Wave reflection.
Source and load.
Although the usual situation for a
CB
enthusiast will be that the radio is the source and the antenna the load, this is
not always the case. The theory applies equally to other situations.
For example, when receiving signals, the antenna is the source and the radio the
load. Reflections and standing waves can occur with a receiver. Reflections may be
seen with
TV receivers as a "ghost"
image, as the reflections give delayed images.
Also a resistor may be used as a load, in place of the antenna or receiver, to give
an ideal non-reactive load for test purposes. If you have an amplifier in line it
becomes a load for the input signal and a source for the output. These are reversed
for receive and transmit modes.
The ideal situation.
In the perfect system, the source, load and feeder all have the same impedance. The
signal will be passed to the feeder which, in turn will pass it to the load. The
load will then accept all the energy, converting it to another form. In the case of
an antenna, this will be electromagnetic energy.
Antenna tuning.
The antenna is like a tuned circuit made up of capacitance, inductance and
resistance.
When the antenna is the correct length for the frequency in use, the reactance
cancels out and the antenna presents only the resistance - which should match the
transmitter impedance.
When the antenna is not tuned to the frequency in use, the reactance will no longer
cancel out, so the antenna impedance now includes both resistance and reactance.
This will alter the total impedance, creating a mismatch.
Wave reflection.
If the load impedance does not match that of the source, the load will not accept
all the energy being sent down the feeder. That which is not accepted by the load
will be reflected back down the line towards the source.
Wave reflection is shown in the diagram on the right, where:
Black line = Forward signal.
Blue line = Reflected signal.
Source end.
Any reflected energy may get all the way back to the source. If the source and feeder
have the same impedance (as they should), this reflected signal will all
be accepted by the source.
Should the source and feeder impedances not match, the signal will be reflected
back along the feeder - once again towards the load which will still not want it
all. 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.
Transmitter issues.
Any reflected energy must now be dissipated in some form and, in a transmitter, this
is likely to become heat. Some of this energy may also find it's way into other
circuits, creating some rather nasty problems (discussed on other pages in the
section).
SWR and reflected energy.
As
SWR is related to
reflected signal, it possible to calculate the reflected signal level from
SWR and forward amplitude.
The first formula, on the right, shows this applied to voltages:
SWR = Standing Wave Ratio.
Vf = Forward voltage.
Vr = Reflected voltage.
As the above applies to voltage and current, the formula is easily modified to
calculate the reflected power:
SWR = Standing Wave Ratio.
Pf = Forward power
Pr = Reflected power.
