Correction in the last formula. To remind you, this is the "pillbox"
antenna having a length of 1 meter and a height of 0.07 meter. The
frequency is 9000 kmc (or GHz if you like).
From: drcuthbert
Sent: Thursday, April 29, 2004 4:57 PM
To: 'Price, Andrew P (UK Basildon)'; IEEE EMC-PSTC E-mail Forum (E-mail)
Subject: Near field/far field calculations
Andy,
I ran another simulation. This time with a collinear array 1/4
wavelength above a reflecting plane (perfect ground). Very similar I
think to the antenna you described to me. The collinear is 0.6 meters
long. I classify this as an aperture antenna. I would have gone to a 1
meter length but the version of NEC I have only supports 20 sources and
I didn't take the time to build the collinear with one source. Here are
the results showing the antenna current on a short dipole and then a
small loop placed at distance DX:
Dipole Loop
DX Sim Normalized Delta Sim Normalized Delta
1m 118uA 365uA 10dB 69uA 215uA 10dB
2 63 182 9 38 108 9
4 69 91 2.4 41 54 2.4
8 41 46 1.0 25 27 0.7
16 22 22 0.4 13 13 0
32 5.7 5.7 0 3.4 3.4 0
64 2.9 2.9 0 1.7 1.7 0
The (2D^2)/lambda formula says the near field extends to 22 meters. This
simulation doesn't show the field deviating from 10LOG(D/d) beyond eight
meters. And in this close the variation from 10LOG(D/d) is due to the
phase difference from the different portions of the antenna. Note that
the current in the short dipole and in the small loop vary almost
identically and do not show that the E/H ratio of the wavefront is
deviating from 377 ohms. I conclude that the (2D^2)/LAMBDA formula does
not describe this situation and that the near field does not extend far
>from the antenna.
Dave Cuthbert
Micron Technology
From: [email protected]
[mailto:[email protected]] On Behalf Of Price, Andrew P
(UK Basildon)
Sent: Tuesday, April 27, 2004 3:10 AM
To: IEEE EMC-PSTC E-mail Forum (E-mail)
Subject: Near field/far field calculation
Hi all,
Can anyone help I am having problems calculating the near field/far
field boundary for a 9GHz Antenna which has a length of 1035mm.
Regards
Andy
Andrew Price
Principal Development Engineer (EMC Specialist)
BAE SYSTEMS Avionics
A125
Christopher Martin Road
Basildon, Essex
SS14 3EL
tel: +44 (0) 1268 883308
email: [email protected]
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