Effect of mismatch on coaxial cable.
Characteristic impedance.
As stated on the previous page, coaxial cable characteristic impedance will only
apply in certain conditions:
1. Both source and load match this characteristic impedance.
2. The coax has infinite length (incredibly unlikely).
3. The coaxial length is a multiple of 1/2 wavelengths at the frequency in use.
This information becomes useful in understanding the strange readings and situations
that sometimes occur...
Correct conditions.
With a correctly tuned and matched antenna, the reactances cancel out. The
impedance is just the radiation resistance, which should match that of the coax
and radio.
As the source and load match, condition 1 (above) is satisfied. The coaxial presents
only the load impedance to the source end. Signals along the coax are equal and
opposite, also cancelling out. The coax acts purely as transport, delivering the
signal to the antenna.
Mismatched load.
If the source, load and feeder do not all match, reflections and standing waves are
present on the coax. The coax will now act as a resonant line (see below), no longer
matching the characteristic impedance.
Loss.
As each trip along the coax incurs loss, the coaxial loss for reflected signals is
doubled. For example, a length of coax with 3dB loss will give 3dB loss on the trip
to the antenna and another 3dB on the way back.
To make matters worse, other effects of these reflections may also cause additional
losses.
Resonant line.
With a mis-matched load, the feeder is now acting as a tuned circuit with reactance
that varies along the length. The impedance presented to the source end will be a
combination of the antenna and coaxial impedances.
At half wavelengths along the line (condition 3, above), the reactance cancels out
and the source end will again see just the load.
More rejection.
Due to the above effects and the feeder length, the feeder may no longer represent the
same impedance as the transmitter. Any reflected signal reaching the transmitter may
be met with a change in impedance, causing some of that signal to once again get
reflected. This is shown in the diagram on the right, where:
Black line = Forward signal.
Blue line = Reflected signal.
Green line = Re-reflected signal going back to load.
This time the signal is going back towards the antenna, where it will again be
subject to reflection.
Radiation.
As mentioned previously, the coax operates by containing the signals within it. This
can only be achieved as long as all signals on the conductors are equal and
opposite.
Antenna problems can cause the balance to be destroyed, allowing the cable to radiate
more signal. Not only does this mean that the signal does not get transmitted
from the antenna, but it can also have larger negative effects on the signal that
the antenna does radiate.
It is planned to cover this issue in more depth on another page, for inclusion
in this section at a later date.
