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Syllabus Sections:-

10e SWR. measurements

10e.1 Identify the circuit of an SWR meter and understand its operation.

Standing Wave Ratio (SWR) meters

We have found three different circuits for the SWR unit which at first glance look very different but you should be able to see similarities and thus be able to readily identify an SWR circuit should it turn up in the exam.

This is the first circuit note the diodes and two meters and the two sections of red coloured wire. The wire is the sensing part of the circuit as a signal passing down the signal wire will induce a signal in the sensing wire in an opposite direction.

Whilst you might think that the right hand diode would indicate the reverse signal you would be wrong as it actually indicates the FORWARD SIGNAL.

With the wire terminated in a resistor having a value of the characteristic impedance the signal will flow without REFLECTION. The diodes rectify the signal and it is displayed on the two meters as a forward and a reverse reading. Only signals flowing in the correct direction will be shown. So if there is no reflected signal on dial will show zero and the other the forward signal. When the signal is not matched by the antenna then a reflected signal will result and be displayed on the reverse reading meter.

This is the second circuit note particularly the two diodes and the similarity with the circuit above.. As with the circuit above there are two sensing wires indicated in RED.

In this diagram the location of the diodes is different but the direction they are facing is similar to the diagram above but here right hand diode indicates the REVERSE SIGNAL and the left one indicates the FORWARD SIGNAL. It is the direction of current flow you need to appreciate.

The Forward and reverse meter would connect between GND and the points marked FWD and REV.

This is the third circuit note the use of transformers, a combining circuit and only a single meter.

So now for more detailed explanation

The SWR meter is fitted in series between the transmitter and the antenna. The meter circuit takes two measurement. one VA is related to RF CURRENT in the centre conductor of the coax, the second, VB is related to the RF VOLTAGE between inner and outer conductors.

Let us assume that the antenna cable is connected to a 50 Ohm DUMMY LOAD - A perfect Match - NO STANDING WAVES and current and voltage are in PHASE.

The combining circuit calculates VB/VA which is equivalent to VRF/IRF , which will be the cable impedance (50 Ohms) the output meter will be ZERO volts indicating a standing wave ration of 1:1

If the antenna cable is connected to any impedance other than 50 Ohms, the above relationship VRF/IRF will not resolve to 50 Ohms, due to the standing waves caused by power reflected back from the unmatched load.

The combining circuit will produce an output proportional to the mismatch calibrated as standing wave ratio (SWR).

Under matched conditions, VB can be processed to indicate power output of transmitter. The combining circuits may also produce an output proportional to reflected power.

For interest, the standing wave ratio can be proven to be :-

RLOAD / Characteristic Cable Impedance = RLOAD / 50 (in our case) where RLOAD is the antenna impedance.

Some more thoughts that might help you

STANDING WAVE NOTE

Standing waves occur when a transmitter feeding a transmission line has a load, or antenna, at the far end, not matched to the line impedance.

The Transmitter Voltage / Current ratio = the Line Impedance.

The unmatched load cannot accept this ratio and some power is reflected back towards the transmitter.

The reflected current and voltage alternately ADD to and SUBTRACT from the out going current and voltage resulting in points of HIGH RF VOLTAGE and points of LOW VOLTAGE along the transmission line which are NOT MOVING! (STANDING WAVES).

If we could run an RF voltmeter along the transmission line we would find points of HIGH Voltage and points of LOW Voltage at regular intervals.

The ratio of high to low voltage is the Standing Wave Ratio.

This is the last page of the course !!!!


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