Re: NEC-LIST: far field distance

Maria -

Here is (IMO) an excellent discussion of the various expressions for the
near field/far field boundary:

http://www.edn.com/contents/images/150828.pdf

   Kevin Doherty
   Stensat Group

> 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
>
> --
> The NEC-List mailing list
> NEC-List_at_robomod.net
> http://www.robomod.net/mailman/listinfo/nec-list
>

-- 
The NEC-List mailing list
NEC-List_at_robomod.net
http://www.robomod.net/mailman/listinfo/nec-list