Do your work right the first time, and your installation will last a lot longer and run more safely. Nowhere is this more so than at the transmitter plant.
Richard Wood is with Resonant Results Ltd. in Wisconsin. Take a look at a site he visited, shown in Fig. 1, and comment on how well it conforms to good engineering practice.
Fig. 1: Test your knowledge. How well does this site conform to good engineering practices? I’ll come back to this image in a moment.
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Ira Wilner is a principal with Wilner Associates, a broadcast engineering and technical services company here in the Northeast.
After seeing pictures of burned contacts in the Dec. 16 Workbench, Ira offered tips for wiring antenna switch interlocks.
Simply wiring the switch interlocks to the transmitter external interlock circuitry may not prevent hot switching damage completely.
Unfortunately, the interlock micro-switches in most RF switches typically do not operate until the RF contacts have started to move.
Some movement of the RF contacts while energized will not result in a catastrophic arc-over but will cause avoidable contact wear and pitting, which, over time, could result in disastrous failure.
The best way to prevent accidental hot switching is to automate the process.
There are two methods. The first is to use the remote control system macros to sequence the changeover by first shutting off the transmitter(s). Then incorporate time delays to permit the RF to decay fully to zero before giving the switch a command to move. Finally, delay the restart of the RF carrier long enough to permit the switch contacts to settle down.
Fig. 2: A ‘smart’ antenna controller, properly wired, ensures against damage caused by hot switching. An alternative is to use an intelligent standalone antenna switch controller such as those made by Tunwall Radio, Dielectric or Kintronic Labs. A Dielectric dual controller is shown in Fig. 2. These panels have this sequence of events built in and will prevent hot switching when wired properly.
Finally, Ira says be careful with new switch installations. He has seen misaligned limit switches cause the controllers and motor drives to hunt! The additional motion might not be healthy when the switch is again carrying RF power.
Drop by Ira’s Web site for another interesting discussion on improving FM reception. The site is www.wilnerassociates.com.
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Ira’s comments about RF transfer switches weren’t the only replies I received to our Dec. 16 column.
More than one engineer speculated about the identity of the transmitter site shown in the photo of an air hood placed over the transmitter to exhaust the rig.
Longtime engineer Milton Holliday started the line of questions as to whether that site was WLW(AM). Actually, the transmitter site we depicted is the home of Bonneville’s WFED(AM), Federal News Radio 1500, the former WTOP(AM), in Washington, where Dave Garner is director of engineering.
Those older transmitters really do catch your eye!
Got an oldie but goodie? Snap a high-resolution picture and e-mail it to me with the particulars. My address is firstname.lastname@example.org. It just might show up in a future column.
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So with reference to Fig. 1, how many things did you spot that could have been done right the first time?
Let’s start with the location of the nitrogen tanks right by the front door — next to the light switch, no less. This is a safety concern; arriving late at night, in a hurry, you easily could knock a tank over while reaching for the switch.
These tanks need to be secured properly to the wall. Remember, the gas is under extreme pressure. Knock a bottle over, snap off the regulator and you have a rocket in your building.
The nitrogen line is run in a haphazard way, another accident waiting to happen. Secure the tanks and then secure the poly tubing feeding your manifold or coax line.
In my opinion, a manufacturer should slope the top of a transformer so users aren’t tempted to turn it into a shelf, like the step-down unit in this picture.
I am reminded of a story I heard years ago about an engineer who had to get to the top of his transmitter. He used the flat top of an externally mounted high-voltage transformer as a stepladder. Unfortunately the hot AC terminals were right under the flimsy sheet-metal top and were pointing up. The top couldn’t support his weight, and he was electrocuted.
Not a pretty thought. Be mindful.
That piece of equipment in the rack atop the transformer is the strobe light controller. “Of course these were designed to be cooked on top of a transformer!” Bad idea.
If there’s no room for a stand-alone rack, look into the selection of wall-mount racks at a source like rackmounts.com, Middle Atlantic Products or your preferred broadcast equipment dealer.
Someone did take time to do some things right here; take a close look at other things in the room, like the disconnects and conduit runs.
However, the absence of labels on the breaker boxes isn’t wise. You may think it’s obvious that the breaker goes to the transformer; but in an emergency, you or someone else may not have time to try to trace electrical conduit to the appropriate box.
On the positive side, the floor does look clean; and it’s smart to keep a bottle of Formula 409 or similar brand cleaner handy to check for leaks.
Did you spot other practices, good or bad? Drop a note to email@example.com.
John Bisset marked his 40th year in broadcasting recently. He is international sales manager for Europe and Southern Africa for Nautel and a past recipient of the SBE’s Educator of the Year Award. Reach him firstname.lastname@example.org.
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