[SBE] NEC/4 Modeling

[Cris Alexander]

Yes, and again, impedance prediction is really the weakness of any
moment-method modeling core. It's a handy feature and it gets you in the
ballpark, but out here in the real world there are many factors that a
moment-method model cannot account for. We use nodal analysis to account for
some of these to good effect, but with the earth and ground system acting as
one big plate of a capacitor in close proximity to the tower base, and
everything in the antenna environment having some amount of incident and
distributed capacitance to that big plate, there are a whole lot of unknowns
that you just cannot solve for.
 
When you've been doing these things for a lot of years, you come to realize
that there is a point of diminishing returns not too far down the model
path. Most of us have pretty much figured out what that is and we don't
waste a lot of time trying to resolve things any further.

Good luck with this project. Modeling is a lot of fun, but it can be a
little tedious, too. Stay with it and you will be rewarded with results that
are more than reasonable.
 
______________________________
W.C. Alexander, CPBE, AMD, DRB
Crawford Broadcasting Company
Denver, CO
(303) 433-0104 Tel.
(303) 433-0905 Fax.
 
From: sbe-bounces at sbe.org [mailto:sbe-bounces at sbe.org] On Behalf Of Paul
Christensen
Sent: Thursday, May 17, 2012 9:23 PM
To: sbe member discussion mail list
Subject: Re: [SBE] NEC/4 Modeling
 
Cris,
 
Good info.  I've also noticed a high degree of computed Z variability under
both NEC-2 and NEC-4.  It seems the computed Z for free space designs (e.g.,
Yagi) model much more accurately than ground-dependent radiators.   
 
Paul
 
 
 
====================================
Paul Christensen, CPBE CBNT 8-VSB AMD
LAW OFFICE of P. B. CHRISTENSEN, P.A.
3749 Southern Hills,  Jacksonville, Florida 32225
Office: (904) 379-7802    Facsimile: (904) 212-0050
pchristensen at ieee.org
 
 
 
 
----- Original Message ----- 
From: Cris Alexander <mailto:crisa at crawfordbroadcasting.com>  
To: 'sbe member discussion mail list' <mailto:sbe at sbe.org>  
Sent: Thursday, May 17, 2012 5:48 PM
Subject: Re: [SBE] NEC/4 Modeling
 
Okay, Paul. I understand.
 
I've done a ton of modeling over the years and in my experience, the far
field E-field differences between a modeled radial system and a perfect
ground plane are hardly worth all the trouble to model it all. We're talking
fractions of a dB at 1 km, and the base Z predictions are certainly in the
ballpark if not the infield. That being said, impedance prediction is
probably the weakest function of NEC/MN.
 
I can heartily endorse J.L.'s book and in fact the equations therein were
the basis for my own MN program (ACSModel).
 
______________________________
W.C. Alexander, CPBE, AMD, DRB
Crawford Broadcasting Company
Denver, CO
(303) 433-0104 Tel.
(303) 433-0905 Fax.
 
From: sbe-bounces at sbe.org [mailto:sbe-bounces at sbe.org] On Behalf Of Paul
Christensen
Sent: Thursday, May 17, 2012 2:30 PM
To: sbe member discussion mail list
Subject: Re: [SBE] NEC/4 Modeling
 
Hi Cris,
 
I'm trying to get accurate base Z results as well as surface wave field
strength predictions.  Within 4Nec2 is a fast radial wizard, but it does not
allow for changing ground placement, albeit the effect may be small.   In
any case, creating a buried system is easy enough through the geometry
builder utility.  The examples would be used to ensure that I've done the
job correctly.  
 
I've had this discussion with Arie Voors, 4Nec2's developer, as well as
Jerry Burke at LLNL.  The consensus is to create a modeled radial system for
best results.   Ben Dawson suggested J.L. Smith's examples, so I can start
there.
 
Regards,

Paul


===================================
Paul Christensen, Esq.
LAW OFFICE of PAUL B. CHRISTENSEN, P.A.
3749 Southern Hills
Jacksonville, Florida 32225
Office: (904) 379-7802
Facsimile: (904) 212-0050
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----- Original Message ----- 
From: Cris Alexander <mailto:crisa at crawfordbroadcasting.com>  
To: 'sbe member discussion mail list' <mailto:sbe at sbe.org>  
Sent: Thursday, May 17, 2012 4:12 PM
Subject: Re: [SBE] NEC/4 Modeling
 
Is this for AM broadcast? If so, set the environment for ground plane and
forget about trying to model a buried ground medium.
 
______________________________
W.C. Alexander, CPBE, AMD, DRB
Crawford Broadcasting Company
Denver, CO
(303) 433-0104 Tel.
(303) 433-0905 Fax.
 
From: sbe-bounces at sbe.org [mailto:sbe-bounces at sbe.org] On Behalf Of Paul
Christensen
Sent: Thursday, May 17, 2012 1:39 PM
To: sbe member discussion mail list
Subject: [SBE] NEC/4 Modeling
 
Looking to validate the buried radial parameters of a .NEC input file for a
non-phased, single monopole radiator.  I've been using the geometry builder
utility within 4Nec2 software, but I want to confirm radial depth results as
well as source/load parameters.  If anyone has an input file to share, it
would be very much appreciated.  I have J.L. Smith's book on order and will
use his examples as well.  Thanks!
 
Regards,
 
Paul
 
 
====================================
Paul Christensen, CPBE  CBNT  8-VSB  AMD
LAW OFFICE of P. B. CHRISTENSEN, P.A.
3749 Southern Hills,  Jacksonville, Florida 32225
Office: (904) 379-7802    Facsimile: (904) 212-0050
 <mailto:pchristensen at ieee.org> pchristensen at ieee.org
 

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