Busy again this week; our FM antenna was damaged by lightning. I thought you might find these images interesting.

This poor guy lost the coin toss; he had to climb the tower to do the initial inspection and to attach the lines for everyone else! It's a long way up; it takes 20-30 minutes to get to the top, 1,000 feet above the city. Because it's 1,600 feet above sea level, the air actually thins out while they climb, by the time he got to the top, he was huffin' and puffin.'

I get dizzy just looking at some of these pictures. The crew has to rig themselves to the very top of the tower and hang in the air to get to the antenna sections (called "bays"). The corrosion on the input connector (called a "bullet") is vaporized Teflon(tm) and copper.

The rubber O-rings are required at each joint because the entire line is pressurized. One of the ways that we can tell that something's wrong, in fact, is a sudden pressure drop. Look at what lightning did to the poor T-section on this bay. Needless to say, the loss of pressure in this case was immediate and total!

Here's the new bay in place. The "crossed arm" configuration gives us circular polarization, so that you can receive a good signal whether your antenna is horizontal (like a TV antenna) or vertical (like the whip on a car).

But hey, there's a nice view! Downtown Birmingham is in the center of this image. We share the hilltop with several other stations; the tower for Magic 96, another 100,000 watt FM, is to the right. The two wires to the left are the winch cables that the first guy rigged up.

You couldn't pay me enough to do what these guys do. I run the equipment and watch from the ground, thank you. (And I still get so nervous I have trouble eating lunch, just watching them!)

-- Anonymous, October 04, 2001

Answers

By the way, I shouldn't have mentioned Magic 96. They're part of the Dark Force. :)

-- Anonymous, October 04, 2001

Thanks, Stephen. I'm absolutely green after looking down.

We have one of those not too far from here (although I think it's somewhat shorter). Is there any chance those things could tumble when hit?

-- Anonymous, October 05, 2001

If you'll hold on, I'll go take a picture of it and maybe you can tell me what it's function is?

-- Anonymous, October 05, 2001

So much for that. My battery went dead after one shot, and all I got was the very top.

Maybe another time.

-- Anonymous, October 05, 2001

LL,

Towers will only fall if damaged by something, such as a direct hit from a tornado or ice falling onto the guy wires.

(The most dangerous time for the latter is when the ice is melting; large chunks can slide down the tower legs and snip the guy wires just like a knife. About 10 years ago, both channel 5 and channel 28 up in Raleigh, NC lost their 1,000 foot sticks to ice on the same day.)

Self-supporting towers aren't at much risk from ice and are the system of choice nowdays for cellular, in spite of the fact that they cost more. The land is so expensive in most urban areas, they still come out ahead.

You can identify towers by look, if you've been in the bidness for a while. AM and shortwave towers will almost always have insulators in the guy lines, because the tower itself is the antenna. FM and TV towers tend to be very tall with the antenna(s) at the top.

The big towers with the large horns at the top are terrestrial microwave. So are the tall towers with little dishes all over them.

Business, fire and police band antennas are usually either sticks or long sticks with loops on them. These are usually on rented space on tall buildings or on other towers.

Cellular towers can immediately be identified from the antennas at the top: the PCS style is group of tall, narrow panels; other systems use a big array that looks for all the world like someone took a giant bedspring, ripped on it with pliers, and placed it on top of the tower. :)

(No one would call a tower "pretty," but the latter are especially UGLY, and are usually the source of City/County "Beautification" committee complaints.[g])

There. More than you ever wanted to know about towers. :)

-- Anonymous, October 05, 2001

No, I found it very interesting, actually.

I can't really tell how high that thing is, but from here it looks like a mile. (However, I have no idea.) The whole thing would have to fall before it would hit me, and I might even be a little further away.

I took this picture today with my 10x lens from up on a hill. It is a very dark, cloudy, rainy day, so the quality isn't great, but can you tell from this what kind it is, and does it emit radiation or anything I should be concerned about?

-- Anonymous, October 05, 2001

I'm sorry, I don't think I made myself clear. I meant do I need to worry about radiation or anything if it topples? I'm not worried about it on a daily basis.

-- Anonymous, October 05, 2001

LL,

It's hard to tell from the photo, but this is a relatively low-power setup. I'd need to know the lengths of the antennas to get an idea of precisely what it is, but that's not a high-powered system.

Radio systems work on the principle of transfer of power. The antenna, feed line and transmitter must all be carefully tuned to radiate; any flaws in the system and the efficiency drops drastically.

(Example: if you're having trouble picking up UHF stations on your rabbit ears, trying collapsing them to make them SHORTER. They're actually way too long for those high frequencies; they're designed for lower VHF frequencies.)

Too much ice or bird poop on an antenna is enough to make a transmitter shut off to protect itself; if the power can't flow into the antenna and out into space, it just "reflects" back into the transmitter and gets dissipated as waste heat.

If a tower falls, the feedlines are instantly damaged; the transmitter has nowhere to radiate its power and immediately shuts off.

Besides, when towers fall, they usually fold in on themselves, much like a carpenter's rule. (Not always, but usually.) They can handle unbelievable stresses straight up and down, but not "sideways" stresses (called "shear"). Once a tower starts to buckle, the weight of the upper sections falls straight down; the lower sections can no longer support the tremendous weight.

You don't want to be anywhere near the base of the tower when that happens (speaking from experience[g]), but an extremely unlikely set of circumstances would have to happen in a precise order for that tower to topple "straight" over and fall like a felled tree intact.

Even then, the odds of the feedlines still being connected at the base of the tower are very high.

And finally, even THEN, once the antenna hit ground, the reflected power would go way up and the transmitter would probably shut off anyway.

-- Anonymous, October 05, 2001

LL, It looks like a cell phone tower, or a tower with cell phone antennas. We have a bunch of them here that have cropped up in the past 5 years.

Stephen, it is too bad that it was towards the end of Dad's life that he got me interested in how frequencies are determined in antennas-towers.

It has been a year now since he died. Seems like just yesterday........

-- Anonymous, October 05, 2001

Thank you, Stephen! I feel much better about it now. However, I went out this morning and took some more pictures of it because I'm still wondering what kind it is. After I downloaded, them, it looks to me like it is kinda leaning to the right. Fortunately, I don't live in that direction but there are people living within range if it falls. Does this look dangerous to you?

The glass bulb on the picture below flashes.

Thank you, Cherri. I'm sorry to hear your father died.

-- Anonymous, October 06, 2001

No, it's not leaning. That's an illusion. It even fools me sometimes on our own towers. :)

This is definitely a cell tower, by the way. See the white panels in the center photos? Those are PCS designs. There are a couple of VHF antennas on it as well.

The total radiation from this tower is less than what you'd experience from living near a modestly-powered AM site.

-- Anonymous, October 06, 2001

Aackkk! What have you done to me? Now when I drive around I wonder what kind of towers those are I see!

This kinda feels like when I first became aware of trains.

Thanks again.

-- Anonymous, October 06, 2001