




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.
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 V_{A} is related to RF CURRENT in the centre conductor of the coax, the second, V_{B} 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 V_{B}/V_{A} which is equivalent to V_{RF}/I_{RF} , 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 V_{RF}/I_{RF} 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, V_{B} 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 : R_{LOAD }/ Characteristic Cable Impedance = R_{LOAD} / 50 (in our case) where R_{LOAD} 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 !!!!






