Some decades ago, the FCC used to require that US amateurs log every single QSO. And those logging requirements could be somewhat complex, especially for third-party traffic with other countries.
It's the law because
those 5 channels are shared with other licensed commercial services. If we interfere with those, then the FCC might pay us a personal visit and see what our ERP is and what it might have been with our 'previous' 60m antenna (indicated, of course, by our log).
Here are the 60m rules.
Amateurs may transmit with an effective radiated power of 100 W or
less, relative to a half-wave dipole. If you’re using a commercial
directional antenna, FCC Rules require you to keep a copy of the
manufacturer’s gain specifications in your station records. If you
built the directional antenna yourself, you must calculate the gain
and keep the results in your station records.
When using a directional antenna, you must take your antenna gain into
account when setting your RF output power. For example, if your
antenna offers 3 dB gain, your maximum legal output power on 60 meters
should be no more than 50 W (50 W plus 3 dB gain equals 100 W
Effective Radiated Power).
It's intended to make sure that the ERP does not exceed those limits.
How can I be sure I don't exceed the power limit?
The FCC said hams
may run 100 W effective radiated power (ERP) on the five 60-meter
channels. The new rules say, "For the purpose of computing ERP, the
transmitter PEP (peak envelope power) will be multiplied by the
antenna gain relative to a dipole or the equivalent calculation in
decibels. A half-wave dipole antenna will be presumed to have a gain
of 0 dBd." This means if you use a half-wave dipole (about 87 feet 3
inches for the "middle" channel according to the formula), set your
transmitter's output power for 100 W PEP (many transceivers' meters
can be set to indicate peaks), and you should be in compliance. The
FCC asks licensees using antennas other than half-wave dipoles to
"maintain in their station records either manufacturer data on the
antenna gain or calculations of the antenna gain." This is a new
record-keeping requirement for amateurs. The "best" antenna
configurations are those with a proven track record on the lower
bands, keeping in mind that using a loop or an array of some kind will
require you to "do the math" to ensure you are not radiating more than
100 W ERP in any direction. For the math you must reduce your power by
the number of decibels your antenna gain exceeds 0 dBd (0 dB relative
to a half-wave dipole). Conversely, you can increase your transmitter
power if your antenna exhibits loss compared to a dipole. Be prepared
to document these situations in your station records.
More information from the Federal Register