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.
The Japan Times But when the AP took a Geiger to the riverbank outside Dambaev's home, the reading surged at the water line and the machine began beeping loudly and continuously. Measurements ranged ... Environmentalists estimate that Mayak
Reuters CNN reported that the Russian jet had come within about 100 feet (30 ) of the U.S. plane and had performed a barrel roll. "All flights of Russian planes are conducted in accordance with international regulations on the use of airspace
Springfield News-Leader “It's all hands on deck,” Boyce said with a smile as he carried a lap into the field. Volunteers included ... “With the crowd, everything was better,” said Marissa Kurtimah, who oke the 200- da
The World Weekly Time is kept by the ticking of Geiger , the hideous heartbeat of gamma rays. "It's quite morbid here," says Alex, from Munich. The well-dressed 20-something takes a ... Behind him, a hundred- high antenna stretches into the s
Patch.com We were convinced that we could meet our objective of not providing free street parking for those heading for a night in Manhattan, rather than using the space for a dinner, movie or exercise class in the Village with the 7pm termi
Four minutes into the match, he put a midfield penalty into touch ten from the visitors' line, and the lineout ball was moved quickly through two phases. Tempelmeyer found Callan Erasmus with a skip ... Goss would immediately get his name on th
Etches quickly countered and placed a soft foot on the ball driving it inside the 5 line where Kennedy scooped it up and ran in for the 5-pointer (22-5, conversion missed). Just minutes later, the Scarlets were ... After moving through a couple
Chicago Tribune To the parking problem, the village proposed placing a three-hour parking limit on two surface parking lots along the east side of Marion Street between Lake Street and North Boulevard. ... In addition to the parking lot rest