Simple Accurate Directional Wattmeter - FDIM2018 Buildathon

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Picture of Simple Accurate Directional Wattmeter - FDIM2018 Buildathon


Keeping with the bench instrumentation theme over the past few years, the project for the FDIM 2018 Buildathon was a Simple and Accurate Directional Wattmeter for measuring forward and reflected power. Once you know the two powers, is is an easy calculation to obtain the true VSWR. The project that we built is an updated version of Roy, W7EL, Lewallens Simple & Accurate Directional Wattmeter from 2/90 QST. Now you only have to acquire a single NICE 1mA meter and a suitable enclosure to complete the project and make it a true bench worthy instrument.

As usual, the Buildathon started about 4:00 on Friday afternoon in the Buildathon room at the FDIM hotel. Of the 40 registered builders, we had 30 actually show up to build with 3 having to leave early to prepare for club night in the ballroom. A few builders ran out of gas so about 25 completed the build and needed the final calibration. Mark Dunning, VK6WV, from Perth Australia volunteered to be the head of the Calibration Department (Hurray for Mark!!!) and we successfully got every project working and calibrated before the builder left.....save 1. MANY sets of eyes checked for errors and we did all kinds of checks but there was simply a hidden short on one unit that we simply could not find. That builders went home with another kit to build with a little more time and patience to insure a successful second build....



The W7EL original design from Feb.1990 QST was truly a homebrew project with a hand made microstrip line to form the 4 port directional couple and all the additional components added using point to point ugly (but beautiful!) construction style. The design was updated by W1REX to make it easier to build and eliminate a difficult to source OpAmp. The power meter portion of the circuit has diode compensation and is accurate from 5mW to 10W in 3 ranges: .1W, 1W 1nd 10W which makes it very useful to QRPers looking at low power measurement. During the Buildathon, Mark Dunning, VE3POA, came up with a good calibration procedure based on the original QST article by Roy. I have included his nice page of notes about calibrating the meter in the documentation links below. We did have to solder extra wires to our builds to calibrate them properly as we used a fixed off-board setup to adjust for calibration and then added fixed resistors in their place to make the calibration permanent.

After I returned home from FDIM, I sent a couple of kits down to Chuck Carpenter, W5USJ, for experimenting and creating a more detailed documention portfolio. Chuck has worked for a couple of weeks and come up with some very thorough sheets! First and foremost, he did lots of measuring and tinkering and discovered that the 4 Port Direction Coupler used in this design is not as HOT as the microstrip line coupler used in the W7EL design so a new adjustment and calibration procedure was required. Please review all the v3b documents to see all the updates. The kits built in the Buildathon need to be readjusted to compensate for the lower output from the coupler circuit. During the Buildathon, we did not run any RF through the coupler so the floating secondary connections on the toriods were NOT caught during the build. You have to scrape the solder mask off between the two pads and solder a bridge between the 2 pads and the ground plane. The pcb fix is documented below.

For kits mailed out, I will be making a few parts changes to the kit, including: SIP sockets for the pots to make it easier to insert the pots and then extract them and insert fixed resistors, if desired, for a more portable meter. I have also added the 2 range adjust pots and another 0 ohm jumper to short the 2 pins of the OpAmp in order to make the calibration possible. I will also be including the more popular BNC connectors for more universal acceptance and parts to construct the 10db attenuator pads for calibration so mailed kits are more complete. All you need to add is and enclosure and appropriate meter.

The Registered Builders were:

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NORM SCHKLAR....................WA4ZXV
CHRIS WALDRUP..................KD4PBJ
RON EIKE.................................KA3PSO
RON HAMBRIC........................N5SBN
JOHN KITCHENS..................NS6X
JOHN DEMUTH......................K8CQA
KARL DEMUTH........................N8PKB
ALAN SHAFER........................AC8AP
MARTHA AUCHARD..............W0ERI
GARY AUCHARD.....................W0MNA
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 10

WALT SCHUKNECHT.............N7IZM
CHRIS SCHUKNECHT.............N5ZIW
RICHARD SCHWENKE............N8GBA
DON SZENINA.........................N8HDT
BILL KOSONEN........................KD8ZEG
BILL HULLEY.............................K7WXW
MARK DUNNING........................VK6WV
JOHN EICHORN........................WD1P
ALBERT PETER........................AC8GY
PETER AYEARST.......................VE3POA
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 20

DAVID WENDT...............................VE3EAC
DAVE RAJNAUTH.......................VE3OOI
ERIC GUNTER...............................KC2TXO
ERIC VINANDE.............................KG6NFJ
JIM FISHER..................................AJ3DI
JOHN KUNZ....................................N8MCA
JASON PONTIOUS.......................
MICHAEL GLENNON.....................KB4JHU
DONAL HEIDENBLAD................K3CUU
CARL HEIDENBLAD.....................N1CUU
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 30

JOE EISENBERG............................K0NEB
JILL COMBS.....................................KD8QGA
JEREMY HONG..................................KD8TUO
PAULETTE QUICK...........................WB9VHF
ROGER STEIN..................................VA1RST / K7SJ
CHARLES BROWN..............................W3CHB
CHARLES OHRSTEDT........................KB8YUM
LARRY MORPHEW................................
DONALD SANDERS..............................
KEVIN MURPHY...................................
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 40
THE BUILDATHON IS OFFICIALLY SOLD OUT!
SEND ME AN EMAIL IS YOU WANT TO BE ON A STANDBY LIST!!
STANDBY LIST
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KITS TO BE PICKED UP @ FDIM
(NO free DMMs included with these kits!)
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JOHN YOUNG..........................KA8KNE






Please pay special attention to the names and calls below! They are volunteering their time & expertise to help make the 2017 FDIM Buildathon a success!

VOLUNTEERS:
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Rex Harper.........W1REX
Graham Firth.......G3MFJ
Rod Wallberg.......W8GRI
Jon Poland...........N0WL
Jack Purdum.........W8TEE

Hope to see you there!
Rex W1REX









Documentation
  • FDIM Buildathon 2018 pcb fix
  • FDIM Buildathon 2018 v3b Instruction
  • FDIM Buildathon 2018 v3b Schematic
  • FDIM Buildathon 2018 v3b Assembly Options
  • FDIM Buildathon 2018 v3b Meter Scales
  • FDIM Buildathon 2018 v3b Meter Scales
  • FDIM Buildathon 2018 v3b Setup
  • FDIM Buildathon 2018 v3b Adjust Setup
  • W5USJ WM-2 Work-a-Like
  • FDIM2018
  • Marks NOTES