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I was digging around the SE region Chart Supplement, and I was reading KSRB (Sparta, TN), and this "VOR unusable" listing is contained in it:

Sparta TN Chart Supplement Page

The sectional chart shows that it is nearly on top of a hill. I've made some visual marks on this to help everyone visualize. Sectional over HCH

Hench Mountain is a Low Altitude VOR, so by normal standards, service should be clear all the way around at 1,000 ft above it's altitude:

Low Altitude VOR Service Volume

Why are there unusable areas more than 1,000Ft above the elevation of the VOR? I've flown over it, and there are no obvious obstructions, it sits on top of a flat hill as you can see in this video I found on Youtube. enter image description here

Ryan Mortensen
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    Google maps looking west... I could see there being obstructions in other directions but this seems pretty clear. Though it is the lowest restriction. – fooot Apr 21 '19 at 04:47
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    I can't cite a specific cause in this case, but VORs are subject to multipath and other propagation errors. These almost always occur at the lower altitudes. If the FAA flight check shows the omnibearing to be outside the allowable variation, that area will be declared unusable. In this area with highly variable terrain a strong reflection from a slope 10 to 40 degrees off a course could cause enough multipath to 'bend' the omnibearing by several degrees. – Gerry Apr 21 '19 at 13:45
  • See guidance reversal. – mins Jul 27 '20 at 18:52

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Either multipath including refraction induced errors or blocking/masking by terrain.

david innes
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