SWR Meter


Rating: 0.00 / 5 
Meters Circuits / Meter Counter Circuits
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.
SWR Meter - schematic

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.

Recommended videos

  • MFJ-828 Digital SWR Watt Frequency Counter HF / CB Meter Overview
    Duration: 15:32.
  • Digital VHF/UHF Power
  • #196: How a Directional Coupler in an SWR meter works
    Duration: 21:47.


  • The Denver Post
    The Denver Post It is an hour before Katie Rainsberger will run her final race for Air Academy High School, the 1,600 at the Colorado state high school meet on an unseasonably hot Saturday afternoon. One of ... Katie is up at 6 a.m. on a Tue
     23 May 2016
  • Miami Herald
    Votel, who has headed U.S. Central Command for just seven weeks, became the highest-ranking U.S. military officer known to have entered Syria since the U.S. began its campaign to the Islamic State in 2014. The circumstance was exceptional
     22 May 2016
  • Officer.com (press release) (registration) (blog)
    Officer.com (press release) (registration) (blog) But this high-tech device of awesomeness doesn't fire bullets - the DroneDefender from Battelle sends out a signal up to 400 away to take advantage of typical standard drone (or UAV if you're
     21 May 2016
  • and win
    Santa Rosa Press Democrat
    Santa Rosa Press Democrat Her bubbly personality is in stark contrast to the cold, hard numbers she puts up: the Empire's second-fastest (by less than a second) all-time 1,600 and third-best 3,200 . Track fans haven't seen a female runner po
     21 May 2016
  • Ames Tribune
    Ames Tribune web1_05-19-R-S-RUNB-Thomas-Noah.jpg Buy Photo. Roland-Story's Noah Thomas will compete in the wheelchair 100-, 200- and 400- events at the state track meet in Des Moines this weekend. File Photo by ... He leaps over another on th
     19 May 2016
  • UPI.com
    UPI.com WASHINGTON, May 18 (UPI) -- Defense research non-profit Battelle showed off its DroneDefender system at this week's Navy Sea-Air-Space expo, a handheld, rifle-like system designed to the growing presence of smaller, commercial drones
     18 May 2016
  • Sports Illustrated
    Dana Vollmer, looking to defend her 100- butterfly gold medal at the Games this summer, continues her incredible comeback after giving birth in March 2015. The two-time Olympian won the 100- freestyle in 53.59, the fastest time of any
     17 May 2016
  • Forbes
    Forbes You will enjoy an exceptional private dining experience in the exclusive Tree Pod, suspended 12 above sea level and featuring Michelin-starred gourmet food. Additionally, you will be treated to a ... Martha Ann features a “swim
     14 May 2016
  • War on the Rocks
    War on the Rocks If Russia was planning a full-scale invasion of the Baltic states, it would also have to plan to take on all of NATO and defend against a -attack. Great powers typically don't attack superpowers .... NATO's biggest challenge
     12 May 2016


Leave Comment

characters left: