High frequency transmission lines

Teoriaa: Systeemi

Error creating thumbnail: libgomp: Thread creation failed: Resource temporarily unavailable

Dipole antennas and transmission coefficient. See the svg file

Antenn system.

Lähetin, johto, antenni, jne. https://www.antenna-theory.com/tutorial/txline/transmissionline.php

https://www.worldradiohistory.com/BOOKSHELF-ARH/Technology/Rider-Books/R-F%20Transmission%20Lines%20-%20Alexander%20Schure.pdf

Transmission line

• 

  The (long) transmission line is modeled as Z₀. If the frequency (wavelength) of the source is too large (small) compared to dimensions of the system, it need to be considered in more detailed. See also the svg file.

• 

  The physical realizations of the transmission lines are usually coaxial cables, twisted cables or twin lead cables.

• 

  The system is analyzed as being differential short pieces. The conductance Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle G} is the conductance between the two wires, which exists because of the high frequency.

If a transmission line has a length greater than about 10% of a wavelength, then the line length will noticeably affect the circuit's impedance. The equation in the above image can be written as

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{align} \frac{\partial v}{dx} &= -L\frac{\partial i}{\partial t} - Ri \\ \frac{\partial i}{dx} &= -C\frac{\partial v}{\partial t} - Gv \end{align} } and these two is easy to combine, and it gives the second degree dy \frac{\partial^2 v}{dx^2} -LC\frac{\partial^2 v}{\partial t^2} = (RC+GL)\frac{\partial x}{\partial t} + GRv </math> and similar to i.

The solutions to the above equations is the sum of forward and backward traveling (reflected) waves: $v(z,t)=v^{+}e^{-\alpha z}e^{\imath (\omega t-\gamma z)}+v^{-}e^{\alpha z}e^{\imath (\omega t+\gamma z)}$ and if we assume that Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle \alpha =0} we have the telegraphers equations https://en.wikipedia.org/wiki/Telegrapher's_equations

$v(z,t)=v^{+}e^{\imath (\omega t-\gamma z)}+v^{-}e^{\imath (\omega t+\gamma z)}$ and a similar for Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle i=i(z,t)} . If we replace $i$ by Ohm law, we get

$i(z,t)={\frac {v^{+}}{Z_{0}}}e^{\imath (\omega t-\gamma z)}+{\frac {v^{-}}{Z_{0}}}e^{\imath (\omega t+\gamma z)}={\frac {v^{+}}{Z_{0}}}e^{\imath (\omega t-\gamma z)}\left(1-{\frac {v^{-}}{v^{+}}}e^{\imath (2\gamma z)}\right)$

The fraction ${\frac {v^{-}}{v^{+}}}$ is called reflection coefficient $\Gamma$

${\frac {v^{-}}{v^{+}}}=\Gamma e^{\imath \phi _{z}}={\frac {Z_{L}-Z_{0}}{Z_{L}+Z_{0}}}$

which gives

$i(z,t)=={\frac {v^{+}}{Z_{0}}}e^{\imath (\omega t-\gamma z)}\left(1-\Gamma e^{\imath (\phi _{z}+2\gamma z)}\right)$

The characteristic impedance is

Thus we have ${\frac {d^{2}v}{dL^{2}}}=\gamma ^{2}v$ and similar for the current. The constant $\gamma ^{2}=(R+\imath \omega L)(G+\imath \omega C)$.

Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle Z_{0}={\frac {v^{+}}{i^{+}}}=-{\frac {v^{-}}{i^{-}}}={\sqrt {\frac {R'+\imath \omega L'}{G'+\imath \omega C'}}}}

For lossless line Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle R'=G'=0} and for distortionless line Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle R'/L'=G'/C'} . The voltage reflection coefficient $\Gamma$

$\Gamma ={\frac {v^{-}}{v^{+}}}=-{\frac {i^{-}}{i^{+}}}={\frac {Z_{L}-Z_{0}}{Z_{L}+Z_{0}}}$ where $Z_{0}$ is the characteristic impedance of transient line, and $Z_{L}$ is the impedance of load (antenna). If $Z_{L}=Z_{0}$, then the line is perfectly matched, and there is no mismatch loss and all power is transferred to the load (antenna).

• An open circuit: Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle Z_{L}=\infty } and Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle \Gamma =+1} .
• A short circuit: Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle Z_L = 0} and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Gamma = -1} , and a phase reversal of the reflected voltage wave.
• A matched load: Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle Z_L = Z_0} , and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Gamma = 0} and no reflections.

The voltage standing wave ratio or VSWR

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \text{VSWR} = \frac{|V|_\text{max}}{||V|_\text{min}|} = \frac{1 + |\Gamma|}{1 - |\Gamma|} }

Siirtolinja (transmission line). Impedanssi. Koaksaalikaapelin impedanssi muodostuu sen kapasitiivisestä rakenteesta. Ei juuri resistiivistä häviötä (impedanssia) https://electronics.stackexchange.com/questions/543100/derivation-of-resistance-of-coaxial-cable. Koaksaalikaapelin ε^r

• 76.7 Ω
• 30 Ω
• The impedance of a centre-fed dipole antenna in free space is 73 Ω, so 75 Ω coax is commonly used for connecting shortwave antennas to receivers.
• Sometimes 300 Ω folded dipole antenna => 4:1 balun transformer is used.

twin-lead transmission lines: the characteristic impedance of is roughly 300 Ω.

Feeding length.

Some transmission lines are

• Coaxial cable
• Two-wire cable
• Microstrip line
• . . .

Skin Effect

The skin effect Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \delta} . The higher the frequency, the more the currents are confined to the surface.

Velocity factor

Caption text

Velocity factor	Line type
0.95	Ladder line
0.82	Twin-lead
0.79	coaxial cable (foam dielectric)
0.75	RG-6 and RG-8 coaxial (thick)
0.66	RG-58 and RG-59 coaxial (thin)