rooby84 said:
I want to simulate a crossed-loop antenna at 100 KHz.
Sounds like a Bellini-Tosi direction finding system to me.
http://en.wikipedia.org/wiki/Radio_direction_finder#Bellini-Tosi
At 100KHz the wavelength will be 3 Km, so I expect your loop diameter will be small when measured in wavelengths. The field pattern will therefore be that of a simple dipole.
Because your loops are orthogonal, (at 90°), they will not interact. For fixed stations it is usual to mount the two loops, one with N-S alignment, the other with E-W. On a ship one is mounted in the fore-aft plane, the other in the port-starboard plane.
This antenna configuration is not usually modeled with NEC type software. Analysis is best done by treating each loop independently, as an inductive loop with a capacitance to bring it to resonance.
There is a problem that comes from 100 KHz being in the middle of the “switching power supply” band. As a single turn loop you will pick up the local LF electric field noise. But if you have an even number of turns in each loop you can Earth the mid point where you take the balanced feed from the ends of the loop, usually at the bottom of the supporting tower. A single capacitor between the balanced feed points will bring the antenna to resonance.
A Faraday shield, (screen), can be installed in the form of a conductive tube or coaxial braid about the wire loop, that is also earthed at the base, but must have a short insulated gap symmetrically positioned, to prevent circulating currents. That system will respond only to the LF magnetic component of the signal. Most local electrical noise will then be rejected.
You will need to know the shape of the loop, it's size and the number of turns. That will permit calculation of loop self inductance and the value of the neutralisation capacitor required. I have equations to solve for regular polygon shapes, but to give the right ones, I will need to know;
What shape and size will your loops be?
You will also need to decide on the bandwidth. That can be adjusted by selecting the diameter and resistivity of the wire used to make the loop, or by using a resistor at the tuning capacitor to lower the Q of the resonance.
If you want to tune to a narrow band or channel you will want high Q. But if you want to use the output of the two loops on an x-y display to get the direction of signals or thunderstorms, you will need to use a much lower Q so as to reduce sensitivity to the phase error between the loops.
What bandwidth do you require?
It is common with crossed DF loops to have a very small third loop at 45° to the two main loops. That small loop is used to generate a milliwatt of test signal while trimming the capacitance to tune the loops to the same phase.
I'm sorry about the protracted process, but it will all be worth it in the end.
