As the weather improves, this is a good time for checking on the health of your transmission lines.
Richard Mertz of Cavell, Mertz and Davis (703-591-0110) provided an interesting SBE program outlining the use of a RF analyzer to document both new and existing RF systems.
In addition to checking the impedance, bandwidth and return loss, the analyzer can serve as a time domain reflectometer to spot any line anomalies.
(click thumbnail)Fig. 1: An AM station’s sampling lines, measured using a Delta Electronics PRH-1 TDR.
Figures 1 through 3 show how this kind of preventive maintenance can help. The photos document an AM station’s sampling lines, which were measured using a Delta Electronics PRH-1, a high-power TDR.
Analysis of the photos turns up some interesting effects. First, a 100-foot test cable was used to couple the TDR output into the sample line at the antenna monitor. The 5cm from the “test cable ends” flag represents the 500-foot run of sampling line. The spike at the far right end of the photo is the sampling toroid signature.
(click thumbnail)Fig. 2: An AM station’s sampling lines, measured using a Delta Electronics PRH-1 TDR.
Figure 2 demonstrates the usefulness of this test. In addition to some faults on the sampling line (seen at 100 and 150 feet from the input), a fault just before the sampling toroid can also be seen. Figure 3 shows the normal echoes, similar to Figure 1.
The analysis demonstrates why it is important to evaluate everything before adjusting a directional array, simply because the antenna monitor is not reading properly.
So where do you find this service? In addition to consulting firms like CMD, tower companies and a number of contract engineers provide this service. This periodic checkup of your lines is invaluable. Because you can’t see inside transmission lines, it’s difficult to spot impending problems.
(click thumbnail)Fig. 3: An AM station’s sampling lines, measured using a Delta Electronics PRH-1 TDR.
I remember measuring a line for a television station and discovering a troublesome “bump” on the display. We measured out to the fault, finding it at the junction of two sections of rigid line. The tower rigger found a severely heated bullet, which, if left alone, would have certainly taken the station off the air.
Just as our bodies need period health checkups, so do our transmission systems.
. . .
Not all transmitter fires end in disaster, but the repair costs can be staggering.
(click thumbnail)Fig. 4: Part of the final cavity of an RCA-BTF-20E1 transmitter damaged by fire.
Many times, the insurance company pushes for repairs rather than replacement. Pictures of the damage, prior to any work being performed, are an engineer’s best protection.
Figure 4 shows a part of the final cavity of an RCA-BTF-20E1 transmitter. Most of the damaged components have been removed, but a series of pictures were taken during each step. Not only will the pictures document the extent of the damage; in this case, because the transmitter was to be rebuilt, the pictures assisted in the reconstruction.
(click thumbnail)Fig. 5: The rebuild is underway.
Figure 5 shows the rebuild in progress. Quite a difference! After all components were removed, the cavity walls and shelf were cleaned thoroughly. All reusable silver-plated parts were cleaned with silver polish. This work was done outside, because of the noxious fumes the polish emits.
Using silver polish isn’t a big deal when you are removing the tarnish from your mother’s silver, but when used in the quantity to clean a final cavity’s parts thoroughly, you need plenty of ventilation.
A rebuild of this scale isn’t an overnight project and shouldn’t be rushed. Taking the time to snap photos of mounting bolts, positions and washer-lockwasher-nut sequences is worth the effort.
And what have you done to plan for a backup while you are rebuilding your main?
Something as simple as some 1/2-inch coax and connector adapters to get your FM exciter on the air or a low-power LPB AM transmitter can be money well spent.
I’ve heard of several transmitter fires over the past couple of months, and except in Florida, they can’t be blamed on lightning strikes. Take some time to clean up your transmitter site; add a smoke and/or fire detector to your remote control system.
Make sure you know the address of your transmitter building. Post the street number so it can be seen from the road; this will assist emergency crews.
When I supervised a remote transmitter site, I made friends with several of the neighbors, passing out T-shirts or baseball caps with the station logo. They had all my contact numbers and I had theirs. We stopped a couple of vandalism incidents working together in this manner.
A stop at your local police precinct and fire/rescue station is a worthy use of time. Make sure they know your new call letters and location.
With all the consolidation, you don’t need emergency response crews driving to a location used three owners and 10 sets of call letters ago. You hope you’ll never need their service, but you’ll never regret your efforts if you need to call 911!
I’ve talked to other engineers who have extended this courtesy to their electrician or HVAC contractor. It works.
One year, I got service for a burst water pipe located in a wiring trough, underneath the transmitter, at midnight on Christmas Eve. Now I know your first reaction would be, “Who would put a water pipe in a wiring trough?” Like most of us, I inherited that problem, and that’s another story!
Of more immediate concern was locating the water meter shutoff valve to stem the spray of water. It was located across the tower field, buried under ice and snow, in a location the plumber knew and I didn’t! The problem got fixed, and I looked like a hero. Once again, the camera documented how terrible things were, and I included the pictures in my annual engineering accomplishments memo when I had my review.
It’s funny, though. I wasn’t anticipating the looming emergency when I had bought lunch for this plumbing company’s crew a month earlier and made sure everyone had a station T-shirt. It was inexpensive insurance, given the magnitude of the catastrophe. The gesture created a bond between the station and the contractor that is still in place.
I imagine my GM would have passed out sacks of money that night, just to get the problem fixed; but planning for an emergency in the middle of a disaster just doesn’t work. Take a few minutes now to make sure your house is in order.
Submissions for this column are encouraged, and qualify for SBE recertification credit. Fax your submission to (703) 323-8044, or send e-mail to email@example.com