Identify and recall the use of coaxial
and twin feeders.
Know that in coaxial cable the RF field
only exits within the cable and is not affected by
objects outside the cable.
Know that feeders cause loss of signal
strength on both transmit and receive. The longer the
cable, the greater the loss.
Know that twin feeder usually has
lower loss that coaxial cable.
Know that loss is measured in dB.
Know how to calculate the power
delivered to an antenna for a given RF output and given
feeder loss (in multiples of 3 dB).
5b Feeder characteristic impedance
Know that the term 'impedance' can be
thought of as a form of AC resistance.
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.
Know about the formation of standing
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
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