Many thanks to Ron WA5QBA for the excellent revision of the following text
Calculating SWR Meter
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
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. 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
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.
Fig.1: SWR meter schematic
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. 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.
Fig. 2: Signals at the OP amp and timer IC outputs
The indicator reads the average of the current pulses. The indicator reacts only to the t1 / t2 ratio and not to the frequency f over a wide dynamic voltage range. At Uv or Ur voltages less than 0.2 V the indicator shivers a bit because the frequency becomes very low due to the long charging and discharging times. With no transmit signal a low quiescent current through the 2.2 MOhm resistor achieves a SWR display of 1 and not infinity. The reflectometer was designed for 0.5 to 10 watt transmit power. Higher or lower power requires little modification.
|Uv [V]||Ur [V]||Ur/Uv||I_rel [%]||standing wave ratio SWR|
The table shows that identical Ur/Uv ratio at different absolute voltages Ur and Uv result in the same SWR readings.
The SWR meter needs an unregulated voltage Ub= 7....15 V. IC3 provides TL071 and NE555 with a regulated voltage of +6 V. The saw tooth amplitude and the pulse current though the indicator are independent from Ub by this measure. +3 V reference voltage (virtual ground) allows symmetrical OP amp operation with a single supply voltage. The circuit current consumption is about 8 mA. For battery operation a current saving NE555 CMOS version is recommended.
|P1||10 kOhm 10 turn potentiometer|
|R1, R2||56 Ohm|
|R4 + R5||2 x 100 kOhm in series|
|R6, 7||100 kOhm|
|C1||3 .. 20 pF trimmer capacitor|
|C3, 4, 5, 6||4.7 nF|
|C7, 10, 11||0.1 uF|
|C8, 9||100 uF, 10 V|
|T1||T50-2 ( applicable for 1.6 to 30 MHz), 1 turn prim. / 50 turns sec.|
|Dr1||FT37-43, 15 turns|
|IC2||NE555 or CMOS type|
|M||100 uA indicator|
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 transmit power should be about 1 to 2 watts for this and the ANT terminal has to be terminated with a non-reactive 50 Ohms load. 50 Ohm transmitter output impedance is obvious.
Back to the menu