By Thomas R. Ray, III CPBE, AMD, DRB
Tom Ray Consulting
NEW YORK — I received a rather disturbing photo via text message from a friend of mine in a fairly small market. Seems this station, as many small market stations do, had not had anyone do maintenance on their transmitter for quite some time. My friend, an on air personality, not an engineer, was shown how to tune the transmitter. And that’s fine. But, there is more to tuning a transmitter than making sure the numbers fall where they should on the meters on the transmitter.
You see, this station is located in an area that gets a lot of snow and ice. And this station does not have radomes or deicers on its FM antenna. Radomes are plastic or fiberglass covers that keep ice off the antenna elements. Deicers heat up the antenna elements to melt ice before it can form. It makes sense that this antenna does not have radomes – radomes have considerable weight (particularly if they are covered with ice). The AM tower this antenna is mounted to is somewhat light weight – and would not be able to support a three bay antenna with radomes.
All I can figure is the former owners of this station did not want to pay additional dollars on their power bill, so they did not install deicers on this antenna. I can tell you that this station operates numerous days over the course of a winter on reduced power because of ice build up on the antenna. All antennas (even AM sticks) are composed of two elements – one that is, for all intents and purposes, attached to the transmission line, the other at ground. These elements are insulated from each other. When ice forms on an FM antenna, this insulation becomes bridged, as the water and therefore the ice it forms is conductive. This creates a situation called high VSWR (Voltage Standing Wave Ratio). Under ideal conditions, you want a VSWR of 1:1. High VSWR can do nasty things.
Here is how to think of VSWR. You take your garden hose. You face the street, point your nozzle at the street, and pull the trigger. Water goes up the driveway and the majority of it hits something, perhaps the street, maybe a flower bed you wish to water. You have accomplished the task. Now, face the garage, and stand facing the garage about 3-4 feet away. Point the hose at the garage door and pull the trigger. The garage door gets wet – and so do you, as the water hits the door and splashes back on you. This is essentially how RF travels down a transmission line. If the cable is properly “matched” to the antenna, the power from the transmitter all goes to the antenna and your listeners radios. If there is something wrong, power will be sent back to the transmitter. This is what happens when there is ice on an FM antenna.
Power coming back to the transmitter, or a high VSWR condition, will cause components to heat up – including the transmission line and the final amplifier components in the transmitter. On the transmission line, “hot spots” may be created, as the wave coming from the transmitter, and going back to the transmitter, has voltage peaks and valleys. The usual result of this condition is either a failure of the amplifier, ie, the tube lets go, or a catastrophic failure of the transmission line.
All transmitters have a VSWR protection circuit. This circuit will usually shut the transmitter down to protect it and the transmission line from a high VSWR. Some, like the transmitter we are discussing, have a “fold back” circuit that will reduce the transmitter power to a safe level before forcing it to shut down.
Tuning the transmitter will take what is being presented to the final amplifier (the load) and make it so that the final amp will run properly into a funky load. And this is where the mistake happened. My friend, not knowing what he was really doing (after all, they just showed him how to tune the transmitter to make it stay on the air if it ices up severely), did as shown. He retuned the transmitter so that it would stay on the air. But, retuning the transmitter does not correct the high VSWR condition.
The icing was so severe that the section of transmission line in the transmitter that connects the final amplifier to the low pass filter literally melted. His picture clearly showed solid copper that looked like candle wax dripping down the pipe. And there were several holes blown in the line. Additionally, this line is located directly behind the power output meter on the transmitter. It melted.
The transmitter is presently off the air – the station is operating at reduced power on their backup. Parts needed to be ordered. And once the burned parts are replaced, they may find other issues with the transmitter. So, no deicers, coupled with telling someone to “just retune the transmitter so it stays on the air”, has resulted in several thousand dollars worth of damage that could have been avoided.
Please. If you are not fully aware of what you are doing and what is involved and why, do not crank on the tuning controls of a transmitter. Not only can you start generating “out of band” products, ie, the transmitter will start producing trash that could affect neighboring stations, but you could find yourself out several thousand dollars in repairs. Icing and high VSWR is nothing to ignore or play around with. This station could have stopped the issues by installing deicers. But, instead, had someone who did not fully comprehend the consequences of what he was doing retune the transmitter to keep it on. And frankly, they got off easy. If the heat generated can melt copper, they are lucky the building did not go up in flames.
While money is tight all over, the last thing you need is your transmitter and/or building to be a smoking hulk. Consult with someone qualified to guide you. Think of it like buying an insurance policy. You may not want to spend the cash, but it could save you a bundle in the long run.
Thomas R. Ray, III CPBE, AMD, DRB is president of Tom Ray Consulting and Technical Editor of TALKERS. He can be phoned at 845-418-5065 or emailed at firstname.lastname@example.org. His website is www.tomrayconsulting.com. Meet Tom Ray at TALKERS New York 2013 on Thursday June 6.