r"""
.. module:: skrf.tlineFunctions
===============================================
tlineFunctions (:mod:`skrf.tlineFunctions`)
===============================================
This module provides functions related to transmission line theory.
Impedance and Reflection Coefficient
--------------------------------------
These functions relate basic transmission line quantities such as
characteristic impedance, input impedance, reflection coefficient, etc.
Each function has two names. One is a long-winded but readable name and
the other is a short-hand variable-like names. Below is a table relating
these two names with each other as well as common mathematical symbols.
==================== ====================== ================================
Symbol Variable Name Long Name
==================== ====================== ================================
:math:`Z_l` z_l load_impedance
:math:`Z_{in}` z_in input_impedance
:math:`\Gamma_0` Gamma_0 reflection_coefficient
:math:`\Gamma_{in}` Gamma_in reflection_coefficient_at_theta
:math:`\theta` theta electrical_length
==================== ====================== ================================
There may be a bit of confusion about the difference between the load
impedance the input impedance. This is because the load impedance **is**
the input impedance at the load. An illustration may provide some
useful reference.
Below is a (bad) illustration of a section of uniform transmission line
of characteristic impedance :math:`Z_0`, and electrical length
:math:`\theta`. The line is terminated on the right with some
load impedance, :math:`Z_l`. The input impedance :math:`Z_{in}` and
input reflection coefficient :math:`\Gamma_{in}` are
looking in towards the load from the distance :math:`\theta` from the
load.
.. math::
Z_0, \theta
\text{o===============o=}[Z_l]
\to\qquad\qquad\qquad\quad\qquad \qquad \to \qquad \quad
Z_{in},\Gamma_{in}\qquad\qquad\qquad\qquad\quad Z_l,\Gamma_0
So, to clarify the confusion,
.. math::
Z_{in}= Z_{l},\qquad\qquad
\Gamma_{in}=\Gamma_l \text{ at } \theta=0
Short names
+++++++++++++
.. autosummary::
:toctree: generated/
theta
zl_2_Gamma0
zl_2_zin
zl_2_Gamma_in
zl_2_swr
zl_2_total_loss
Gamma0_2_zl
Gamma0_2_Gamma_in
Gamma0_2_zin
Gamma0_2_swr
Long-names
++++++++++++++
.. autosummary::
:toctree: generated/
electrical_length
distance_2_electrical_length
electrical_length_2_distance
reflection_coefficient_at_theta
reflection_coefficient_2_input_impedance
reflection_coefficient_2_input_impedance_at_theta
reflection_coefficient_2_propagation_constant
input_impedance_at_theta
load_impedance_2_reflection_coefficient
load_impedance_2_reflection_coefficient_at_theta
voltage_current_propagation
Distributed Circuit and Wave Quantities
----------------------------------------
.. autosummary::
:toctree: generated/
distributed_circuit_2_propagation_impedance
propagation_impedance_2_distributed_circuit
Transmission Line Physics
---------------------------------
.. autosummary::
:toctree: generated/
skin_depth
surface_resistivity
"""
from . constants import NumberLike, INF, ONE
import numpy as npy
from numpy import pi, sqrt, exp, array, real
from scipy.constants import mu_0
from . import mathFunctions as mf
[docs]
def skin_depth(f: NumberLike, rho: float, mu_r: float):
r"""
Skin depth for a material.
The skin depth is calculated as:
.. math::
\delta = \sqrt{\frac{ \rho }{ \pi f \mu_r \mu_0 }}
See www.microwaves101.com [#]_ or wikipedia [#]_ for more info.
Parameters
----------
f : number or array-like
frequency, in Hz
rho : number of array-like
bulk resistivity of material, in ohm*m
mu_r : number or array-like
relative permeability of material
Returns
-------
skin depth : number or array-like
the skin depth, in meter
References
----------
.. [#] https://www.microwaves101.com/encyclopedias/skin-depth
.. [#] http://en.wikipedia.org/wiki/Skin_effect
See Also
--------
surface_resistivity
"""
return sqrt(rho/(pi*f*mu_r*mu_0))
[docs]
def surface_resistivity(f: NumberLike, rho: float, mu_r: float):
r"""
Surface resistivity.
The surface resistivity is calculated as:
.. math::
\frac{ \rho }{ \delta }
where :math:`\delta` is the skin depth from :func:`skin_depth`.
See www.microwaves101.com [#]_ or wikipedia [#]_ for more info.
Parameters
----------
f : number or array-like
frequency, in Hz
rho : number or array-like
bulk resistivity of material, in ohm*m
mu_r : number or array-like
relative permeability of material
Returns
-------
surface resistivity : number of array-like
Surface resistivity in ohms/square
References
----------
.. [#] https://www.microwaves101.com/encyclopedias/sheet-resistance
.. [#] https://en.wikipedia.org/wiki/Sheet_resistance
See Also
--------
skin_depth
"""
return rho/skin_depth(rho=rho, f=f, mu_r=mu_r)
[docs]
def distributed_circuit_2_propagation_impedance(distributed_admittance: NumberLike,
distributed_impedance: NumberLike):
r"""
Convert distributed circuit values to wave quantities.
This converts complex distributed impedance and admittance to
propagation constant and characteristic impedance. The relation is
.. math::
Z_0 = \sqrt{ \frac{Z^{'}}{Y^{'}}}
\quad\quad
\gamma = \sqrt{ Z^{'} Y^{'}}
Parameters
----------
distributed_admittance : number, array-like
distributed admittance
distributed_impedance : number, array-like
distributed impedance
Returns
-------
propagation_constant : number, array-like
distributed impedance
characteristic_impedance : number, array-like
distributed impedance
See Also
--------
propagation_impedance_2_distributed_circuit : opposite conversion
"""
propagation_constant = \
sqrt(distributed_impedance*distributed_admittance)
characteristic_impedance = \
sqrt(distributed_impedance/distributed_admittance)
return (propagation_constant, characteristic_impedance)
[docs]
def propagation_impedance_2_distributed_circuit(propagation_constant: NumberLike,
characteristic_impedance: NumberLike):
r"""
Convert wave quantities to distributed circuit values.
Convert complex propagation constant and characteristic impedance
to distributed impedance and admittance. The relation is,
.. math::
Z^{'} = \gamma Z_0 \quad\quad
Y^{'} = \frac{\gamma}{Z_0}
Parameters
----------
propagation_constant : number, array-like
distributed impedance
characteristic_impedance : number, array-like
distributed impedance
Returns
-------
distributed_admittance : number, array-like
distributed admittance
distributed_impedance : number, array-like
distributed impedance
See Also
--------
distributed_circuit_2_propagation_impedance : opposite conversion
"""
distributed_admittance = propagation_constant/characteristic_impedance
distributed_impedance = propagation_constant*characteristic_impedance
return (distributed_admittance, distributed_impedance)
[docs]
def electrical_length(gamma: NumberLike, f: NumberLike, d: NumberLike, deg: bool = False):
r"""
Electrical length of a section of transmission line.
.. math::
\theta = \gamma(f) \cdot d
Parameters
----------
gamma : number, array-like or function
propagation constant. See Notes.
If passed as a function, takes frequency in Hz as a sole argument.
f : number or array-like
frequency at which to calculate
d : number or array-like
length of line, in meters
deg : Boolean
return in degrees or not.
Returns
-------
theta : number or array-like
electrical length in radians or degrees, depending on value of deg.
See Also
--------
electrical_length_2_distance : opposite conversion
Note
----
The convention has been chosen that forward propagation is
represented by the positive imaginary part of the value returned by
the gamma function.
"""
# if gamma is not a function, create a dummy function which return gamma
if not callable(gamma):
_gamma = gamma
def gamma(f0): return _gamma
# typecast to a 1D array
f = array(f, dtype=float).reshape(-1)
d = array(d, dtype=float).reshape(-1)
if not deg:
return gamma(f)*d
else:
return mf.radian_2_degree(gamma(f)*d )
[docs]
def electrical_length_2_distance(theta: NumberLike, gamma: NumberLike, f0: NumberLike, deg: bool = True):
r"""
Convert electrical length to a physical distance.
.. math::
d = \frac{\theta}{\gamma(f_0)}
Parameters
----------
theta : number or array-like
electrical length. units depend on `deg` option
gamma : number, array-like or function
propagation constant. See Notes.
If passed as a function, takes frequency in Hz as a sole argument.
f0 : number or array-like
frequency at which to calculate gamma
deg : Boolean
return in degrees or not.
Returns
-------
d : number or array-like (real)
physical distance in m
Note
----
The convention has been chosen that forward propagation is
represented by the positive imaginary part of the value returned by
the gamma function.
See Also
--------
distance_2_electrical_length: opposite conversion
"""
# if gamma is not a function, create a dummy function which return gamma
if not callable(gamma):
_gamma = gamma
def gamma(f0): return _gamma
if deg:
theta = mf.degree_2_radian(theta)
return real(theta / gamma(f0))
[docs]
def load_impedance_2_reflection_coefficient(z0: NumberLike, zl: NumberLike):
r"""
Reflection coefficient from a load impedance.
Return the reflection coefficient for a given load impedance, and
characteristic impedance.
For a transmission line of characteristic impedance :math:`Z_0`
terminated with load impedance :math:`Z_l`, the complex reflection
coefficient is given by,
.. math::
\Gamma = \frac {Z_l - Z_0}{Z_l + Z_0}
Parameters
----------
z0 : number or array-like
characteristic impedance
zl : number or array-like
load impedance (aka input impedance)
Returns
-------
gamma : number or array-like
reflection coefficient
See Also
--------
Gamma0_2_zl : reflection coefficient to load impedance
Note
----
Inputs are typecasted to 1D complex array.
"""
# typecast to a complex 1D array. this makes everything easier
z0 = array(z0, dtype=complex).reshape(-1)
zl = array(zl, dtype=complex).reshape(-1)
# handle singularity by numerically representing inf as big number
zl[(zl == npy.inf)] = INF
return ((zl - z0)/(zl + z0))
[docs]
def reflection_coefficient_at_theta(Gamma0: NumberLike, theta: NumberLike):
r"""
Reflection coefficient at a given electrical length.
.. math::
\Gamma_{in} = \Gamma_0 e^{-2 \theta}
Parameters
----------
Gamma0 : number or array-like
reflection coefficient at theta=0
theta : number or array-like
electrical length (may be complex)
Returns
-------
Gamma_in : number or array-like
input reflection coefficient
"""
Gamma0 = array(Gamma0, dtype=complex).reshape(-1)
theta = array(theta, dtype=complex).reshape(-1)
return Gamma0 * exp(-2*theta)
[docs]
def load_impedance_2_reflection_coefficient_at_theta(z0: NumberLike, zl: NumberLike, theta: NumberLike):
"""
Reflection coefficient of load at a given electrical length.
Reflection coefficient of load impedance zl at a given electrical length,
given characteristic impedance z0.
Parameters
----------
z0 : number or array-like
characteristic impedance.
zl : number or array-like
load impedance
theta : number or array-like
electrical length of the line (may be complex).
Returns
-------
Gamma_in : number or array-like
input reflection coefficient at theta
"""
Gamma0 = load_impedance_2_reflection_coefficient(z0=z0, zl=zl)
Gamma_in = reflection_coefficient_at_theta(Gamma0=Gamma0, theta=theta)
return Gamma_in
[docs]
def reflection_coefficient_2_propagation_constant(Gamma_in: NumberLike, Gamma_l: NumberLike, d: NumberLike):
r"""
Propagation constant from line input and load reflection coefficients.
Calculate the propagation constant of a line of length d, given the
reflection coefficient and characteristic impedance of the medium.
.. math::
\Gamma_{in} = \Gamma_l e^{-2 j \gamma \cdot d}
\to \gamma = -\frac{1}{2 d} \ln \left ( \frac{ \Gamma_{in} }{ \Gamma_l } \right )
Parameters
----------
Gamma_in : number or array-like
input reflection coefficient
Gamma_l : number or array-like
load reflection coefficient
d : number or array-like
length of line, in meters
Returns
-------
gamma : number (complex) or array-like
propagation constant (see notes)
Note
----
The convention has been chosen that forward propagation is
represented by the positive imaginary part of gamma.
"""
gamma = -1/(2*d) * npy.log(Gamma_in/Gamma_l)
# the imaginary part of gamma (=beta) cannot be negative with the given
# definition of gamma. Thus one should take the first modulo positive value
gamma.imag = gamma.imag % (pi/d)
return gamma
[docs]
def Gamma0_2_swr(Gamma0: NumberLike):
r"""
Standing Wave Ratio (SWR) for a given reflection coefficient.
Standing Wave Ratio value is defined by:
.. math::
VSWR = \frac{1 + |\Gamma_0|}{1 - |\Gamma_0|}
Parameters
----------
Gamma0 : number or array-like
Reflection coefficient
Returns
-------
swr : number or array-like
Standing Wave Ratio.
"""
return (1 + npy.abs(Gamma0)) / (1 - npy.abs(Gamma0))
[docs]
def zl_2_swr(z0: NumberLike, zl: NumberLike):
r"""
Standing Wave Ratio (SWR) for a given load impedance.
Standing Wave Ratio value is defined by:
.. math::
VSWR = \frac{1 + |\Gamma|}{1 - |\Gamma|}
where
.. math::
\Gamma = \frac{Z_L - Z_0}{Z_L + Z_0}
Parameters
----------
z0 : number or array-like
line characteristic impedance [Ohm]
zl : number or array-like
load impedance [Ohm]
Returns
-------
swr : number or array-like
Standing Wave Ratio.
"""
Gamma0 = load_impedance_2_reflection_coefficient(z0, zl)
return Gamma0_2_swr(Gamma0)
[docs]
def voltage_current_propagation(v1: NumberLike, i1: NumberLike, z0: NumberLike, theta: NumberLike):
"""
Voltages and currents calculated on electrical length theta of a transmission line.
Give voltage v2 and current i1 at theta, given voltage v1
and current i1 at theta=0 and given characteristic parameters gamma and z0.
::
i1 i2
○-->---------------------->--○
v1 gamma,z0 v2
○----------------------------○
<------------ d ------------->
theta=0 theta
Uses (inverse) ABCD parameters of a transmission line.
Parameters
----------
v1 : array-like (nfreqs,)
total voltage at z=0
i1 : array-like (nfreqs,)
total current at z=0, directed toward the transmission line
z0: array-like (nfreqs,)
characteristic impedance
theta : number or array-like (nfreq, ntheta)
electrical length of the line (may be complex).
Return
------
v2 : array-like (nfreqs, ntheta)
total voltage at z=d
i2 : array-like (nfreqs, ndtheta
total current at z=d, directed outward the transmission line
"""
# outer product by broadcasting of the electrical length
# theta = gamma[:, npy.newaxis] * d # (nbfreqs x nbd)
# ABCD parameters of a transmission line (gamma, z0)
A = npy.cosh(theta)
B = z0*npy.sinh(theta)
C = npy.sinh(theta)/z0
D = npy.cosh(theta)
# transpose and de-transpose operations are necessary
# for linalg.inv to inverse square matrices
ABCD = npy.array([[A, B],[C, D]]).transpose()
inv_ABCD = npy.linalg.inv(ABCD).transpose()
v2 = inv_ABCD[0,0] * v1 + inv_ABCD[0,1] * i1
i2 = inv_ABCD[1,0] * v1 + inv_ABCD[1,1] * i1
return v2, i2
[docs]
def zl_2_total_loss(z0: NumberLike, zl: NumberLike, theta: NumberLike):
r"""
Total loss of a terminated transmission line (in natural unit).
The total loss expressed in terms of the load impedance is [#]_ :
.. math::
TL = \frac{R_{in}}{R_L} \left| \cosh \theta + \frac{Z_L}{Z_0} \sinh\theta \right|^2
Parameters
----------
z0 : number or array-like
characteristic impedance.
zl : number or array-like
load impedance
theta : number or array-like
electrical length of the line (may be complex).
Returns
-------
total_loss: number or array-like
total loss in natural unit
References
----------
.. [#] Steve Stearns (K6OIK), Transmission Line Power Paradox and Its Resolution.
ARRL PacificonAntenna Seminar, Santa Clara, CA, October 10-12, 2014.
https://www.fars.k6ya.org/docs/K6OIK-A_Transmission_Line_Power_Paradox_and_Its_Resolution.pdf
"""
Rin = npy.real(zl_2_zin(z0, zl, theta))
total_loss = Rin/npy.real(zl)*npy.abs(npy.cosh(theta) + zl/z0*npy.sinh(theta))**2
return total_loss
# short hand convenience.
# admittedly these follow no logical naming scheme, but they closely
# correspond to common symbolic conventions, and are convenient
theta = electrical_length
distance_2_electrical_length = electrical_length
zl_2_Gamma0 = load_impedance_2_reflection_coefficient
Gamma0_2_zl = reflection_coefficient_2_input_impedance
zl_2_zin = input_impedance_at_theta
zl_2_Gamma_in = load_impedance_2_reflection_coefficient_at_theta
Gamma0_2_Gamma_in = reflection_coefficient_at_theta
Gamma0_2_zin = reflection_coefficient_2_input_impedance_at_theta