Re: NEC-LIST: far field distance

Quoting Maria Pia Di Donato <m.didonato_at_cira.it>:

> 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?

Posso aiutare?

The above equation is similar to the effective aperture (but not quite),
which can be given as:

(Gain * [lambda^2])/(4*pi)

So your equation is AE/(2*lambda)

Now, "a rule of thumb" is that effective aperture is about half the
physical aperture, so *if* the physical aperture is taken to be square
with side length d, then AE =~ (d^2)/2 so I think your equation
approximates to:

(d^2)/(4*lambda)

Now the traditional far field approximation is 2*(d^2)/lambda, giving
22.5 degrees max phase error, but for rough work (i.e. EMC, rather than
antenna
measurements!!) it is common to use (d^2)/(2*lambda), which tolerates 90
degrees max phase error. Thus your equation tolerates an even bigger
phase error, but I would not be surprised at this, as it is a *maximum*
error and the real-world value will usually be much smaller.

I guess that the document you have quoted is for some very approximate
application, such as RF hazards or EMC, not antenna measurements.

Spero che quest e' usevole! (E senza errore)

Peter Excell
Bradford University and University of Wales, UK

>
>
>
> 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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