- High frequency fields include both electric and magnetic fields and cannot
be separated and are always in phase for waves traveling in empty space.
- The dynamic constants of the effective values of capacitance, inductance
and resistance at high frequency are different from the static constants of
the actual values of capacitance, inductance and resistance at lower
frequencies (i.e. 60Hz) or DC.
- The difference between the dynamic and static values may be due to a
nonuniform distribution of electric charges along the surface of a conductor
or to a nonuniform distribution of current and potential throughout the
conductor. For example, a voltmeter connected at different places
between an antenna and ground would indicate quite different values at a
high frequency, whereas its indication for a 60-cycle current would be
practically constant along the entire length.
- Radiation is mostly due to reflections of electromagnetic disturbances
traveling along a wire. The reflections at the ends of a line become a
maximum where a stationary wave system is developed on the wire.
Stationary waves happen when the wire is tuned since the electromagnetic
disturbance traveling toward the open end is reflected there and travels
back to ground with the same velocity. The radiation becomes more
pronounced the higher the frequency of the exciting current.
- A condenser of even small capacitance offers an easy path for
high-frequency currents, while an ordinary air-core coil may constitute a
very high impedance in a high frequency circuit. In bifilar wound
coils an injurious cross current may flow if not properly designed. This is
due to capacitance effect of the coil in spite of the fact that the coil
still exercises its choking effect.
- In a properly designed coil the dynamic inductance is higher than the
static inductance at high frequency.
- In condensers designed with spacious electrodes and/or that produce corona
and/or that use imperfect dielectrics, the dynamic capacitance is lower than
the static capacitance at high frequency.
- The dynamic value of resistance is higher than its static value at high
frequency and is conveniently determined by direct measurement.
- When placing high frequency across a poorly built coil with some of the
turns within other turns, the coil loses inductance due to eddy currents
which may act as the secondary of a transformer and decrease the effective
lines of flux. Actual measurements are the only safe criterion for the
effective inductance of a coil.
- Since both continuous and damped electric waves can be used in
high-frequency work, they may be conveniently classed as:
a. Sustained waves, sinusoidal or any other form, produced by ferric and
nonferric generators.
b. Discrete wave trains, damped sinusoidal, generally produced by spark and
buzzer exciters.
- The current flowing into a condenser when distorted gives a higher
effective reading, since currents of double, triple, and other harmonic
frequency are less impeded by the condenser than its fundamental which is
supposed to be measured.
- When a coil is used, the effective voltage produced across its terminals
is higher when a distorted current wave exists, since the choking effect of
an inductance is all the more pronounced for higher harmonics and produces
corresponding harmonic voltages across the inductances.
- When a resistance is connected in parallel with a condenser, the effective
capacitance is greater (by the amount 1/(wCR)˛)
than without the shunt resistance. At the same time the
effective resistance becomes considerably smaller.
- A coil with self-capacitance can be imagined as being due to an inductance
with a capacitance in parallel.
- The effective resistance at resonance is the reason why a
parallel-resonance circuit can transfer energy from a primary to a secondary
coil coupled to it. A secondary circuit will increase the effective
resistance corresponding to the energy transfer.
- In order to transfer energy from one high frequency circuit to another,
the circuits must be coupled. There are four ways to couple high
frequency circuits: direct reactance coupling, direct inductance coupling,
magnetic inductance coupling, and capacity coupling.
- In the case of magnetic inductance coupling (typical Tesla Coil
primary/secondary configuration) maximum coupling occurs when the impedance
of the primary circuit times the impedance of the secondary circuit equals
the square of the transformer impedance (Z1*Z2=Zm˛).
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