WGBH Upgrades COMARK PARALLAX VHF DTV Band 1 Transmitter

Public media producer GBH has upgraded its VHF Band 1 PARALLAX liquid cooled DTV transmitter for up to 47.8kW of power.

As part of the FCC repack, COMARK supplied GBH a PARALLAX VHF band 1 liquid cooled transmitter on channel D5. This transmitter system was initially delivered as a three-cabinet system with four power amplifiers (PA’s) per cabinet (12 total) for operation at 6.2kW Transmitter Output Power (TPO).

In 2018, GBH purchased their transmitter system as “upgradeable,” allowing future power upgrades to accommodate an increase in the station’s Effective Radiated Power (ERP). PARALLAX was designed to accommodate from 3 to 16 final power amplifiers per cabinet depending on the required TPO. Each VHF Band 1 liquid cooled PA is capable of 1.5kW. Upgradeable PARALLAX transmitter cabinets (offered as an option) were pre-wired to facilitate quick upgrades with additional PA’s and PA combiners. The RF mask filter system was originally supplied with an anticipated power level of ~55kW TPO.

Fast forward a few years, GBH applied for and received authorization from the FCC to increase the station’s ERP from 6.7kW to 34kW to better serve their community of license. GBH ordered the system upgrade package from COMARK that included 24 additional amplifiers and the associated materials, providing TPO capability of up to 47.8kW. The new configuration of the GBH transmitter now uses 12 PA’s per cabinet or 36 PA’s total. After COMARK completed the transmitter upgrade, the final proof of performance was conducted at 31.4kW TPO. The GBH transmitter system offers plenty of additional headroom allowing GBH to run two of the three cabinets for maintenance periods.

“Planning is the key. We had a lot of very detailed customer discussions during the initial phases of this system,” says Joe Turbolski, VP of Sales and Marketing at COMARK. “GBH had a long-term plan for their post repack D5 allocation for this transmitter system. With PARALLAX, we were able to offer both the technology and the architecture that GBH required to make this project successful.”

“When we originally ordered the COMARK PARALLAX transmitter, our FCC engineering consultant informed us that there was a strong possibility we could be granted a future increase in effective radiated power up to 50Kw,” said GBH Transmitter Supervisor Jon Frank. “However, we had no idea precisely what that increased power figure would be. As result, we purchased the upgradeable cabinet option with our original transmitter. When we were granted a 34 Kw ERP, COMARK’s modular transmitter design allowed us to easily increase the transmitter’s power output in the field and to purchase only as many amplifier modules and combiners as we required.”

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