Electromagnetic test method

An electromagnetic test method simulates the effects of plane electromagnetic wave illumination on a conducting body. A conductive sheet is placed in the vicinity of the body and connected to an electrical ground. Alternating current from a current generator is injected into the conducting body via electrodes positioned at spatially separated points on the body's surface. The fields generated by the injected currents and the mirror currents in the grounded conducting sheet induce fields which create an overall current distribution in the conducting body substantially identical with that resulting from the incidence of a plane electromagnetic wave upon the body.

BACKGROUND TO THE INVENTION: 
The present invention relates to a test technique to simulate the plane 
electromagnetic (EM) wave illumination of a conducting body. Such 
techniques are used in the testing of military vehicles such as aircraft 
for resistance to the effects of high intensity wide-band electromagnetic 
radiation. In the past such testing has been carried out simply by using 
antennae and appropriate generating equipment. In order to generate high 
intensity fields around a body as large as an aircraft such systems have 
necessarily been physically large and have had very high power consumption 
levels. In addition to being expensive to build and run such systems 
suffer the further disadvantage that they create high levels of radio 
frequency interference. 
Illumination of conducting bodies at radio frequencies extending up to 
substantially 30 MHz is particularly difficult because of the large size 
of efficient radiation structures for these frequencies and the difficulty 
of focusing the radiated power on the body to be illuminated. Moreover it 
is found that at frequencies where the wavelength is 10's or 100's of 
metres that inductive coupling between the antenna and the aircraft occurs 
thereby distorting the electromagnetic field creating fields with 
spherical wavefronts and non-uniform field strengths. This distortion of 
the field severly restricts the diagnostic value of the test procedure. 
SUMMARY OF THE INVENTION 
According to the present invention, a test technique for the simulation of 
plane electromagnetic wave (EM) illumination of a conducting body includes 
the step of connecting a high frequency current to said body via a 
multi-point feed system arranged to produce a distribution of surface 
current on the body substantially identical to that generated by the 
body's exposure to a plain electromagnetic wave. In order to provide the 
required field characteristics a suitably shaped conducting sheet is 
placed close to the body but not in electrical contact with the body. 
If the surface current distribution on the body is the same as that 
obtained during illumination by a plane electromagnetic wave then the 
internal electromagnetic environment will be identical. The coupling of 
energy from the external field to currents induced on internal wiring 
within the conducting body is therefore simulated accurately. Any 
apertures in the body may be ignored from the point of view of electric 
field polarizibility where the materials concerned are moderate to good 
shields in the frequency hands used. If this is not the case then the 
value of normal field strength around the body must also be arranged to be 
equal to the scattered normal field in addition to the current 
distribution.

DESCRIPTION OF THE PREFERRED EXAMPLE: 
An apparatus for testing conducting bodies to simulate the effects of plane 
electromagnetic illumination of such bodies includes an alternating 
current generator capable of generating current at frequencies up to 30 
MHz and electrodes 2 connected to the generator. The apparatus further 
comprises a planar metal sheet 3 having dimensions of substantially the 
same order of magnitude as the body to be tested. 
In use the electrodes 2 are placed on an outer surface of the body to be 
tested. This body will typically be an aircraft fuselage 4. The conducting 
sheet 3 is placed close to the fuselage 4 but spaced apart from the 
fuselage 4 so that it is not in electrical contact. The conducting sheet 3 
is then electrically grounded. 
Current from the generator 1 is injected via the electrodes 2 into the 
surface of the fuselage 4. These currents in turn induce mirror currents 
in the conducting sheet 3. These in turn modify the current flow in the 
surface of the fuselage 4 to produce the desired current distribution in 
the surface corresponding to that produced by a plane electromagnetic wave 
impinging on the surface. For given boundary conditions around a closed 
surface there is a unique solution for the electric field within the 
surface. Therefore by creating the boundary conditions on the surface of 
the fuselage 4 characteristic of those produced by a plane electromagnetic 
wave a field is created within the fuselage 4 which is substantially 
identical to that resulting from illumination by a plane electromagnetic 
wave. The present apparatus is therefore able to simulate, for example, 
the effects of such a wave on components of the aircraft such as internal 
wiring routes 5. 
Although in the example described the conducting sheet 3 is flat other 
forms, such as a sheet configured to follow the curvature of the body 
being tested, may be used. The relationship between the conducting sheet 3 
and the position and magnitude of the injected current necessary to 
effectively simulate a plane electromagnetic wave are discussed more fully 
in the detailed theoretical discussion contained in the appendix. 
APPENDIX 
1. Introduction 
The purpose of this document is to describe the exact purpose of direct 
current injection (DCI), the concept of the DCI technique and the 
underlying mathematical theory. 
2. The Concept of DCI 
The following sections will explain; 
(a) the idea of natural impedance paths on a conducting body 
(b) how these impedance values may be used to compute the current 
distribution on a body undergoing EM illumination 
(c) how the presence of a nearby conducting ground plane affects the 
impedance on a body 
(d) a mathematical statement of the objective of DCI in terms of the above 
(e) considerations for an injection arrangement to achieve the simulation 
of plane wave illumination for two and three dimensional shapes 
2.1. Natural Impedance Paths 
Consider the cross section through a rectangular conductor (such as the CFC 
cylinder) as shown in FIG. (). Let RF current be introduced so that it 
flows along the length (z-axis, into the paper) of the conductor. It is 
known that the current will distribute itself circumferentially around the 
perimeter of the rectangle, and that a high frequencies (&gt;100 KHz) the 
majority of the current will flow along the sharp corner regions. The 
reason for this is easily explained using elementary physics. 
Take the cross section and divide it into N equal width strips. The applied 
voltage along the unit length (into the paper) of any one of the strips is 
equal for each and every incremental strip, yet the current flowing along 
each will differ according to position. The impedance presented by each 
strip is therefore dependent upon position. The impedance of any 
conducting path at high frequency will be determined by the magnitude of 
the TOTAL magnetic flux density linking that path. This is simply a 
statement of Lenz's law 
##EQU1## 
Thus, those paths linking fewest lines of magnetic flux, 0, will have the 
lowest impedance (inductive reactant). It can be easily seen therefore 
that the corner regions will link the smallest amount of total flux as 
they are the furthest removed regions on the body and flux-fields fall 
away with increased spacing. 
The impedance of any general strip, m, is given by 
##EQU2## 
where L(m) is the self-inductive reactance of path m, and is the same for 
each of the equal strip widths and may be written as L, and M(n,m) is the 
mutual inductive coupling reactance between strip n and strip m. 
The form of L and M(n,m) has been given in reference [3] and is given by 
##EQU3## 
where 
P is the total perimeter length of the body (m) 
N is the number of strip segments 
K is the wave number for the frequency (a) 
n is the free space wave impedance (Ohm) 
Y is Eulers constant and 
Ho.sup.(1) (K.vertline.Rn-Rm.vertline.) is a Hankel function (zero order, 
2nd kind) relating the field or flux quantity at vector position Rn to the 
specified point Rm. Ho.sup.(2) is a complex combination of Bessel 
functions and is a term most suitable for representing the damped and 
sinusoidally-varying real and complex terms of an emanating wave field. 
Thus the term that actually determines whether a path will have high or low 
impedance is the second summation in equation (2), and each of those terms 
are related to the spacing, .vertline.Rn-Rm.vertline.. The larger 
.vertline.Rn-Rm.vertline., the smaller the magnitude of Ho.sup.(2) 
(K.vertline.Rn-Rm.vertline.). 
Equation (2) will yield a value of Z for each and everyone of the N paths 
and we may write this in the matrix form below for the total surface 
impedance linking matrix 
##EQU4## 
This may be termed the `natural` impedance of the body (portioned into N 
segments) and will allow us to calculate the (N) induced surface current 
values distributed on the body via either plane wave EM illumination or 
indeed by end- current injection. 
It is this natural impedance compounded with the incident field conditions 
that determines the surface currents induced on the exterior of the body 
when illuminated by a plane wave EM field. 
2.2 Current Induced by EM Illumination 
We have computed the `natural` impedance of the body. Without entering the 
realms of the numerical technique in too great a detail, the induced 
surface currents on the exterior of the conducting body may be computed 
from the simple matrix equation 
EQU [E.sup.i]=[Zn].multidot.[Js] (6) 
where 
[E] is a matrix of values of the incident plane wave EM field over the 
surface 
[Z] is the natural impedance matrix 
[Js] is the induced currents 
Hence, we can solve for the plane-wave induced surface currents at the N 
segment locations simply by inverting the [Z]matrix and producing with the 
[E]matrix: 
EQU [E.sup.i].multidot.[Zn].sup.-1=[Js] (7) 
2.3. Modifications of Natural Impedance Paths 
It is possible to modify or perturb the natural impedance path values of 
all, or some, of the segments of the body by the use of a ground or 
earthed plane. 
When we site a conducting body above an earthed plane, and induce RF 
current flow on the body, a scattered EM field will be set up around the 
object with the provision that the TOTAL electric field tangential to the 
surface of the earth plane will be zero for all points on it's surface. 
The most common and simplest way of visualising this is to imagine the 
creation of a virtual image of the conducting object created on equal 
depth away from the ground plane but on the opposite side to the real 
object. See FIG. (2). This concept is identical to that used in mirror 
optics. 
The plane-tangential currents in the virtual image will flow in the 
opposite sense to those in the actual conductor and hence the total 
electric field at the mid point between them (ie. the earth plane) is 
entirely cancelled. 
This also has the effect that the flux coupling with each of the N 
segmental paths on the real conducting body is modified since the 
effective flux coupling with each now takes the form; 
##EQU5## 
where M'(n,m) is the inductive coupling reactance with the image in the 
earth plane, and Z'(m) refers to the modified impedance of path m, and 
differs from the natural impedance Zn(m) by the third term in equation 
(8). 
It should be noted that the magnitude of this new third term is dependent 
upon the closeness of the conducting body to the earth plane: it is a 
function of the spacing, R. 
Now consider what would happen if we did not use a planar ground sheet, but 
an arbitrary surface instead. The currents flowing on the conducting body 
would still form an image, of distorted shape certainly, to counteract the 
scattered fields so that at the ground plane, the total tangential 
electric field components remained zero. The impedance paths on the body 
will be some other value, Z"(m), different from those both in equations 
(2) and (8), and the current distribution for a given EM stress will be 
distinctly different once again. 
We now begin to see that we may manipulate, in theory at least, the 
impedance paths on the body to be ANY value we so choose, thus controlling 
the current distribution on the body for any given EM stress conditions, 
whether that may be plane wave illumination or current injection. 
2.4. The objective of DCI: A Mathematical Statement 
It is the stated objective of the Direct Current Injection (DCI) technique 
to simulate the external surface current distribution on the conducting 
body by means other than by free-field. HF band plane-wave illumination 
which is very demanding to reproduce at the field strength levels required 
in the HF band (far field). 
From a consideration of the previous three sections, it should now be clear 
that given we can stimulate or excite the airframe at a specified, and 
limited, number of points on the body, the exciting matrix set [Ee]is thus 
set. 
If we are to achieve the desired current distribution, represented in it's 
matrix form at the N locations on the body as [Js], then we must achieve 
the balance given by 
EQU [Js]=[Zn].multidot..sup.-1[E.sup.i]=[Zp.multidot..sup.-1[Ee](9) 
which is a statement of 
##EQU6## 
The perturbed impedance matrix set, [Zp], to be sought is therefore 
mathematically defined in equation (9). 
2.5. Considerations for an Injection System 
We are free to choose either the ground plane shape and then accept the 
injection point positions, levels and relative phases so defined, or 
alternatively, fix the injection system and thus accept the ground plan 
shape defined. 
It is more than likely that the injection arrangement and ground plane 
shape will vary with frequency, but in the light of experience from RAE 
predictions on the CFC fuselage, we do not expect the shape or 
distribution of either to vary considerably over the LIMITED frequency 
range (1-30 MHz) of our interest. 
We believe that a `sensible` choice of ground plane should be chosen to 
envelop the body, compute the perturbed impedance and hence the injection 
levels. This may need to be an iterative process.