Bill Hayes /
01.10.2007 12:00AM
Digital Journal: Keeping Analog Alive
JOHNSTON, IOWA

The Iowa Public Television network consists of nine full-powered transmitters and eight translators that effectively cover the entire state. Two of the full-powered analog stations are high-band VHF and the remaining seven analogs are in-core UHF stations.

All nine full-powered stations have in-core UHF DTV assignments. The analog VHF stations both have solid-state transmitters while six of the UHF analogs have very old integral cavity klystron transmitters. One of the analog UHFs was acquired from a community college a few years ago and has a low-power solid-state analog transmitter.

Based on our experience with our solid-state VHF transmitters, the age of our analog transmitters and the power levels of our DTV assignments, we purchased seven solid-state DTV transmitters for our UHF analog facilities and two IOT-based transmitters for our VHF facilities. At the end of analog broadcasting, we would migrate DTV service to our high-band VHF transmitters in two markets and keep the new DTV assignments in our U-U markets with the new solid-state transmitters. When analog service ends, the IPTV network would consist of two solid-state VHF DTV transmitters and seven solid-state UHF transmitters. Given the redundant nature in solid-state design, we would see the same higher reliability of service at our UHFs that we have experienced at our VHFs and we would no longer deal with replacing tubes.

Now that we have several years of DTV operation at six of our sites, our analysis so far has revealed some interesting and unexpected data. As our first solid-state UHF DTV transmitters went on line, we experienced some of the typical problems associated with most new transmitter installations.

What has been more surprising is that some of the problems have continued for long enough that they don't appear to be what would typically be described as "infantile" failures. In a number of the solid-state UHFs, we have seen problems related to capacitors that appear to point to insufficient temperature ratings for the devices selected. The designs also seem to run the active devices very close to their maximum rated operating point.

I am not a design engineer, but I have been told by the design engineers that the devices become more efficient the closer to the red line that they are operated. While this may be true, it appears to fly in the face of the more conservative design approach that in transmitters leads to longevity.

Our cumulative data for the last few years shows that at several of our DTV sites, a 25-plus year-old integral cavity klystron spends more time operating at 100 percent power than the related solid-state DTV transmitters.

Now obviously, the solid-state transmitters spend less time at 0 percent power than the older klystrons, but it still is disturbing how frequently these solid-state transmitters are running with at least one RF pallet out of service.

Even though we have entered into the digital age throughout the industry, complete with its three-year replacement cycles and maintenance agreements, we still expect 20 to 25 years of life expectancy for out transmitters, and this doesn't bode well.

On the other hand, our DTV IOT experience has been somewhat better. We have seen some minor problems associated with a new installation but for the most part they are understandable and correctable.

REPLACEMENT PLAN

While the DTV conversion was being planned and projects were underway, we were also looking at the condition of our analog service. As I mentioned earlier, six of our klystrons are more than 25 years old and even though they continue to operate, their serviceable life has been exceeded and then some. Before Congress mandated the Feb. 18, 2009 analog shutoff date, we were still laboring under the unrealistic 2006 shutoff date and its nebulous 85 percent market penetration, while those of us in the industry knew that the actual conversion date was closer to the 2012-15 timeframe.

Over three years ago, IPTV presented a plan to the State of Iowa to replace the existing UHF analog complement of transmitters with new IOT-based UHF transmitters that would allow us to reliably serve Iowans until analog broadcasting ended and those transmitters could be moved to DTV service on the new channels. The side benefit is that it would--for the first time--also provide a backup transmitter at those sites.

IOTs were initially selected because the peak power requirements made solid-state options too unrealistic, and based on some of the experiences I noted earlier, we were beginning to doubt some of the benefits of solid state in this particular application. In addition, since these transmitters would be going into the space occupied by the old integral cavity klystrons. The amount of redesign and expense that would go into adapting the buildings from the glycol-based cooling to high-volume air cooling required by solid states would be excessive. Trust me--that this is not an inconsequential consideration in planning a facility, and we struggled mightily with vendors trying to get this right.

THE MODIFICATION PLAN

Much to my surprise, the analog replacement plan that we presented was accepted allowing us to replace all six of the integral cavity klystron transmitters. As I write this, we are getting ready to release the RFP for the replacement transmitters.

The specifications call for the six transmitters to be delivered and set up for analog operation, but they are to include all of the components necessary to convert the transmitter for DTV operation in the field.

That has always been part of the plan for these replacement transmitters, although at the time, we expected them to spend considerably more of their initial operating life in analog service; who knows what will happen as the 2009 deadline approaches?

The only real change is that we have determined that at the end of analog broadcasting, the IOTs will become the primary transmitter at each site and the solid states will fulfill the back up role.

In addition to the concerns that I expressed earlier, there is also the fact that in a backup role, unless it is a hot standby, an IOT transmitter would be shut down for long periods of time and the time to cycle the transmitter on would be excessive.

So, we'll be taking down fairly ancient single-ended klystron transmitters and replacing them with new IOT-based transmitters. Obviously, there will be some time involved in pulling out the old system, preparing the room for the new transmitter and then installing and proofing the new transmitter, what happens to the service area.

This is where we get into what I am sure some of my RF engineers consider being one of my more creative ideas. IPTV currently has a contract for replacing all eight of our existing translators which have all been displaced. We have CPs to change channels and increase the power at those sites. I have accelerated the purchase of one of the 5 kW units and we have purchased a truck.

We will install the translator in the truck and drive it to the site to function as the auxiliary transmitter that will fill in for the main while it is replaced. Since we will be doing this at six sites scattered throughout the state, making this a mobile system seems to me to be the best alternative. Time will tell whether or not this is an act of genius or lunacy. I plan on writing more about the mobile idea in a future Digital Journal.


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