Don't Procrastinate - Schedule An RF System Sweep Now
I had several converstations at the 2014 NAB show about RF system sweeps and inspections as a method to reduce the potential for expensive burnouts. Three of the conversations stuck with me and inspired the thoughts shared here. In addition, Mark Aitken's presentation at the Dielectric breakfast led me to read the Widelity report to the FCC. In the discussion below, I will address the table of prohibited channels shown on page 84 of the Widelity response to the FCC, and why you shouldn't take it at face value.
First Conversation: I stopped to say hello to a friend that is a well-respected RF engineer, and we went almost immediately to the topic of transmission line sweeps to detect watchband failures prior to a total burnout. Amazingly enough, while I am evangelical with respect to preventive maintenance, he initiated the topic. We agree that sweeping transmission lines on a regular basis can detect watchband and other failures before a total burnout. We also agree that when a questionable connection is detected, it must be repaired as soon as possible after it has been detected.
While not absolutely necessary, it is desirable to have reference data for the system to which the new data can be compared. However, if the stimulus and certain other parameters are unknown, the reference data may not be very helpful. A good system report should state the number of data points used, the start and stop freqencies used in broadband TDR plots, if any velocity factor is applied to distance measurements, if averaging is employed, where connection to the system was made, information about the test transition used to connect to the system, and how the instrument was calibrated. This all helps maintain consistency in future testing. Copies of RF system reports should be maintained at the transmitter site for future reference.
Second Conversation: As a result of having paid attention to Dan Fallon's NAB presentation, a friend and regular customer of mine requested an additional service be performed while I'm on site sweeping the transmission lines at his stations this year. This is also the DOE that requested the line sweep as part of a due diligence prior to station purchased discussed on this page. And, Oh yeah, that ugly looking elbow was from another of his sites; the one that made him a believer in preventive sweeps! For those of you that don't know, Dan Fallon is Senior RF Engineer at Dielectric.
I don't know why this didn't occur to me, but many thanks to Bob for asking: He would like swept data showing the summing frequencies on each of his transmission lines. So, I will detour now to discuss the "prohibited channels" chart in the Widelity report. The chart has at least 20 channels listed that will NOT necessarily have issues operating with the line sections indicated. There is a simple explanation for this discrepancy; When selling a new system, manufacturers need to be confident the line will not have issues when it is installed, so they employ a 3 MHz guardband in their charts. This is an excellent idea for NEW installations. Not so much for saving you money in the upcoming conversion. I will say that the various manufacturers will advise you to have your line swept to check for summing before you replace it.
As an additional example, math indicates that 20' line sections sum at roughly 2.3 MHz above the upper edge of channel 16. Why can't you use it for channel 16? Darned if I know! So before you throw out your transmission line just because it's on a chart, do the math. Then, if the summing frequency is outside your 6 MHz channel, but within 3 MHz of either edge, have a qualified engineer sweep the line to determine if it can be used for your new digital channel. Better yet, get your line swept as a preventive measure, and have the summing frequencies mapped as part of the service. You will sleep better knowing the line is in good condition, and you'll know for certain which channels won't play nice on your transmission line.
If you would like to have some fun with various channels and line lengths, see my ERP, TPO and Line Length workbook on the links and goodies page. The spreadsheet already has the channel 16 on 20' line example above loaded into it. You can see recommended line lengths for the various channels, as well as use the calculator to see if a channel will experience flange summing on a given line length. You can use it online, or download it so you have it available as you start building your new channel. You can also call for assistance with your RF ssytem questions. The phone number is on the About Us - Contact Us page.
Also, your broadband optimized transmission line may not necessarily be optimized for your new channel. Again, having your line swept for flange summing is an excellent idea. As we approach the auction and subsequent repack, demand on qualified field service engineers will increase. It is strongly recommended you have you line swept before the repacking begins. You may find there are insufficient resources after the transition.
Third Conversation: Another friend that is a field service engineer suggested station engineers are reluctant to do preventive maintenance on a transmission line that may not be compatible with their new channel. I get that. What I don't get is taking the risk that he might need to explain having lost airtime and a $100,000 repair bill for a line burnout, then telling the boss a couple of years later that line isn't going to work moving forward. The smart money is still on annual sweeps to prevent the lost air time and added expense incurred while multiple sections of damaged line are replaced and the entire run is disassembled for soot and debris removal. Add in the bonus of having your line mapped for summing frequencies at the same time, and it doesn't make sense to put off sweeping your transmission line.
Other Passive System Measurements: In addition to the broadband TDR measurements, we also look at system performance within the channel. I'm amazed at the number of systems that were either improperly optimized or not optimized at all.
Of course, verifying proper pressurization, building temperatures, fan operation, and other mechanical parameters are addressed in our periodic maintenance visits.
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