Monday, June 2, 2014

Why do antenna towers top out at about 2000 feet?

I thought this was interesting, so I'm passing it on - that's the purpose of a blog anyway, isn't it?

As you look at an aviation sectional chart, you will see more and more tall antenna towers. I know of one northwest of Dallas that I once came across while flying - the top was in the clouds but I was in perfectly legal VFR. That's tall! It's this one on the chart, topping out at 1999 ft AGL, 2859 MSL.

As you look around the chart, you see a lot of towers that are 1999 or 2000 feet tall, and some that are a little bit taller - 2008 feet for example. Or this one at 2063 northwest of Fargo, North Dakota (it's also the tallest structure in the U.S., and the third tallest in the world).

But you don't see any that are taller than that. You don't see any towers 2300 feet tall, or 2500 feet tall, or even taller. Why not?

Aside from the engineering challenges and massive expense of a tower that tall, there's another reason - regulatory. Both the FCC and the FAA have established a "rebuttable presumption" against structures over 2,000 feet tall. This means that unless proven otherwise by the proponent, the FCC considers antennas taller than this to be inconsistent with public interest, and the FAA will presume anything taller to be a hazard to air navigation.

From the FCC's part of the CFRs - 47 CFR 1.61, the note at the end:

Applications for antenna towers higher than 2,000 feet above ground will be presumed to be inconsistent with the public interest, and the applicant will have a burden of overcoming that strong presumption.

It states that this has been their view since 1965.

The FAA has similar wording in 14 CFR 77.7(d). (Some references still say 77.17(c), this was changed in 2011.)

If you propose construction or alteration to an existing structure that exceeds 2,000 ft. in height above ground level (AGL), the FAA presumes it to be a hazard to air navigation that results in an inefficient use of airspace. You must include details explaining both why the proposal would not constitute a hazard to air navigation and why it would not cause an inefficient use of airspace.

One more note about that 2063-foot tall tower in North Dakota - it was constructed in 1963, before the FCC's establishment of their current policy. Might there be a connection?

More information at the FCC's web site - see the last two paragraphs at:

(Notice the outdated reference to 14 CFR 77.17(c).)

Look out for those towers!

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