Terman treats decibels on page 8 of the 1955 third edition of
"Electronic and Radio Engineering".

The value of the decibel is 10 log of the power ratio.

When the ratio of the powers is 4, the decibel value is 6. This 6 dB
value applies among other things to doubling the distance between a
transmitting and receiving antenna. Half as many volts will be induced
in the receiver and will result in half as many amps. This is 1/4 the
power induced at half the distance between antennas..This may be
expressed as a positive power ratio of 4 to 1, or 6 dB.

Best regards, Richard Harrison, KB5WZI