An antenna sat unused as unexpected questions arose in Iowa
Nothing's for certain high above the Iowa plains.
Have you ever had an experience where you made all the right decisions and moves only to have things still go wrong and upon reflection realized that you didn't start from where you thought you were? If that appears to be a little vague, let me clarify this.
About three years ago, Iowa Public TV purchased a new antenna and transmission line inners for our station KHIN in Red Oak, Iowa. The purchase was part of the DTV conversion project but we were anxious to get the job done. At this site our DTV channel assignment is 35 and our analog channel is 36, so the addition of DTV provided an opportunity to improve our analog service as well. Unfortunately, money got tight right after the new antenna was delivered and we ended up storing it on the ground at the site. Now, we finally have the money available to finish the installation.
A LITTLE HISTORY
Let me tell you a little history of the site and what our thinking was before I go into what has happened. The Red Oak tower is a 1,450-foot Stainless erected in 1975. The original top-mounted Channel 36 antenna is an RCA TFU-36J. When the site was originally planned and the tower designed, there was also going to be a sidemounted FM antenna directly below the UHF antenna.
When we were looking for an appropriate DTV/analog antenna we had some very basic criteria. First we needed an antenna that would carry both signals. Second we did not want to see any significant decrease in ERP for our analog signal. The new antenna had to have radomes since our experience with heating elements and icing has been very bad. And most importantly, the wind load of the new antenna must match the old antenna so that no tower modifications were required. Based on this information, we selected a Dielectric TFU-28GTH-R.
(click thumbnail)Iowa Public TV stores KHIN's Dielectric antenna while it works out its tower issues.
Comparing the structural design specifications showed that there were some differences in weight and diameter between the two antennas. The numbers were not huge but they were enough that we decided we needed an expert opinion. Now I have to confess at this point I am a little uncertain of what happened and how but we ended up with a letter from the structural engineers at Kline Tower stating that the load changes generated by the proposed Dielectric antenna were insignificant. I am somewhat confused by why we have a letter from Kline telling us that the Dielectric antenna is okay for our Stainless tower. In retrospect, we probably should have gone back to Stainless for its expert opinion, as the company has access to all the original documentation. Suffice it to say that we continued with the project with the assurance that the antenna was a non-issue. First lesson learned: In tower work there are no non-issues.
Last year funding became available and we put out for bid a project to remove the old antenna, replace it with the new antenna, replace the transmission line inners and do some maintenance work on the strobe system. When writing the bid specification we made a small error in that we specified the wrong Dielectric antenna model and SpectraSite, one of the bidders sent a note stating that if they were the winning bidder they would want a tower study done before any work commenced because of the load changes. This had us scratching our heads since the load changes appeared to be insignificant. A quick review of the bid revealed the problem and we sent out a correction. When the bids came in, SpectraSite's bid still had the caveat that a tower study would have to be done prior to any work. This was the point where we started thinking about the basic foundation upon which this project was conceived and decided that a tower study was probably a prudent step in the process to make sure that everyone was comfortable. There have been several tower collapses in the last few months in Nebraska and no one wants to make a mistake that might cost lives.
Once the decision was made to do the tower study, I placed one caveat on the bidder that probably upset a couple of them. I specified that any company bidding on doing the study could not bid on the installation project. We have had experience with companies running up bills with change orders and I felt that an independent structural engineer could give an unbiased evaluation of the tower. The bid was out and Entrex won the contract. We supplied them with all of the documentation and waited for them to come back with the good news. Lesson number two learned: Don't expect good news about your tower.
In the report, Entrex analyzed the tower under the original EIA-222-C structural and the most current TIA/EIA-222-F. We honestly didn't expect to meet the F specification but were fully expecting to meet the C spec. Imagine our concern when the results of the study demonstrated that we didn't meet either specification and it was not feasible to modify the tower to meet those specifications. Since bad news is generally delivered on a Friday, and this was no exception, my staff and I had a whole weekend to ponder this dilemma. Now I am no structural engineer but as I reviewed the documents I was confused at how such a minor change in load could take the tower so far out of specifications. It appeared to me that the tower might not be in specifications even with the existing load-and that was without the FM antenna that was planned but never installed. That couldn't be right, especially since the tower has been standing for 28 years and has seen wind and ice forces greater than those specified in revision C.
The next step for me was to call Entrex and get some clarification and try and find out what the problems and solutions were. Talking with the structural engineer at Entrex did a lot to assuage some of my concerns and he offered some insights regarding what Stainless could have done to strengthen the tower so that it would meet the C specification. I contacted Stainless and got a very quick response that I seriously thought cleared up everything. The tower wasn't built under revision C but rather revision B and according to their analysis, it met revision B. I made a quick call back to Entrex to tell them it was our mistake since we had assumed that the tower was constructed under C and not B. We'd ask them to rerun the study under B and all would be right. Lesson three learned: Standards tend to be fuzzy. According to Entrex the revision B standard didn't give the tower a clean bill of health or really make any significant change in the study.
At this point I decided that Entrex, Stainless and IPTV needed to conference and discuss this situation and figure out the solution. I'd like to tell you that answer but as of this writing, the conference call has yet to be scheduled. Stainless declined to meet unless they could look at the Entrex study, which I sent them. Hopefully in the next couple of weeks that call will take place and if it does, I will share the results in this column. In the meantime, what can a reader take away from this? First and foremost, there are no small tower projects. Second, towers tend to last for decades so if you are considering any minor changes, invest in a tower study to understand the total impact on the structure. It is a minor cost over the life span of the structure. If you have an older tower or a newer tower for that matter, having an independent confirmation that the structure meets the specification under which it was constructed can provide some serious peace of mind down the road.
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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