5c.1 Recall that the feed
point impedance of an antenna is related to the dimensions of the
antenna and the wavelength of the applied signal.
Recall that the current
flowing into an antenna is related to the feed point impedance and the
potential difference of the applied signal.
Recall that an antenna will only present
the correct feed point 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 Ω when used at its designed frequency.
Recall that if the feed point impedance of
the antenna does not match that of the feeder, energy will be reflected
back down the feeder; the proportion reflected depending upon the degree of mismatch. Dipole antennas
A dipole antenna is one that is cut to half the wavelength of the
frequency and cut at the centre and connected to the feeder. A dipole's
impedance will depend on the dimensions of the dipole and the frequency
used.
For example a dipole for 7.1MHz is approximately 66 feet long. If this
is cut in the centre, the impedance will be 50 Ohms when a 7.1 MHZ
signal is applied. This 50 Ohm impedance can be changed by:
Changing the length of the antenna
Changing the frequency applied
The current flowing into the
antenna will depend on the feed point impedance of the antenna and the
potential difference of the signal applied at the feed point.
A half wave dipole has a feed
point impedance of about 50ohms at its design frequency. So a half wave
dipole for 7.100MHz will have a 50ohm impedance only at this frequency.
Standing waves
5d.1 Understand that the signal reflected back down the feeder is not
lost but will combine with the waves travelling up the feeder from the
transmitter leading to the formation of standing waves.
Recall that the reflected signal will change the input impedance of the
feeder so that it is no longer the characteristic impedance and the
feeder will not then present the correct impedance to the transmitter.
Antenna tuning unit
Standing Waves Imagine
we have an antenna cut so that it has an impedance of 50 Ohms at 7.100
MHz. If we change frequency to 10.150MHz the impedance will be much
different.
If there is a mismatch between
the impedance of the antenna and the frequency then some of the energy
is reflected back down the feeder. The amount reflected depends on the
degree of mismatch.
The reflected energy from a mismatched antenna is not lost. It combines
with the signal travelling up the feeder from the transmitter to create
a standing wave.
The reflected wave will change the feedpoint impedance at the feed
point. This means that the transmitter, which has been designed to have
a 50 Ohm output, will not match the feeder impedance. This could cause
damage to the output transistors in a transistor PA.
The losses in the coax will be much higher.
5e.1 Recall that a transmitter is designed to transfer energy into a specific impedance.
Recall that an antenna tuner unit (ATU) can change the impedance
presented to the transmitter and may also reduce harmonic radiation. An antenna tuning unit
ATU (better described as an antenna matching unit AMU) if correctly
designed will match the changed impedance at the feedpoint to the 50
ohms impedance of the transmitter output. So an ATU acts as a variable impedance matching device. Here are some scenarios:
Impedance at bottom of feeder
Impedance of RX/TX socket
Comments
50 Ohms
50 Ohms
No ATU required
200 Ohms
50 Ohms
ATU reduces the 200 Ohms to 50 Ohms
800 Ohms
50 Ohms
ATU reduces the 800 Ohms to 50 Ohms
1600 Ohms
50 Ohms
ATU reduces the 1600 Ohms to 50 Ohms
If using coax cable the standing waves on the coax will lead to greater
attenuation in the feedline. If using twin feeder there will be a
much lower attenuation in the feeder.
NB an ATU cannot change the impedance at the feedpoint of the antenna.
All it can do is match the impedance at the end of the feeder to the
impedance required by the transmitter.
Harmonic suppression
The final amplifier in a transmitter not only produces the transmit
frequency wave, but also multiples called harmonics. The design of many
ATUs enable them to suppress these harmonics, the ATU acting like
a band pass filter tuned to pass the required frequency but attenuating
other frequencies.
