NEC-LIST: far field distance

I am looking for some book, note, paper or something else where it would
clarified the following formula to evaluate the far field distance:

Far field distance= (antenna gain * wave length)/(8*p)

What above is prescribed in Federal Aviation Administration AC 20-68B.

Please, can someone help me?

Thanks

Dr. Maria Pia Di Donato
Space Program - Avionics
CIRA Via Maiorise
81043 Capua (CE)
ph. ++39 0823 623254
fax ++39 0823 623335
e-mail <mailto:m.didonato_at_cira.it%20> m.didonato_at_cira.it

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