The importance of maintaining open dialogue between manufacturers and broadcasters
In last month's article I shared part of a conversation that one of my associates had with the manufacturer of our broadband antenna system at the Iowa Public TV facility in Cedar Rapids/Waterloo regarding the measured VSWR performance. I quoted some reflection coefficient numbers as they related to return loss as percentages of reflected power. It may appear that I am nitpicking, talking about reflected power percentages that are less than 1 percent of the total antenna input power. Maybe I am, but in an environment in which I read about receiver manufacturers working diligently to gain tenths of dB headroom in receiver performance, it seems foolish to accept any unexpected and unexplained system losses.
The primary driver for me in writing this series of articles is to share with other engineers our failures and successes as we move forward in the DTV conversion. That is not always easy or popular to do because I am doing it in a public forum that opens all of the parties involved to public scrutiny. To that end, I have pointed out a number of errors that vendors and manufacturers have made in various areas and I would be remiss if I didn't follow through with this situation.
GOOD NEWS, BAD NEWS
There are two issues that I want to discuss in this column: the technical problem with its solution and the communication problem with its solution.
As a follow-up to the conversation that was the impetus for last month's column, we recently had a conference call with some of the Dielectric engineers and sales representatives to talk about the problems with the antenna system and to formulate a plan to move forward to improve performance. In truth, I wish that the conference call had happened earlier because it would have done a lot to assuage some of the concerns we had and to ease the tension among the IPTV engineering staff over the holidays.
There is both good news and bad news in how we are going to proceed. The good news is that we are going to improve the antenna system performance. How good it can be made remains to be seen. Talking with Kerry Cozad of Dielectric is for me always a pleasure and an education because he is both knowledgeable and willing to explain to and teach the people he is speaking to. He is also honest and straightforward, which is critical to me because many times he is talking about concepts and science of which I only understand the very basic principles.
The bad news is that after we have made all the adjustments possible, the antenna system is still not going to perform at the 1.05:1 VSWR that Harris would like to see. The simple facts are is that we would be hard-pressed to get a single-channel antenna system on top of a 2,000-foot tower tweaked in to meet a 1.05:1 VSWR. What we are dealing with is a broadband antenna that not only has to work for our DTV Channel 35 but also for two other DTV stations at Channels 51 and 52. As I have pointed out before when discussing performance improvements, there is a fundamental truth that "you don't get something for nothing."
Any adjustment to improve performance on IPTV's Channel 35 will result in a change in performance somewhere else in the pass-band of the system. Because we're dealing with an antenna system that has a passband of approximately 120 MHz, we could easily degrade performance on the other channels sharing the antenna, so we'll need to balance performance as a function of all stations involved. What this translates to is that at best, we'll probably see VSWR of 1.15:1, which is kind of scary when you consider that this particular antenna is at the end of a 2,000-foot-plus transmission line in an area that gets significant ice during the winter.
A DARK SCENARIO
The fact that the VSWR is higher than I expected has been troubling to me, but the truth is that the VSWR number, in and of itself, is not the end of the world. The problem for me is that the errors add up and are extremely difficult to differentiate, so it is critical to the overall health and reception capability of the DTV system that we make every element as transparent as possible. We have no real control over the signal once it leaves the transmitting antenna and free space and the home receive system can be hostile.
I can see a scenario whereby viewers in eastern Delaware County in Iowa are receiving a fine signal from KWWL-DT and nothing from KRIN-DT, because in their area our receive signal is a tenth of a dB below threshold for their 35-year-old Channel Master antennas and the RG-59 that are coming into the Radio Shack four-way splitter in their houses. The antenna has a couple of twisted elements from the high winds and is pointed down about 30 degrees from all ice it has had to support over the years, but the fact is that their analog service still looks good and they actually get DTV from one of the stations that is transmitting from the same area as we are. How could a reception problem possibly be their antenna system? By the way, these viewers aren't getting KCRG-DT or KGAN-DT either, which happen to be transmitting using the same antenna as KRIN-DT.
I admit I am painting a pretty dark picture here. And in truth, I don't think any of the manufacturers we are dealing with for our DTV conversion would leave us with a system they are not confident is performing to the best of its capabilities. However, a lot of what IPTV and other stations are doing in the DTV conversion is somewhat experimental and there is still a lot of trial-and-error work going on. Having worked in broadcasting for 20-plus years, I have high expectations. I need to recognize that experimental nature of DTV and adjust my expectations to recognize that what I am used to may not be what I get, and that may not be bad. On the other hand, the manufacturers need to be completely upfront as well and, from my point of view, more proactive in solving the problems that are virtually guaranteed to pop up because of the experimental nature of DTV.
The bottom-line question I had to ask myself after talking with all the concerned parties today is what would have made this situation less of a crisis, because I have a lot of confidence in the people and companies I am dealing with. If you remember from last month's column I started off by paraphrasing a question that we were asked after it was explained to us that the VSWR was 1.19:1 and that with some tweaking it could be brought down to around 1.15:1: "What would we be satisfied with?"
Now in the light of conversation and communications, I'm sure we're on the way to solving this technical problem. But what about the next problem? I mean, we've got six more DTV sites to convert and I don't think any of them will go off without a hitch. What can be done to improve the dynamics of the situation to make sure that when the next problem occurs, it is viewed as a challenge to overcome as opposed to an impending train wreck?
Remember that perception is reality, so how we perceive a situation will have a huge impact on how we react. Suppose we received a call about this antenna that said something like "Listen, we've looked at the test data from the antenna installation and we're not satisfied with what we're seeing. The system will operate but we are sure we can make it better. We'd like to schedule some time to come out and work with the transmitter manufacturer to optimize the system. What time frame works for you?"
This is essentially the course of action that is going to be followed; the manufacturer is now being proactive at suggesting the solution as opposed to making me feel like I had to fight to get what we paid for.
Bill Hayes is the director of engineering and technology for Iowa PBS and a SMPTE Fellow. He is also a member of SBE, IEEE-BTS and SCTE and a recipient of Future’s 2021 Tech Leadership Award.
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