A couple of days ago during high wind conditions, I moved our den antenna around while tuning in the stations located to the east of Tulsa, near Coweta. (Generate this map for your location on this page at fcc.gov). I get excellent signal strength from all of these stations.
But some were affected by the wind. This is due to the movement of trees between my antenna and the transmitter.
Comparing my results with the data in the chart at bottom (click to enlarge), I observed that the stations with signal readings unaffected by wind were lower in frequency. The stations whose signals intermittently dropped to zero were higher frequency.
This is understandable. The higher the frequency, the more “line of sight” it is. High frequency FM radio has a much shorter range than low frequency AM for this reason.
Our “legacy” channels are KJRH-2, KOTV-6, KTUL-8 and KOED-11. Their antennas are roughly in the same place to the east (8 is a few miles away from the others).
When these channels went on the air in the 1950s (KOTV in late 1949), channels 2, 6, 8 and 11 were all VHF channels, on the lower end of the frequencies allocated for TV by the FCC.
Of those four, only KOTV-6 reception was affected by the wind. Why would it be different from the others?
It turns out that KOTV is the only one on a high frequency channel, despite its low number.
In Tulsa, we didn’t have any higher-band UHF channels until the 1980s (with one brief exception in 1954, KCEB).
Today in the digital era, there are lots of channels across the VHF and especially the UHF band, and most have subchannels.
For technical and administrative reasons, the need and desire was there for some stations to change frequencies. But it would be confusing for KOTV, billed as Channel 6 for decades, to show up on your TV as channel 55 (where it was in 2004), then channel 45, where it is today.
A way had been developed to let KOTV remain Channel 6. Though KOTV broadcasts on real channel 45, it has an alias of channel 6, its “virtual” channel.
During the transition to digital, real channels were shuffled around, but viewers continued to see 2, 6, 8 and 11 (or in the new subchannel nomenclature, 2.1, 6.1, 8.1 and 11.1).
Currently, virtual channel numbers 2, 6, 8 and 11 are assigned to real channels 8, 45, 10 and 11, respectively (only 11 has the same virtual and real channel). Virtual 6/real 45 alone is up in the high frequency range (see the table below).
Thus KOTV is the one of the four more vulnerable to dropouts in high wind conditions (at least as seen by my antenna from my house). If it bothers me enough, I could move my antenna to the attic, or outside on a high mast.
The maps and tables are a great help when you are looking for your house’s sweet spots, and understanding why they are where they are.