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July 24, 2024. Some Design Thoughts

[Late Breaking News! Dean, KK4DAS now has resolved the MCU problem -- it was a library issue. Check out this link on his build and the resolution.

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In one week, we will be looking to change the Calendar to August. Time to start thinking about a winter project. Do we channel the thinking toward something we need like a new rig for the shack or to an item of test equipment. 

If the wheel of fortune lands on test gear, then one of the most useful pieces is a SWR Bridge. In the most recent SolderSmoke Podcast (#252) Dean, KK4DAS shared some tales of woe regarding a SWR Power Meter that would be built into his homebrew SDR Bitx (SBitx) transceiver.

Essentially, Raspberry Pi 4 software poles a sensor (Stockton Bridge) and the voltage values, Forward and Reverse, are read and then massaged in software to derive an output for display on the screen. He has been wrestling this gorilla for several weeks. The issue is tilting toward a software problem with the onboard ATTiny MCU in the sensor.

Back up in the sewer for half a moment with a starting place of what is a SWR Bridge Circuit. For a more than two-word sentence from the Co-Pilot AI, check out the K6JCA Blog It does have a lot of math but will also give the clueless a clue about SWR Bridges.

One very popular bridge circuit amongst the QRP crowd is the Stockton Bridge named after a UK ham who presented this approach in a G-QRP Club SPRAT article in 1989/90. The key feature of this bridge is it can remain in line without being a dummy load and works down to very low power levels.

You are in Luck as W8DIZ (SK) in his website has kits (about $12) as well as bare boards ($4). Look for the Universal SWR Bridge.






This kit gives you two voltages, one forward and one reverse. If you simply install two meters for these readings, then you will need a magic decoder ring to translate the readings into SWR. If you are lucky enough to own a Drake W4 wattmeter it originally came with a nomograph to make that decode. Otherwise, you can create your own nomograph using an Excel Spreadsheet.

This is where a MCU and a Raspberry Pi come into the picture as the voltage readings sent to Analog pins on the R Pi can then be calculated and displayed even on a simple 16X2 LCD.

Now before you get too excited like seeing Elisabeth Hurley in a faux leopard skin bikini, the kit is only good thru HF and a max of 100 watts.

1:1 SWR

I don't have the other shoe, as yet, regarding the MCU and RPi but will endeavor to find same and share with you. For those who loathe digital stuff all you need is the kit and two meters and a bit of time with a spreadsheet. 

TYGNUBNAINNVNA.

73's
Pete N6QW

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