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identify and recall the use of coaxial and twin
feeders.
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Konw that in coaxial cable the RF field only exits
within the cable and is not affected by objects outside the cable.
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Know that feeders cause loss of signal strength on
both transmit and receive. The longer the cable, the greater the loss.
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Know that twin feeder usually has lower loss
that coaxial cable.
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Know that loss is measured in dB.
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Know how to calculate the power delivered to an antenna
for a given RF output and given feeder loss (in multiples of 3 dB).
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5b Feeder characteristic impedance
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Know that the term 'impedance' can be thought of as
a form of AC resistance.
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Know that feeders have a characteristic impedance
which depends upon the diameter and spacing of the conductors and those commonly
available.
-
Know that an antenna has a feed point impedance
which is determined by the relative RF potential difference and current at
that point.
-
Know that the impedance of an antenna depends
upon its dimensions related to the wavelength of the applied signal.
-
Know that an antenna will only present the correct
impedance when fed with the frequency for which it is designed, and that
a half-wave dipole has a feed point impedance of approximately 50 ohms when
used at its designed frequency.
-
Know that if the impedance of the antenna does not
match the impedance of the feeder, then some energy will be reflected back
down the feeder and that the proportion reflected will depend upon the degree
of mismatch.
-
Know that the signal reflected back down the
feeder is not lost but will combine with the waves travelling up the feeder
from the transmitter.
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Know about the formation of standing waves.
-
Know that standing waves will affect the impedance
presented to the transmitter causing a mismatch between the transmitter and
the feeder.5e Antenna tuning unit
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Know that a transmitter is designed to transfer
energy into a specific impedance.
-
An antenna tuner unit (ATU) can change the impedance
that a feeder presents to the transmitter, and that when correctly matched,
the ATU may reduce harmonic radiation.
