TopBand: NEC-4 info

[Earl W Cunningham]

On Tue, 17 Mar 1998 20:09:01 EST km1h@juno.com (km1h @ juno.com) writes:
>I have been advised that NEC4.1 computations have been included with 
>an Al Christman  article in the ARRL Antenna  Compendium # 5. It is 
>supposed  to include extensive math for those so inclined .
=================
I just read the Christman article in Compendium #5.  Some very
interesting comparisons were made via modeling software (no math at all
in the article, but many tables of comparison data).

EZNEC (NEC2-based) was used to model a 160m elevated 1/4-wave vertical
with 4 to 36 elevated 1/4-wave radials at heights from 10 to 60 feet
above "good" (.004 S/m) ground.  It was apparent from the modeling that
going beyond 8 radials yielded very little advantage in gain.

Christman used NEC4.1 to model a ground-mounted 1/4-wave vertical with
from 4 to 120  1/4-wave radials (buried 2" beneath the surface) to
compare with the elevated models.

Results showed that the 10-foot high elevated model with 8 radials was
only 0.13 dB down from the ground-mounted vertical with 64 buried
1/4-wave radials.  Going to 120  1/4-wave radials improved the
ground-mounted vertical by 0.11 dB.
(I wish he had used 4/10-wave radials in the 120-radial model, since
studies by Brown et al. show that to be the optimum length for 120
radials.)

One of Christman's conclusions is, "When modeling elevated vertical
antennas, EZNEC (and presumably, other NEC2-based software packages)
appears to give results which are nearly identical to those derived from
the more recently developed NEC-4.1".  His gain comparisons using EZNEC
vs NEC-4.1 of the same elevated radial models were within 0.02 dB of each
other (and within 0.02 dB of my elevated radial models using EZNEC -- our
impedance and takeoff angle figures are virtually identical).

BTW, EZNEC cannot model buried radials, but NEC-4.1 can.

In the thread about elevated radials, I keep reading about gain
improvements of 5 dB or so when going fom X to Y number of radials. 
Christman's NEC-4.1 models of the ground-mounted vertical show a gain
improvement of only 1.89 dB when going from 4 to 120 buried 1/4-wave
radials (another reason I wish he had tried 4/10-wave radials).

For elevated radials in the models, the gain improvement for going from 4
to 36 radials is 0.125 dB for the 160m vertical 10 feet above ground and
0.09 dB for the same vertical 60 feet above ground.

73, de Earl, K6SE


_____________________________________________________________________
You don't need to buy Internet access to use free Internet e-mail.
Get completely free e-mail from Juno at http://www.juno.com
Or call Juno at (800) 654-JUNO [654-5866]


--
FAQ on WWW:               http://www.contesting.com/topband.html
Submissions:              topband@contesting.com
Administrative requests:  topband-REQUEST@contesting.com
Problems:                 owner-topband@contesting.com