Reflection and standing waves.

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:
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:
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:
As the above applies to voltage and current, the formula is easily modified to calculate the reflected power:


Standing wave.
When energy is reflected back towards the source, the combination of the forward and reflected signals create a third wave. The picture on the right is a rough demonstration of the creation of a standing wave.
The forward signal (black) combines with the reflected (blue) to create the standing wave (green) which, although stood still, is continually varying in amplitude.


Standing wave ratio.
This is the ratio between the amplitude of the node and the peak amplitude of the adjacent antinode of a standing wave. A node is a point where the amplitude has least change, an antinode is a point where the amplitude sees maximum swing.
This is shown in the diagram on the right, where: As reflected signal increases, the antinode amplitude increases while the node is reduced - resulting in a higher ratio. At 100% reflection, the node is zero making the ratio equal to infinity.
Test instruments do not directly measure the standing wave, it is calculated from other measurements. Methods are shown below.

Measuring with the SWR meter.
A typical SWR meter will measure forward and reflected signals.
From these, it is possible to calculate the node and antinode values. As SWR is the ratio of these two figures, it may be calculated as shown.
The scale printed behind within the meter is designed to give you the reading after this calculation is done.

Measuring SWR with a power meter.
Another method is to use forward and reflected power figures to calculate the ratio. This one may be more useful, as you can measure forward and reflected power by reversing the power meter coax connections:
Measuring SWR from impedances.
As the standing wave ratio and impedance ratio are equal, it can also be obtained by measuring the transmitter and antenna impedance then dividing the larger figure by the smaller one: