QRP Directional WattmeterPrice: $50.00
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.
We had 25 builders complete 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....
original FDIM2018 Buildathon kit
After I returned home from FDIM, I sent a couple of kits down to Chuck Carpenter, W5USJ, for experimenting and creating a more detailed documentation 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 did not produce as much output voltage as the microstrip line coupler used in the W7EL design so a new adjustment and calibration procedure was required.
(The kits built in the Buildathon need to be readjusted to compensate for the lower output from the coupler circuit. Please review all the v3b documents under the 2018 Buildathon ITEM to see all the updates required to make the Buildathon kit operate fully.
I decided to make some changes to the Buildathon kit for a more universal appeal. Included changes are: BNC connectors instead of RCAs, a full set of trimmer pots for easy range adjustment, panel mounted switches to make the kit easier to put in an enclosure. All you need to add is an enclosure and an appropriate meter. I found a nice little meter for about 7 bucks on Amazon. You can buy them cheaper with shipping from China or fulfilled by Amazon for a little more money. Mine came from Amazon and arrived 3 days after ordering it.
The QRP-DWM presented here is a nice piece of test equipment that should be welcomed on every QRPers bench.
BUILDERs NOTE: the germanium diodes now supplied with the Directional Power Meter are VINTAGE RUSSIAN point contact diodes that are equivalent to 1N34As. Please note in the 9D9 Diode Marking image below that the Anodes are marked with a WHITE dot instead of the bar on the Cathode end of 1N34As. Install the 9D9 diodes opposite as to the markings for a 1N34A diode on the silk screen!