Radio Towers

[ActionDave]

How dangerous can it be working around a A.M. radio transmitting tower?

 

[mpoulton]

How dangerous can it be working around a A.M. radio transmitting tower?

The entire tower is the active element of the antenna, and contacting it would result in instant death by incineration, like contacting a transmission line. Unlike 60Hz power lines, you cannot "bond on" to an AM tower and be safe while in contact with it. Why? Because the high frequency results in a large current flow into your body even without any ground path (your body's capacitance is enough). So, in order to work on or near an AM tower the transmitter will be shut off, and you better lock it out just like any other equipment you would be working on! The FCC establishes workplace limits for RF field exposure (though their limit levels are WAY below anything that has ever been proven to cause harm). The station licensee should manage compliance with these rules and shut off the transmitter if you will be within the prohibited area. Aside from the RF-field exposure (a non-hazard in many people's opinion) and accidental contact, there are no other serious issues. If you are near the tower you may find that you draw small arcs or receive minor burns from touching metal objects. This is more a nuisance than a danger.

 

[ActionDave]

We've been working at a cell site that shares space with an A.M. transmitter. I knew to stay away from the tower but wasn't sure why.

 

[mpoulton]

We've been working at a cell site that shares space with an A.M. transmitter. I knew to stay away from the tower but wasn't sure why.

Yeah, sure don't touch it! Usually AM towers have a fence right around the base of the tower, inside the fence for the site, as a second line of protection. Sites are also required to have a fence that surrounds the no-entry area defined by the FCC field limits, with warning signs posted. However, many sites do not comply with these rules since AM towers were usually built before those rules were enacted. Unlike the NEC, old transmitter sites must stay in compliance with all new FCC rules. The FCC enforces this vigorously (like a bureaucratic sledgehammer) when informed of a violation, but rarely investigate on their own.

If the AM tower is quite old, it may be fed with open "ladder line" strung between transmitter shack and the tower base equipment. This RF transmission line consists of two bare conductors spaced a certain distance apart from each other, on insulators like a power line. Treat it like a power line - it will do the same thing if you touch it or get too close. Unlike a power line, the ladder line may be very low to the ground, unprotected, and will not trip any protective device if you get in the way. It will just burn you out of the way.

 

[ActionDave]

That is exactly what this place looks like. The site has a fence with a gate and an inner fence around the tower with an old rusty sign that says "WARNING".

The transmitter shack is realy a shack with transmission lines as described. The tower was insulated from the ground which made me think something was up.

 

[mpoulton]

That is exactly what this place looks like. The site has a fence with a gate and an inner fence around the tower with an old rusty sign that says "WARNING".

The transmitter shack is realy a shack with transmission lines as described. The tower was insulated from the ground which made me think something was up.

Yeah, definitely watch yourself around that. Here's a fairly amazing video of a fault on an AM antenna feed:

http://www.youtube.com/watch?v=6pjrWjlBf4Q

That's a 100kW transmitter, which is fairly typical for AM equipment. Notice that it just doesn't stop - the transmitter keeps on pumping a steady 100kW into the arc until somebody turns it off.

 

[sgunsel]

Any radio frequency transmitter antenna for AM, FM, TV, Radar, etc. can be hazardous when operating, because of electromagnetic radiation, which definitely does NOT require contact to be a hazard. The danger zone depends on the frequency, power level, antenna radiation pattern. It can extend some distance from the antenna. Your microwave oven cooks with radio frequency emissions. The FCC has established radio frequency exposure guidelines. I would not consider working anywhere near an active antenna without reviewing potential exposures with the station's engineer.

 

[techntrek]

Any radio frequency transmitter antenna for AM, FM, TV, Radar, etc. can be hazardous when operating, because of electromagnetic radiation, which definitely does NOT require contact to be a hazard. The danger zone depends on the frequency, power level, antenna radiation pattern. It can extend some distance from the antenna. Your microwave oven cooks with radio frequency emissions. The FCC has established radio frequency exposure guidelines. I would not consider working anywhere near an active antenna without reviewing potential exposures with the station's engineer.

I saw the perfect example of this at a microwave site in western MD about 15 years ago. It was obvious that at the time of the site installation vegetation had been removed all around but nothing had been done since. The lowest horn on the tower was fairly low, and new pines had grown 20-25' feet up... and into the radiation pattern from the horn. There was a line of trees that had no live growth above the point where they intersected the RF. What was a little scary was that there was a tall rock outcropping which let you walk within about 15 feet of this obvious danger zone (the trees were growing up from below that rock outcropping and you were staring at the dead ends 15 feet in front of you).

 

[electricmanscott]

We've been working at a cell site that shares space with an A.M. transmitter. I knew to stay away from the tower but wasn't sure why.

I don't know the answers but I guess it's good that your asking. Maybe you should have asked before you were around them. :grin:

 

[ActionDave]

I wasn't that close.

Also, most days I don't know where I'm going till I get there and I don't carry a lap top to ask this fine group any questions during the work day.

Been around a fair amount of cell and repeater towers and this one was enough different to rouse my interest. Twenty years ago I would have gone for a closer look. Not today. Now I ask questions first.

 

[cadpoint]

Your microwave oven cooks with radio frequency emissions.

Your microwave cooks within the magnetic electroc spectrum (I'm probably stating it wrong/ per the phrase)...

 

[mpoulton]

Your microwave cooks within the magnetic electroc spectrum (I'm probably stating it wrong/ per the phrase)...

The electromagnetic spectrum extends from sub-radio-frequency all the way to X-rays. The radio spectrum is at the "beginning" of it - the low energy end of the spectrum. Microwave ovens operate at right around 2.4 gigahertz, which is towards the upper end of the radio frequency spectrum.

 

[grich]

The primary threat from AM RF energy is burns when touching energized parts, along with the other potentially deadly things that happen when you become part of a circuit. That's the biggest reason for the fence right around the tower, and why the FCC is anal about that fence being in good shape.

Higher frequencies have greater heating effects on the body. Standing next to a 50 KW AM tower will have practically no effect on you, but if you climb up a tower and hang next to a FM antenna radiating 50KW, you will feel it...the old-time tower climbers tell me that you will first feel heating in your eyes and testes. The RF exposure standards are weighted based on frequency...the higher the frequency, the greater the risk for tissue damage due to heating. There's more to it than that, but it's too much to discuss here.

I recently attended a seminar on RF safety, and they cautioned one of the biggest RF hazards an electrician or service tech will come across is working on the roof of a tall building bristling with two-way radio repeaters and cell antennas.

http://www.fcc.gov/oet/rfsafety/Welcome.html will start directing you to some information.

 

[grich]

The electromagnetic spectrum extends from sub-radio-frequency all the way to X-rays. The radio spectrum is at the "beginning" of it - the low energy end of the spectrum. Microwave ovens operate at right around 2.4 gigahertz, which is towards the upper end of the radio frequency spectrum.

The power level and time of exposure mean a lot, too...current thinking is, your cell handset next to your head (frequencies from 0.8 to 2 GHz) doesn't cause a problem because the tiny amount of RF power causes a tiny amount of heating which is easily dissipated by the body. Stand next to a cell transmit antenna running a few hundred watts, and it's Radarange time.