A simple SWR meter first needs a calibration to Uv = 100 % (forward power). When switching over to Ur (reflected power) the SWR is indicated. This procedure has to be repeated every time a change is made to the transmit power, antenna tuner or frequency etc. With the help of the following circuit one can read the SWR directly without repeating the calibration after every change. The timer IC NE555 is used as a window comparator in this application. It controls the charging and discharging of capacitor C6. The discharge period, t1, begins when VT1 is switched off. Both NE555 outputs 7 and 3 go `high` when the OP amp output voltage falls below the 1/3 threshold of the comparator.
It works according to the principal that the ratio of capacitor charging time t2 to discharging time t1 is inversely proportional to the ratio |-Ur| / (Uv - Ur). t2/t1 = (Uv-Ur) / |-Ur|). The relationship between time and voltage is as follows:
t1 ~ 1 / (Uv - Ur) = A
t2 ~ 1 / |Ur| = B
T = t1 + t2 ~ 1 / (Uv - Ur) + 1 / |Ur| = A + B
The relative current I_rel [%] through the indicator is:
I_rel = t1 / T ~ A / (A+B)=1 / (1 + B/A) = 1 / [1 + (Uv - Ur) / Ur ] = Ur / Uv
I_rel [%] = 100% x Ur / Uv
VT1 short-circuits Uv to the +3 V virtual ground potential and therefore only -Ur is active. The negative voltage, -Ur, begins the t2 charge period until the OP amp output voltage exceeds the 2/3 comparator threshold and the outputs 7 and 3 go `low`. VT1 is switched off again and the process repeats indefinitely. `High` at pin 3 means no current flow and `low` means 100 uA current flow through current limiter R9 and the indicator.
All necessary calculations are made by a small analogue electronic circuit (divider). This circuit isn't new, it was used at a time before microprocessors or cheap logarithmic ICs were available.
To align, set the Uv terminal to +2 V and the Ur terminal to -1 V. The voltage rates are referring to the + 3 V virtual ground. To get the fixed voltages, use two potentiometers and feed each with +6 V against real ground ( 0V). Connect one slider to the Uv terminal and the other one to the -Ur terminal. Against the real 0 V ground the slider voltages should be +5 V and +2 V. The related SWR calculation looks like this:
SWR = (Uv + Ur) / (Uv - Ur) = (2V + 1V) / (2V - 1V) = 3 / 1 = 3.
Because (Uv - Ur) and |-Ur| are equal, the pulse to break ratio will be equal too (symmetrical rectangle signal at IC2 pin 7). The indicator pointer should stay in the 50% position. This corresponds to the before manually calculated SWR of 3. A deviation from the center position may require a balancing resistor in series or in parallel to R9.
Now remove both potentiometers and adjust the reflectometer by C1 to very little or no |-Ur|.
The assignment between SWR and the relative current is done graphically when calibrating the indicator scale. At Ur = 0 V no current flows through the indicator and the pointer remains at 0% and shows a SWR of 1. Ur = 0.5 x Uv makes a current flow over 50% of the period time T. The pointer stays at the center position corresponding to SWR = 3. At Ur = Uv the 100% end position (SWR = infinity) is reached.
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