A Brief History: DTV Transmitter Transition at KCTS 9

We’ve received several e-mails and phone calls lately asking about the technical changes we are making to our broadcast for the DTV transition.

I’d like to explain what changes KCTS 9 has made to our transmitter facility to prepare for the analog shutoff on June 12, 2009.

First some notes about licensed transmitter power.

Licensed transmitter power lies in Effective Radiated Power or ERP. ERP is the power being radiated from the antenna which is transmitter power output or TPO minus transmission line loss plus antenna gain. All of the power ratings below are given in ERP.

UHF transmitters require a much higher ERP than VHF transmitters to achieve the same or similar coverage. Analog television transmission requires much higher power than digital television to achieve the same coverage. This means you can’t compare the ERP of KCTS 9 analog to KCTS-DT on channel 41 or what KCTS 9 will be broadcasting on digital channel 9 starting June 13, 2009. The FCC uses the Longley-Rice method to predict coverage and grants power levels on DTV to replicate existing analog coverage.

KCTS 9 was assigned a pre-transition DTV channel on UHF channel 41 by the FCC. KCTS-DT went on the air in 1999 with a Harris single tube DTV UHF transmitter operating at 427kW.

Harris single tube DTV UHF transmitter at KCTS 9.

To accommodate transmission on analog channel 9 and digital channel 41, KCTS 9 had two new antennas installed on the KCTS tower located at 18th and Madison in Seattle. The channel 41 antenna sits on top of the channel 9 antenna. Both the KCTS 9 analog channel (ch. 9) and KCTS 9 DTV channel (ch. 41) are below channel 53 which was the new upper channel limit after the DTV transition. KCTS 9 was given the option to continue operating DTV on UHF channel 41 or relocate DTV transmission to VHF channel 9 after analog shutoff.

Transmission on UHF channels require significantly more power then VHF transmission to achieve the same coverage area. UHF transmitters with high power requirements usually use IOT or Klystron tube amplifiers which are much less efficient to operate and have regular maintenance costs to rebuild or replace the tubes. The lower power levels required for VHF coverage make it economically feasible to build solid state, transistor based, transmitters which are more efficient and don’t require regular costly maintenance. Solid state transmitters are more reliable since there isn’t one or two amplifiers as in a tube transmitter but it is made up of several amplifier modules. If one amplifier module in the transmitter fails the transmitter can usually make at least 90% power. With a single tube transmitter if the tube fails the transmitter is off-air. With a two tube transmitter, if one tube fails the transmitter will be operating at one half or one quarter power depending on the type of combiner being used.

KCTS 9 elected to relocate DTV transmission to VHF channel 9 after the analog shutoff now scheduled for June 12, 2009. In Seattle, KCPQ (channel 13) and KSTW (channel 11) also elected to return to their VHF channels.

The original Construction Permit (CP) for KCTS-DT on channel 9 was granted with a power of 7kW. KCTS 9 filed for DTV maximization requesting a power increase to 21.7kW. The maximization request was granted and on July 22, 2008 a new CP was issued to allow KCTS 9 to operate at 21.7kW.

KCTS 9 filed for a Special Temporary Authority (STA) with the FCC to reduce power on our analog to half our licensed powered. The STA was granted and on October 6, 2008 KCTS 9 reduced analog power from 316kW to 158kW. KCTS 9 communicated this power reduction to our viewers through on-air announcements as required by the FCC. We heard from very few viewers that this power reduction resulted in a noticeable difference in the KCTS 9 signal.

The KCTS 9 analog transmitter is an NEC transmitter installed in 1985. There were two transmitters installed each with 1 final power amplifier tube. The tubes are air cooled requiring large ducting and a blower room. The output of each transmitter fed an RF switcher/combiner system. The switcher/combiner allowed us to use either transmitter A or B individually or combine them. Our normal operation was with transmitter A and B combined which totaled 316kW.

On October 6, 2008 our Transmitter Engineer, Tim Schall, changed the switcher/combiner to put only transmitter B on air, thus our power was reduced one half to 158kW. This was necessary so transmitter A could be removed from the building to make room for our new DTV VHF transmitter. Here’s a picture of transmitter A before it was removed.

Transmitter A before it was removed.

The rack on the far right side of the picture is used to control the transmitters and switcher/combiner. Transmitter B is to the right of the control rack and isn’t visible in this picture.

Transmitter A was disassembled in place and removed from the building in pieces. All of the internal copper transmission line was sold as scrap. The metal was all recycled.

Here's a picture of a partially disassembled transmitter.

The middle cabinet is where the tube was mounted. The tube is mounted in the octagonal “can” just to the left of the copper transmission line which is the RF output of the tube. To the left of where the tube sits is a square duct coming down and under the tube which is the supply air coming from the blower. The top of the middle cabinet has a round duct which is the exhaust duct for the tube. The blower then exhausts the hot air outside.

Space vacated by transmitter A.

Here’s a picture of space vacated by transmitter A, you can see the control cabinet for the NEC transmitter B still in place.

KCTS 9 purchased a Rohde & Schwarz VHF DTV transmitter to operate after the analog shutoff. This is a single cabinet solid state transmitter that is liquid cooled. Tim traveled to the Rohde factory in Germany to perform the customer acceptance of the new transmitter. Tim witnessed the transmitter respond to several real world conditions such as a loss of one phase on the AC input. Tim was very impressed with the Rohde transmitter and it performed very well.

On November 21, 2008, the new DTV VHF Rohde transmitter arrived at the KCTS 9 transmitter site.

New DTV VHF Rohde transmitter arrives at KCTS 9.

The new transmitter was unloaded and installed by an engineer from Rohde, an electrician from CES and Tim the KCTS 9 Transmitter Engineer.

The new DTV VHF Rohde transmitter.

Here’s a picture of the transmitter cabinet in place still in shrink wrap. The amplifier modules will be installed in the empty section in the middle of the cabinet. Against the wall behind the transmitter is the pump stand. The transmitter is liquid cooled with antifreeze that is pumped through the amplifier modules and then cooled outside in a heat exchanger. Notice how much smaller this transmitter is than one half of the NEC analog transmitter formerly in the space.

The transmitter installation was completed and the proof of performance was carried out with the transmitter transmitting into the dummy load.

Transmitter with amplifiers installed.

Transmitter with one amplifier partially removed

Tim thought it would be interesting to see one of the dead tubes from the analog channel 9 transmitter with an amplifier module from the DTV channel 9 transmitter.

Dead tubes from the analog channel 9 transmitter with an amplifier module from the DTV channel 9 transmitter

On January 4, 2009, at approximately 3:00 a.m., the DTV VHF was put on-air for the first time with test signals. Since it operates on channel 9 it can’t be operated at the same time as the KCTS 9 analog transmitter. The DTV VHF transmitter performed very well. It was tested on-air three more times in January and February to give cable headend operators time to install and test their DTV receiving equipment for VHF.

So on June 12, 2009 at 11:59 p.m., KCTS 9 will shut off our analog channel 9 transmitter and our digital channel 41 transmitter. After making a few changes to the transmission lines inside the building to permanently connect the DTV VHF transmitter to the antenna, the DTV VHF will be put on the air.

Jabran Soubeih is the Executive Director of Engineering and Technical Planning at KCTS 9. Jabran's weekly blog covers all things digital TV. Post a comment or concern about your TV signal or the digital transition and Jabran will answer it in this space.