*Changing Out A Broadcast Antenna*
Bullets and RF Don’t Mix
05 September 2007
Although this isn’t “survival” related per say, many people find it interesting. Actually it can be “survival”. By not taking a spill off a tower or soaking up extremely dangerous amounts of RF energy I am “surviving”.
Somebody(s) thought it would be entertaining to put a bullet through one of the antennas at a radio station’s broadcast tower near here. Here’s what happens when a bullet goes through 49.5Kw of RF power. Yes, this station is pumping out 49,500 watts, which isn’t a lot actually. Some stations are running 100,000 watts and more.
An insulator on the antenna melted and there was so much heat that the radome melted. Notice the dripping gob of plastic. Fortunately the antenna wasn’t physically broken. The entire transmission line and pipe and the antennas are pressurized. It was still holding pressure.
Onwards and upwards literally. The tower is just shy of 200’ and the fried antenna was about 175’. There are 5 antennas and I had to replace the middle one. It’s impossible to remove only the middle one because of how the pipe connects. The bottom two had to come off first. The pipe that the antennas are mounted on is also the transmission line. The sections slide together. The stub on top of a section slides in to the hole in the bottom of the upper piece of pipe.
The antenna mounts on to the pipe.
|Bottom of the pipe
|Top of pipe
These two mate together
Now….how much does this stuff cost? Well....
There are 5 antennas, 5 pipes plus the tuning cans plus the heliax running down to the transmitter. BIG bux. Have you ever wanted to own a radio station?
|Here’s a $5,000 antenna...
|...and a $2,000 transmission pipe
The antenna at the bottom of the right picture is a microwave link to the studio. If you can find a radio station’s studio transmitter link (STL) frequency, you can listen to the broadcast before it goes through the transmitter
|Two stations sharing a tower
|3 antennas off
The heliax running up the tower hooks on to the pipes just below the tuning cans.
The swap out project way WAY too well. Total tower time was only 4 hours. Kinda cool to be at 175’ and leaning out near horizontal and facing the ground.
|New antenna ready to go up
|Tuning cans are at the base of the antennas
The white cabinet is 107.7 MHz and running 20,000 watts. The blue cabinet is 97.7 MHz running 49,500 watts. The two big white boxes are band pass band restrict (BpBr) filters. The one on the left blocks the 97.7 signal from getting in to the 107.7 transmitter and the one on the right blocks the 107.7 signal from getting in to the 97.7 transmitter. With all that RF up there right next to each other there’s a bunch of it going down the opposite feed line. If the filters weren’t in place, the RF going down the feed lines would fry the opposite transmitter.
That’s the general idea of how a broadcast radio station’s transmitter site works. There’s a lot of things that go on behind the scenes so that you have radio to listen to. ;o)
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