- Basic theory
- Octave script
- Bessel's in c-code
- Ad hoc formula
- Coaxial tube
- Hankel: A solution for the outer tube
- 9. Bessel-bignum

The E-field is a vector, but in the discussion below only the component parallel to the axis is of interest.

The usual technique to solve such an equation is to extract a fixed frequency time function and write the equation as

For a metal wire the second term in this equation can be dropped, unless we are interested in the optical range.

For a circular wire it is convenient to use cylindrical coordinates:

In cylindrical coordinates, if we assume a rotational symmetry around the z-axis, and z-independence, the equation can be written

This is Bessel's differential equation with n=0, and solutions are the Bessel functions J0 and Y0. The one of interest here is

The other solution Y0, has a singularity in the center and is not of interest for a solid wire, but useful when we need the solution for e.g. the outer conductor of a coaxial tube.

More about Bessel functions.

With this solution as a base, the field can be written as proportional to it:

The expression gives the electric field inside the wire, and the current density is proportional to the field according to Ohms law.

For simplicity, let **A** denote the root-expression:

Note that A is complex and E, J0, J1, g and z are complex entities.

To get he total current in the wire integrate the current density from
radius 0 to radius R. R denotes the boundary radius, i.e. half the wire diameter.

The interesting solution is the one that fulfills the boundary condition at the wire surface r = R. When one volt is connected over a wire of one meter length, the field at the boundary is one V/m.

Assuming this boundary field we get the current i_ac in the wire as the first formula below, and the wires impedance z is simply 1/g, as in the second formula below. (There is a recurrence formula for the Bessel functions implying that the integral J0(Ar)*r*dr is simply r/A*J1(r). [Standard Mathematical Tables]

Also check a nice animation (for a plane wave) at
<http://fermi.la.asu.edu/w9cf/skin/index.html >

Markus Meisinger has pointed out to me that the wire conductance 1/z, can also be represented as a sum over roots to the Bessel functions.

Please read more about roots here.

Here you can find an Octave script Bessel that does the job.

For a wire of

freq, delta, L=imag(z)/omega, real(z)/R_DC, tube model, Col4/Col5 Hz [m] [H] [ratio] [ratio] [ratio] 1e+00 0.066006 -5.000000e-08 1.000000 1.000000 1.000000 1e+01 0.020873 -5.000000e-08 1.000000 1.000000 1.000000 1e+02 0.006601 -4.999998e-08 1.000001 1.000000 1.000001 1e+03 0.002087 -4.999829e-08 1.000069 1.000000 1.000069 1e+04 0.000660 -4.982952e-08 1.006822 1.000000 1.006822 1e+05 0.000209 -3.918752e-08 1.451263 1.513669 0.958772 1e+06 0.000066 -1.315192e-08 4.049728 4.055194 0.998652 1e+07 0.000021 -4.173172e-09 12.231119 12.232539 0.999884 1e+08 0.000007 -1.320079e-09 38.126500 38.126924 0.999989

The following plots are produced from this script are showing
the wire impedance, R/R_dc and L, for various frequencies.

These entities are plotted versus **beta*r**,
where

**beta = sqrt(omega*sigma*u0/2) = 1/skin_depth ;**

These functions only take real arguments, but we need to use complex arguments to calculate wire impedance.

Fortunately they can be expressed as a series that can be
expanded with complex arguments.

Here is a c-routine:
**jn_series.c** for this.

The series expansion method is problematic for arguments > 20 because the terms grow.

For arguments > 20 it is better to use the formula

and do the integration numerically.

Here is a c-routine: **jn_exp_int.c** for this.

Unfortunately this routine is not very fast, but it will only take a few milliseconds to run
on a computer.

Find also a **program** including functions that
computes a wires impedance per meter.

The formula is accurate to eleven digits over the whole range, although the the accuracy remains to be proved for beta*R-values over 500. There is no reason to believe that something strange should happen there, however.

It would be possible to make the formula a little faster by adapting polynomials to lower ranges of beta*R, but the number of coefficients grow. The formula above is a reasonable compromise.

Those who do not believe in these calculation can find a
**raw integration**
of the wave equation. You can compare the result, and for high values of
integration steps (N) the results should approach each other. Check it out.

The term

L_internal is what is shown in the plot above. It is what the Octave script says.

L_external is easy to compute:

L_internal is sometimes given as

It is amazing to observe
that the internal inductance is not only exactly **u0/8pi**, it
is also exactly **50 nH/m**. The only deviation from exactness
is the fact that the metal might have some
relative permeability, if so we use **L_internal = u0.ur/8pi**,
and the fact we do not live long enough to reach zero frequency.

Assume we have a circular symmetric coax tube with radii R1, R2 and R3.

R1 is the radius of the inner conductor, R2 is the inner radius of the
tube and R3 is the outer.

Since there is no restriction regarding the center, the Bessel function J0 is not the only solution to the equation. The general solution is a linear combination of J0 and Y0.

As the Y0 reach high values for big abs(arg), it would seem more natural to choose the Hankel function as a base. The Hankel H0 of first kind is a certain combination of J0 and Y0 that reach low values for big abs(arg). It represents an outgoing wave.

So our ansatz will be:

- Ez= c0*J0 + c1*H0
- Find the values of c0 and c1 that meet the proper conditions.

The boundary conditions are not directly available, but let's assume that the coax tube is short compared to the wavelength and that the tube is not grounded at both ends. Under this assumption the current flowing into the central conductor will return through the outer tube. It is not strictly true, because part of the current can flow as a displacement current in the outer space, but if the tube i short, this fraction can be neglected. Let us arbitrarily set the current through the tube to one ampere.

So the following two conditions are to be met:

- The current in the tube is one ampere
- The magnetic B-field at R3 is zero

Now, the theta component of curl(E) is dEr/dz - dEz/dr. The first term is zero at the boundary R3, so the condition gives that dEz/dr is also zero at the surface R3. Accepting this, we can solve the matrix and obtain c0 and c1.

Fortunately, as seen in section 1, it is easy to take the derivative of the Bessel functions,
and also to integrate them.

It is not so easy to compute the Bessel and Hankel functions, but they are
available in the Octave program.
You can find a calculation of the coax impedance in the Octave-script
Coax.
The program is also available in c-code in the file
** hankel.c.gz, **
which also includes c-code algorithms to compute Bessel and Hankel functions.

Compile it with the command `'gcc -lm hankel.c'`

.

Here is an example of what that program can produce:

The impedance of a coax. Loss and delay Coax dimensions in mm: R1=5.00e-04 R2=1.00e-03 R3=1.50e-03 DC-values: R=2.627966e-02 C=8.025903e-11 AC-values: f[Hz] L[H/m] R[ohm/m] Z0[ohm] loss[dB/m] delay[s/m] 1.0e+00 2.2137e-07 2.6280e-02 5.2519e+01 4.3463e-03 4.22e-09 1.0e+01 2.2137e-07 2.6280e-02 5.2519e+01 4.3463e-03 4.22e-09 1.0e+02 2.2137e-07 2.6280e-02 5.2519e+01 4.3463e-03 4.22e-09 1.0e+03 2.2137e-07 2.6283e-02 5.2518e+01 4.3469e-03 4.22e-09 1.0e+04 2.2094e-07 2.6582e-02 5.2467e+01 4.4005e-03 4.21e-09 1.0e+05 1.9891e-07 4.3432e-02 4.9782e+01 7.5779e-03 4.00e-09 1.0e+06 1.5838e-07 1.2883e-01 4.4422e+01 2.5189e-02 3.57e-09 1.0e+07 1.4489e-07 3.9765e-01 4.2489e+01 8.1291e-02 3.41e-09 1.0e+08 1.4061e-07 1.2483e+00 4.1856e+01 2.5905e-01 3.36e-09 1.0e+09 1.3926e-07 3.9386e+00 4.1654e+01 8.2129e-01 3.34e-09 1.0e+10 1.3883e-07 1.2446e+01 4.1590e+01 2.5993e+00 3.34e-09 1.0e+11 1.3869e-07 3.9349e+01 4.1570e+01 8.2218e+00 3.34e-09

In the table above, the characteristic impedance Z0 is set to
Z0 = sqrtl (L /C).

This may be wrong at low frequencies.
If we set Z0 = sqrtl ((R + omega*L )/(G + omega*C)),

and assume G is infinity, we get the following table:

Hankel: The impedance of a coax. Loss and delay Coax dimensions in mm: R1=5.00e-04 R2=1.00e-03 R3=1.50e-03 DC-values: R=2.627966e-02 C=8.025903e-11 AC-values: f[Hz] L[H/m] R[ohm/m] Z0[ohm] loss[dB/m] delay[s/m] 1.0e+00 2.2137e-07 2.6280e-02 7.2191e+03 3.1619e-05 4.22e-09 1.0e+01 2.2137e-07 2.6280e-02 2.2834e+03 9.9965e-05 4.22e-09 1.0e+02 2.2137e-07 2.6280e-02 7.2380e+02 3.1537e-04 4.22e-09 1.0e+03 2.2137e-07 2.6283e-02 2.3426e+02 9.7452e-04 4.22e-09 1.0e+04 2.2094e-07 2.6582e-02 8.9577e+01 2.5775e-03 4.21e-09 1.0e+05 1.9891e-07 4.3432e-02 5.7789e+01 6.5280e-03 4.00e-09 1.0e+06 1.5838e-07 1.2883e-01 4.7210e+01 2.3702e-02 3.57e-09 1.0e+07 1.4489e-07 3.9765e-01 4.3407e+01 7.9572e-02 3.41e-09 1.0e+08 1.4061e-07 1.2483e+00 4.2151e+01 2.5724e-01 3.36e-09 1.0e+09 1.3926e-07 3.9386e+00 4.1748e+01 8.1944e-01 3.34e-09 1.0e+10 1.3883e-07 1.2446e+01 4.1620e+01 2.5974e+00 3.34e-09 1.0e+11 1.3869e-07 3.9349e+01 4.1579e+01 8.2199e+00 3.34e-09When L and R are split into components for the inner conductor, the air gap and outer tube:

AC-values: f L_inner L_gap L_outer R_inner R_outer [Hz] [H/m] [H/m] [H/m] [ohm/m] [ohm/m] 1.0e+00 5.0000e-08 1.3863e-07 3.2741e-08 2.1900e-02 4.3799e-03 1.0e+01 5.0000e-08 1.3863e-07 3.2741e-08 2.1900e-02 4.3799e-03 1.0e+02 5.0000e-08 1.3863e-07 3.2741e-08 2.1900e-02 4.3800e-03 1.0e+03 4.9998e-08 1.3863e-07 3.2739e-08 2.1901e-02 4.3815e-03 1.0e+04 4.9830e-08 1.3863e-07 3.2480e-08 2.2049e-02 4.5325e-03 1.0e+05 3.9188e-08 1.3863e-07 2.1089e-08 3.1782e-02 1.1650e-02 1.0e+06 1.3152e-08 1.3863e-07 6.5956e-09 8.8688e-02 4.0138e-02 1.0e+07 4.1732e-09 1.3863e-07 2.0871e-09 2.6786e-01 1.2979e-01 1.0e+08 1.3201e-09 1.3863e-07 6.6006e-10 8.3496e-01 4.1336e-01 1.0e+09 4.1746e-10 1.3863e-07 2.0873e-10 2.6285e+00 1.3101e+00 1.0e+10 1.3201e-10 1.3863e-07 6.6006e-11 8.3001e+00 4.1459e+00 1.0e+11 4.1746e-11 1.3863e-07 2.0873e-11 2.6235e+01 1.3114e+01

**Note:** If you would like to do programming based on bessel-bignum, please first take
a look at e.g.
** MPFR**
, a library containing a lot of useful functions. Most of them faster and more refined than the slower counterparts used in bessel-bignum.
However, to my knowledge, it does not contain Bessel functions, but faster log, exp and euler-gamma.

E-mail: <sven.wilhelmssonNOSPAMswipnet.se>

Back to home-page, Updated Apr 30 2010