Re: Combining fields.

This is a reply on a question on nec-list with some additional
information provided by Peter Richeson reproduced below:

> From: richesop_at_matrix.hv.boeing.com (Peter Richeson)
> Subject: Combining fields.
> To: fourie_at_odie.ee.wits.ac.za
> Date: Wed, 6 Mar 96 7:18:22 CST

> What I have is a closed cylinder that has cables penetrating it. The cables on
> the external are exposed to 60V/M. I need to determine what the fields are
> inside the cylinder.
>
> I had planed to do it in two steps. 1) model the cables on the outside and get
> a current. Use that current as the driving force (turn up the voltage untel the
> current on the first segment inside is equal to the current I got from the
> external model.
>
> Cylinder is 8 meters long and 4 meters in diameter.
> Freq. I am using is 300 MHz. Freq of concern is 200 MHz to 10GHz.
> Number of cables is 10.
>
>
> I am asuming the fields inside will be worst (highest) at the low freq. due to
> cable losses.

The simplest way of modelling you situation would be to actually
create a NEC2 model (wire grid for the cylinder) and pass the
penetrating wires through "holes" in the grid. Then use the applied
E-field excitation (normally 1 V/m) and request near fields from NEC2
for the inside of the cylinder. These fields should be quite
representative except for points close to the wire grid surface
itself where the discrete nature of the wire grid model gives
inaccuracies compared to actual solid surfaces. My "gut feel" for how
close is for points removed further than 2/10th wavelength (perhaps
Gerry Burke will suggest a more experienced limit). The other
potential problem is that of "numerical or non-real resonances"
inside the cylinder cavity. I believe that these are less likely when
you have wires inside the cylinder.

The NEC2 model will be quite achievable at 200 MHz - which will
produce, as you noted, the highes levels of E-fields inside. The
problem size I roughly estimate would require between 2500-5000
segments (50 MByte and 200 Mbyte matrix storage respectively).
We have modelled quite a few aircraft with similar dimensions at
these frequencies (Dakota or DC3, Helicopters, C-130 etc). Some
of the larger problems required a multi-workstation approach, and
we have a self written NEC variant which is capable of executing one
problem in parallel on workstations connected to a TCPIP LAN. If some
symmetry exist it will naturally reduce memory and time constraints further.

We also have a gridding package, Structure Interpolation and Gridding
(SIG), which can be used to grid the cylinder and wires
simulataneously. It will require a bit of care (or manual removal of
grid segments to ensure that the penetrating wires are far enough away
from the grid wires (normally we will actually use the package to
ensure connection to some gridded structure!).

I hope some of this is useful and will appreciate other members
comments on this approach.

Regards
Andre Fourie

e-mail: fourie_at_odie.ee.wits.ac.za
Tel: intl + 27 11 716 5386
Fax: intl + 27 11 339 4610
Address: Dr. APC Fourie, Dept Elec Eng, PO WITS, 2050, South Africa
Received on Thu Mar 07 1996 - 21:53:00 EST