TheEC: Tower Climbing & RF Safety

Apr 13, 2015

AT&T tower climbers (circled in green), 102.7FM bottommost antenna bay is circled at top.
Credit Aaron Read RIPR

This past Saturday, AT&T Wireless performed a hardware upgrade to their cellphone antennas that're on the same tower RIPR's 102.7FM signal is, in Narragansett.   This entailed two tower climbers spending nearly six hours up on the tower, within about 20-30 feet of our Shively 3-bay antenna.  Fortunately between the laws of antenna physics, and judicious reduction in transmitter power, RIPR could stay on the air without jeopardizing worker safety!

Working in conjunction with the tower climbers, we determined that if we reduced transmitter power as low as the Nautel V7.5 would go...650 watts transmitter power output (TPO), or about 500 watts effective radiated power (ERP) coming out of the antenna...then things would be safe for them to stay up there.

Tower climbers all carry personal RF safety meters, and in this case they also had a precision Narda RF meter, so they don't end up getting an RF burn on their skin.  Quite literally, the energy from an FM transmitter can be powerful enough to heat up skin cells until they die from overheating...not unlike using a microwave to cook food in your kitchen.

AT&T antenna (upper right) being raised up to the top of the tower via a rope & pulley system, anchored to the truck. Allows hefty antennas to be lifted without risk of banging into the tower or other antennas on the way up.
Credit Aaron Read RIPR

  In WRNI-FM's case, the power levels are relatively low, but it's the proximity that's a problem: RF power levels decrease rapidly with distance; they're governed by the inverse square law.   That means you can move a short distance away and see RF levels drop by large amounts.  But it also means the reverse: when you get real close, you're getting it all.

One method we have to help deal with that is using antenna arrays instead of a single antenna.  One antenna bay results in a roughly spherical distribution of signal energy.   This is not ideal; why would you want to waste power straight up into the sky, or down into the tower structure?  Instead, you can use multiple antennas, spaced just right, to "focus" the energy horizontally; more like a donut than a sphere.  

The spacing is determined by the frequency you transmit at, because frequency and wavelength are mathematically intertwined: (wavelength, in meters) = (speed of light, 300,000,000 m/s) / (frequency, in Hz).  For FM frequencies (88.1 to 107.9 MHz) that means 300,000,000 / 102,700,000 = 2.9211 m...or a bit under ten feet.   If you space your antenna bays one full wavelength apart, you can create gain, meaning you'll need less transmitter power to achieve the same effective radiated power.

FM Signal Emission Pattern for a 3 bay, half-wavelength-spaced antenna array.
Credit Shively Labs

 On the other hand, if you space your antenna bays a HALF wavelength apart, as WRNI-FM is, then you actually have negative gain but you dramatically reduce upward/downward radiation...nearly to zero...as you can see in this graphic at right.  The bulk of the RF is coming out at zero degrees elevation (horizontally) and it rapidly drops off as azimuth tilts up/down.

Think of the pattern past the 80 degree azimuth as being a "cone" that encompasses the tower itself, where the climbers are.  That's only 0.05 of the total field emissions.  So take (500 watts ERP) x (0.05) and that's just 25 watts, or less!

It's this antenna pattern that lets tower climbers stay safe despite being only 20-30ft below an antenna pumping out 500 watts of power!

Granted, 500 watts is lower than our normal power of 1950 watts.  We did get one report from a Fall River listener who noted markedly more static than usual.  But that's pretty far on the fringe of our listening area, so that's to be expected.   The trick here is that wattage is a measurement of power and power is a logarithmic expression.  Ergo, the difference between 1950 and 500 watts is only -6 dB.  That's not a huge difference to most radios!  For example, a car radio has to constantly deal with much bigger swings than 6 dB all the time as the car moves around and signal levels change.  You could walk in front of a tabletop radio's antenna and your body could cause a much bigger drop than -6 dB.  That's just the nature of the beast.

So apologies if you noticed a slightly-weaker-than-usual 102.7 this past Saturday, but know it was for a good cause: avoiding having some nice tower guys get barbequed while doing their jobs!  :)