[AMRadio] beating ice off antenna


John Lyles jtml at losalamos.com
Sun Mar 9 13:20:56 EST 2008


Back in my college days at WUVT FM, we had a 430 watt FM transmitter feeding a 2 bay horizonal polarized antenna. It was on the roof of the 7th floor of a dorm, and the transmitter would blow a screen supply fuse if the VSWR rose too high. So it was routine for the engineer on call (usually me at first, until I trained a few other hams and budding EE's) to go up there and beat on the mast with a 2 x 4 to clear the icing. The TX didn't make enough RF power to burn it off. 

Later, when I went to work for Broadcast Electronics and helped design the FM transmitter line in the 1980s, we developed VSWR-driven foldback of the screen power supply, a feature which, at that time, was unique in a broadcast rig. I remember a bunch of network O&O's in Dallas had gotten hammered with icing, and their FM30s were tripping offline. Our foldback system was applied to the entire product line, you could just watch it reduce power, then it would ramp back up as the ice melted (either for RF or from the weather). I remember getting satisfied reports back from engineers in Texas and Lousiana after they got iced, but stayed on the air through it all. This feature became a standard for all big FM rigs after that, if they had a way to easily reduce power (like a variable screen power supply for a tetrode). 

Here I just grab the center coax up to my dipole, and pull it a couple of times, and the ice seems to drop off, if it isn't too thick. We don't get the kind of thick rind ice around wires and trees that hit more humid areas.
73
John K5PRO


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