It all started the first winter I was in charge of a broadcast transmitter. Winter precipitation can wreak havoc on the signal of a broadcast station. Snow doesn't bother the antenna but ice does. Causing all sort of problems for transmitters because as the ice accumulates on the antenna, it electrically changes the characteristics as if someone had added to its length and therefore changing the frequency that the antenna operates on. The first time I encountered this I worked for an AM/FM station. The AM didn't mind too much the ice, unless it got really thick and started to deform the radiating wires that hung off the tower. It was the FM antenna that was always susceptible.
The reason is simple. A single AM radio wave is several hundred feet long. Usually the antenna is the tower you see. This is called a series fed tower, a tower that is insulated from the ground with a porcelain insulator and the height is critical as it is a mathematical portion of the actual wave length. The other kind that is often used when an FM antenna is located on it is called a shunt fed or a tower that is grounded and wires are hung from the tower and the tower height is not critical and can be much shorter than the wave length. We had a shunt fed and so if the ice grew too thick, the wire supports would sage and cause the wires to lose their shape and the antenna would change electrical characteristics of tuning, not frequency change because the distance of the wires from the tower would be different.
In FM (and TV) the waves are only a few feet long and as the antenna changes frequency, the efficiency of the transfer of RF energy from the transmission line to the antenna drastically reduces. Imagine a water hose that is passing the same amount of water that is being put into it. Reduce the size of the hose at one place along the line without decreasing the water flow and the back pressure increases. The same basic concept happens in RF. But what happens is because the FM and TV waves are so small, the waves are reflected back down the transmission line like the ripples in a lake that a rock has been thrown into and the waves bounce off the shoreline and move back to the originating point. Unused RF is always dissipated as heat. Transmitters are rated to handled a certain amount of extra heat that could be radiated back, but if the change is too much, the transmitter can't handle it and it literally burns itself up.
To combat that, transmitter powers have to be reduced to a level that the transmitter can handle the extra heat until the icing is over to keep from burning the transmitter up. Now a days antennas have what is called a "radome." Radomes are a special RF passing polymer covers that fit over the FM and TV antennas. You also see radomes on the large telephone microwave antennas on microwave towers and satellite dishes. This keeps the ice and snow out of the dish so the antenna performance is not effected. Radomes used to be considered an "accessory" (they add a cost of antenna materials and also add a small loading to the tower so the tower has to be a little larger, something that was of paramount importance in the early days when FMs and TVs were put on existing towers that had not been designed to handle the extra loading) that could be done without. Over time, with the money that was lost due to being off the air or reduced power, radomes became part of the antenna and the extra price for material and a slightly beefier tower was just included in the cost. Now, this brings us back to the here and now. Our old channel 8 antenna from 1963 doesn't have a radome on it. The new antennas on the new tower have radomes.
The winter weather we had this past week not only stopped the finishing work on the new tower, it also caused us to reduce power on the old antenna because at 1200 feet, there was ice on the cold steel, even though there was no ice on the ground. We had hoped that last year would have been the last year we would have to reduce power during an ice storm since the new antennas have radomes and we figured we would be on the new tower this winter. Well so far we are still on the old tower so we have to deal with icing. So no matter what time the ice starts to form, day or night, I need to be aware of it and be sure the Master Control people are on their toes so the transmitter is reduced in power when the icing starts and to get it back to full power as soon as the ice is over. Plus if there is a problem at the site, I have to go down there and fix it, hopefully BEFORE or AFTER the ice falls off the tower. After a good storm, you can get chunks up to 100 lbs and 30 feet in length falling at 150 mph and if it hits you, you are dead. I don't go to the tower unless we are off the air when the ice is falling.
A shower of ice
The building workers were amazed how much ice was falling off the tower on Tuesday. It really wasn't much, but they would just stand there and watch it fall off the tower. I kept telling them that they really didn't need to be out watching it and it could hurt them if a large enough chunk broke loose. Of course we didn't have any large chunks but I wanted them to think about it in case we do get any real ice before the building is completed, they won't get hurt. The general contractor admitted that they didn't really understand why we were so adamant about the ice shields on the tower and the building. After Tuesday, they completely understand now. I kept telling them this wasn't even an "ice storm." If they want to see an "ice storm" wait until we get some REAL ice and then come by and watch!
Ice Knife
No comments:
Post a Comment