[TowerTalk] Modeling Ladderline as an Antenna Element

[Jim Lux]

At 10:05 AM 10/24/2005, Joe Giacobello wrote:
>Jim and Tom, many thanks for your replies.  Let me tell you what I'm up to.
>
>I would like to put up an 80M EDZ between two towers.  However, my towers 
>are only 300' apart and I need about 340'.  I have considered drooping the 
>ends vertically, but I thought either a folded dipole or linear loading a 
>few feet on the ends with ladderline would allow me to fit it inside the 
>300' span.  I use EZNEC 4+ for modeling.
>
>I wonder if minimal lengths of ladderline for linear loading on each end 
>would provide the shortening while making the high resistance losses 
>arising from lower Q negligible?  I haven't tried modeling it yet.  I 
>thought I'd first see if anyone had successfully modeled ladderline as an 
>antenna element.

I'd try approximating it as a couple insulated wires with insulation 
thickness comparable to the average thickness and see where it gets 
you.  Then, try changing the thickness a bit and see if it makes a 
significant (i.e. more than a percent) difference in the efficiency.

If you're interested in looking into how NEC4 models insulated wires in 
more detail, L.B.Cebik's site talks about it a bit, and you can also look 
for Burke's paper on it
Might be at: http://www.llnl.gov/tid/lof/documents/pdf/210389.pdf

The stuff on insulated wires starts on page 15 of the pdf (section 4.4 of 
the report).  They validated the NEC model by comparing against a dipole 
with insulated wires, as well as some other approaches.

Here's the bibliographic reference if you need to search further.
Title:
Recent advances to NEC (Numerical Electromagnetics Code): Applications and 
validation
Authors:
Burke, G. J.
Affiliation:
Lawrence Livermore National Lab., CA.
Journal:
Presented at the Conference on Modern Antenna Design Using Computers and 
Measurement Application To Antenna Problems of Military Interest, Ankara, 
Turkey, 19-20 Oct. 1989
Publication Date:
03/1989
Category:
Communications and Radar
Origin:
STI
NASA/STI Keywords:
ANTENNA DESIGN, COMPUTER AIDED DESIGN, COMPUTER PROGRAMS, ELECTROMAGNETISM, 
MATHEMATICAL MODELS, ELECTRICAL INSULATION, INTEGRAL EQUATIONS, WIRING
Bibliographic Code:
1989STIN...9011917B

Abstract

Capabilities of the antenna modeling code NEC are reviewed and results are 
presented to illustrate typical applications. Recent developments are 
discussed that will improve accuracy in modeling electrically small 
antennas, stepped-radius wires and junctions of tightly coupled wires, and 
also a new capability for modeling insulated wires in air or earth is 
described. These advances will be included in a future release of NEC, 
while for now the results serve to illustrate limitations of the present 
code. NEC results are compared with independent analytical and numerical 
solutions and measurements to validate the model for wires near ground and 
for insulated wires.


>73, Joe
>
>P.S. Tom, I wanted to comment on your replies regarding  tuners with balun 
>inputs. I will do so in a separate post later.
>
>Jim Lux wrote:
>>
>>At 02:32 PM 10/23/2005, Joe Giacobello wrote:
>>
>>>
>>>I am thinking about using ladderline for a folded dipole or for linear
>>>loading a wire antenna.  I wonder if anyone has successfully modeled
>>>ladderline and, if so, what parameters they used for the insulation.  I
>>>assume that the insulation is PVC, but what is the thickness and how are
>>>the alternating windows accounted for?
>>>
>>>I'd appreciate any information that members of the reflector can provide
>>>on this subject.
>>>
>>>73, Joe
>>>K2XX
>>>
>>
>>
>>what program are you using to model it?
>>If NEC, insulation models assume uniform thickness around the conductor,
>>which is not representative of ladder line, so the "fine scale" details of
>>the field in proximity to the ladder line will not be correct.
>>
>>
>>Since the windows are very much less than a wavelength long, you can
>>probably model it as a suitably insulated pair of wires with insulation
>>properties chosen to match the actual properties of the wire.
>>
>>Even better, you could probably represent the ladder line as a suitably
>>built NT card for the transmission line properties, and a single insulated
>>wire to represent the "common mode" properties.
>>
>>If you're hoping to model the effects of things within, say, 5 times the
>>cross sectional size of the ladder line, NEC is probably not a good tool.
>>You need something like HFSS or one of the other programs that models
>>dielectrics.  Method of Moments (MoM) just isn't all that great at handling
>>dielectrics.
>>
>>
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