standard  mandates the use of the same maximum transmit
power with and without wide channels. In an OFDM system,
the transmit energy is uniformly distributed across the subcarriers.
Since wide channels use 114 subcarriers and the total
transmit power remains the same, the energy per subcarrier
is theoretically reduced to 49% of that of 20 MHz bands
(approximately halved). Expressed in dB, this translates to
about a 3 dB reduction in the energy per sub-carrier."
Did I read the PDF wrong? "40 compared to 20 is a 3 dB reduction in the energy per sub-carrier"
This is a quote from the CWNP Study Guides: "An even simpler way to estimate free space path loss (FSPL) is called the 6 dB rule (remember for now that decibels are a measure of gain or loss, and further details of dB are covered extensively in Chapter 3). The 6 dB rule states that doubling the distance will result in a loss of amplitude of 6 dB."