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the dielectric, L' is the inductance per unit length,
and C' is the capacitance per unit length.
η
π
μ
π
πϵ
In an ideal coaxial cable, the electromagnetic
fields propagate in TEM mode, meaning electric
and magnetic fields are perpendicular to each
other and the direction of travel. However, at
higher frequencies, additional modes (like TE11)
start to appear, impacting signal integrity.
It is also possible to estimate the loss per unit length
of a coaxial transmission line using the ohmic and
dielectric losses of the conductors and dielectrics
used in the coaxial line construction. The following
equation—where is the attenuation per unit
length,
c
is the ohmic losses of the conductors,
d
is the dielectric losses, R
s
is the resistivity of
the conductors, and tan is the loss tangent of the
dielectric—shows that the attenuation of a coaxial
line is greater for smaller dimensions of a and b.
α α α
η
ω
δ
Coaxial cables are broadband interconnects, which
can operate down to DC (0Hz). However, their upper
cutoff frequency is determined by their physical
geometry, as higher frequencies can introduce
additional propagation modes beyond the fundamental
TEM mode. Specifically for coax, the TE11 mode
begins to propagate above a certain cutoff frequency
and is approximated by the following equation,
where f
c
is the cutoff frequency, c is the speed
of light, λ
c
is the cutoff wavelength, a and b are
dimensions of the transmission line, μ
r
is the relative
permeability, and ε
r
is the relative permittivity:
λ
π μ ϵ
If modes other than TEM are operating within the
transmission lines, the reflections of the higher-order
modes can cause interference, increase loss, and
reduce the transmission line's voltage standing wave
ratio (VSWR) performance. Larger coaxial cables
have a lower cutoff frequency but can handle higher
voltages and power while exhibiting lower attenuation
per unit length compared to smaller coaxial cables.
ADL8113 Low Noise
Amplifier
mouser.com/adi-adl8113-low-noise-amp
