The components that connect, interface,
transfer, and filter RF energy within a given system or between systems are
critical elements in the operation of vacuum tube devices. Such hardware,
usually passive, determines to a large extent the overall performance of the RF
generator.
To optimize the performance of power vacuum
devices, it is first necessary to understand and optimize the components upon
which the tube depends. The mechanical and electrical characteristics of the
transmission line, waveguide, and associated hardware that carry power from a
power source (usually a transmitter) to the load (usually an antenna) are
critical to proper operation of any RF system.
Mechanical considerations determine the
ability of the components to withstand temperature extremes, lightning, rain,
and wind, that is, they determine the overall reliability of the system.
The effective resistance offered by a given
conductor to radio frequencies is considerably higher than the ohmic resistance
measured with direct current. This is because of an action known as the skin
effect, which causes the currents to be concentrated in certain parts of the
conductor and leaves the remainder of the cross-section to contribute little or
nothing toward carrying the applied current.
When a conductor carries an alternating
current, a magnetic field is produced that surrounds the wire. This field
continually expands and contracts as the ac wave increases from zero to its
maximum positive value and back to zero, then through its negative half-cycle.
The changing magnetic lines of force
cutting the conductor induce a voltage in the conductor in a direction that
tends to retard the normal flow of current in the wire. This effect is more
pronounced at the center of the conductor.
Thus, current within the conductor tends to
flow more easily toward the surface of the wire. The higher the frequency, the
greater the tendency for current to flow at the surface. The depth of current
flow d is a function of frequency and is determined from the following
equation:
d = 2.6/ √(μf)
where d is the depth of current in mils, μ
is the permeability (copper=1, steel=300), and f is the frequency of signal in
MHz. It can be calculated that at a frequency of 100 kHz, current flow
penetrates a conductor by 8 mils.
At 1 MHz, the skin effect causes current to
travel in only the top 2.6 mils in copper, and even less in almost all other
conductors. Therefore, the series impedance of conductors at high frequencies
is significantly higher than at low frequencies.
When a circuit is operating at high
frequencies, the skin effect causes the current to be redistributed over the
conductor cross-section in such a way as to make most of the current flow where
it is encircled by the smallest number of flux lines. This general principle
controls the distribution of current, regardless of the shape of the conductor
involved.
There are two kinds of electrical transmission lines, Transmission line hardware that conduct power from generating source substations, and distribution lines that send power ton to individual consumers.
ReplyDeleteIn some electrical transmission lines communities, the distribution lines are overhead and you can see them.
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