Brats club logo 


Syllabus Sections:-

Measurements

  9A7   56   Identify the circuit of an SWR meter using either a sense wire between inner and outer conductors of coaxial line or a current transformer and capacitive voltage tap.

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.

Circuit 1



This is the first circuit note the diodes and two meters and the two sections of red coloured wire. The red coloured wire is the sensing part of the circuit as an incident signal passes down the coax centre as shown in blue, the signal wire, it  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 ( 50Ω) the incident signal will flow without REFLECTION. The diodes rectify the incident  and reflected signals and they are  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 incident signal is not matched by the antenna then a reflected signal will result and be displayed on the reverse reading meter.

Circuit 2


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 which again go under the braid in coax ( which is not shown in this drawing).

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 meters would connect between GND and the points marked FWD and REV.

Circuit 3



Take a look at

page 98 Fig 14.8
This is the third circuit note the use of a transformer CT and the voltage is tapped off by C3. The braid is only connected at one end but the unit must be in a metal box to enclose the RF and thus prevent RF from escaping ( through the screening box ).


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.

-------------------------------------------------------------------------------

9A7 56 continued Understand in simple terms how this leads to an SWR reading on devices using a single meter, twin meters or cross-needle twin meter.

If you have a meter with only one meter then there will need to be a change over switch to change from Forward to Reverse SWR readings.

With a twin (two separate meters) meter unit one meter will show you Forward and the other Meter Reverse SWR but this time simultaneously reading can be seen.


With a Cross-needle Twin meter such as the one shown above. One needle arcs up from bottom left the Forward reading needle and the other from the bottom right the reverse reading needle. The meter is calibrated to show forward and reverse AVG or PEP power  according to the setting on the selector button. The other three buttons are calibrated for maximum deflection of  respectively 20W, 200W and 2kW forward power levels.

Additionally there is a group of marking called SWR rising at changing angles from near the mid point where in the picture the needles cross. When in use the Forward needle  will rise and also if there is any Reflected power the Reverse Needle will rise. Where they cross you can then read off the relative SWR, as approximated by the legend, on the meter.

The author of this web site has a preference for this type of meter but also in the shack are single and dual meter SWR units.





brats copyright logo