Internally, the HPX transmitter layout is clean and simple. The power supply and cavity are accessible and easy to maintain. All controller cards are grouped in the front. The transmitters are sophisticated with multi-mode operation, adding new functions and adjustments beyond the standard Class C FM transmitter. This sophistication required study and experimentation to understand the interaction of the various parameters and cavity linearity.

Other changes were minimal. Davicom and Bird systems cleanly handle the remote control and onsite power readings, respectively, making meter and status readings available on demand. ERI 6dB mBoxM3 combiners connect and mix the two transmitters. The HPX transmitters and ERI combiners were added to existing RF plumbing using non-pressurized elbows, couplers and an additional 20' length of 3-1/8" non-flanged transmission line. No new transmission lines were required for the towers.

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