The dog days of summer upon us and so are all the cooling deficiencies and thermal instabilities of broadcast equipment.
Michael Brown of Brown Broadcast Services in Portland, Ore., is shocked by the number of transmitter rooms -- some in large markets -- that still don't have remotely readable and alarmed room temperature sensors, let alone fire alarms. Mike says this is like driving a car without a temperature gauge. At least with the vehicle, you might see steam rising from the engine.
(click thumbnail)Fig. 1: This is an exhaust port on a Continental transmitter. A heat sensor has been placed over the stack.
Anyone who thinks that the use of solid-state equipment in the broadcast equipment would lessen heat concerns is wrong. In fact, most every engineer has resorted to at least one clip-on or box fan babying some thermally persnickety computer server, audio processor, satellite receiver or exciter.
So besides stashing a couple of spare fans, what's an engineer to do? Michael has fought the heat battles at his contract stations by doubling all rack space plans. This is particularly necessary because most single-space items need breathing room above the device. Keep in mind that those microprocessors may have millions of semiconductor junctions inside, each creating its own picowatt of heat. The numbers still boggle the mind.
What can the engineer do to fight heat? First, follow the manufacturer's suggestions. This is true particularly for transmitter equipment.
Fig. 1 shows an exhaust port on a Continental transmitter. Note that the exhaust duct is not fixed to the top of the transmitter. This space helps counteract problems caused by outside back pressure. In the photo, a heat sensor has been placed over the stack. This thermostat controls an exhaust fan, routing the air outside the transmitter building, once a pre-set limit is reached.
(click thumbnail)Fig. 2: Ductwork runs to a ceiling exhaust fan. Filtered outside air comes through the rear of the transmitter, heated air exits from the top.
Fig. 2 shows another version of ductwork that runs to a ceiling exhaust fan. In this case, the fan runs all the time. Filtered outside air is brought in through the rear of the transmitter, heated air exhausts from the top.
If you're planning a new transmitter building, and especially if you're intending to use one of the pre-fabricated concrete shelters, consider low -- and high -- temperature limit switches. In addition to controlling the building cooling system, building manufacturers offer remote control options.
Pictured in Fig. 3, the low and high limit switches can be brought out to a block to permit interfacing to the remote control system.
If you're on a budget, not to worry. A well-stocked hardware store has a "hi-lo" thermometer, like the one manufactured by Taylor Thermometer and shown in Fig. 4.
(click thumbnail)Fig. 3: Low -- and high -- temperature limit switches can be remote-controlled.
This thermometer has two small iron vanes within the thermometer tube. The vanes are pushed by the mercury as the temperature changes. One vane indicates the highest temperature, the second vane indicates the lowest temperature. Either vane can be manually "reset" by placing the red magnet, held in the top of the thermometer, over the iron vane and moving it.
(click thumbnail)Fig. 4: "Hi-lo" thermometers like this one from Taylor Thermometer are available at hardware stores.
It's not necessarily high-tech, but it will give you an accurate display of temperature excursions, helpful in determining whether additional airflow is needed.
. . .
Most of our "can you top that" stories in recent issues have focused on the operator. I'm finding there are just as many stories where the engineer is the focus.
Consider the CE who enters the studio to perform some minor maintenance during the final minutes of the morning show. The jock comes unglued, shouting, "You can't be working in the studio while I'm on the air!"
"Fine, then," the CE responds. He turns to the remote control and shuts down the transmitter. "O. K., you're not on the air any more." The chief did what he had to do, then turned the transmitter back on and left the studio.
What's the best part of the story? The GM and owner backed the engineer's actions! Oh, for the good old days.
. . .
For most of the country, this has been a year of rain. One of the places rain can really hurt is in the STL system.
Bob Hawkins, chief of WNAP(FM) in Indianapolis, writes that not everyone has access to a time domain reflectometer or even a Bird Wattmeter that works accurately at 950 MHz. More often than not, when STL receive-signal levels deteriorate gradually, the problem is water in the dish dipole element or the jumper cable from the element to the main feed line.
To find out if water ingress has taken place, all you need is a Simpson 260 or similar ohmmeter. Measure the resistance between the center and outer conductors of the Heliax at the bottom end, where the cable plugs into the receiver. In a typical system, using Anixter-Mark antennas, there will be no measurable resistance on the Rx10,000 scale of the Simpson 260. If the meter shows any indication whatsoever, further investigation is warranted.
Keep in mind that STL pigtails can fail. They are exposed to some horrific conditions and will not last forever, even if they are properly weatherproofed. A few spares on hand can be a lifesaver.
This method may work with other brands of STL dishes, as long as the normal resistance reading of the dipole element is infinity. A call to the manufacturer will yield this information. Bob also cautions that the measurement is made before any cavities, filters or isocouplers that may affect the reading.
At WNAP(FM) a few years ago, Bob observed a gradual decline in received signal strength, down to 300uV. An ohmmeter check showed 15k ohms of resistance between the center and outer conductors.
A climb up the STL tower revealed that the dipole had two drops of water in the connector end, accounting for the 15k of resistance. Thirty seconds with a hair dryer changed the 15k to infinity. The pigtail jumper measured 500k ohms and was replaced. The result of these two corrective actions was 2000uV of received signal.
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