Hamfesters’ Hamfest 2017; Homebrew RF Power Meter

This morning, I trekked out to the Hamfesters’ Hamfest in Peotone, IL for the second year in a row. Though the rain threatened to put a damper on the day, the sun had burned through by 10am and left everyone alone.

There was an interesting mix of vendors at this year’s Hamfesters – lots of assorted “hardware” (tool, dental picks, kitchen sink), but also a good number of radios new and old. There was a table absolutely stacked with older 2M gear, mostly Heathkit. A couple newer ICOM rigs, the usual FT-101e’s… nothing too exciting. I scooped up a handful of beefy heatsinks meant for CPU’s, but will be re-purposing them as LED heatsinks in future projects.

I did find two real scores at the Hamfest today. First, from Patrick-J, a secondhand, gov’t-surplus Tektronix TDS644a. It’s a 4-channel digitizing scope, with 500MHz bandwidth at 2GS/s, and I got it for a song.

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Not unexpectedly, when I got it home and opened it up, it’s in need of a good re-cap-ing (see, for example, W6KWF’s recent video on this subject.) I’ve already got two packs of electrolytics on their way from the wide wide eBay, so I’ll pick this project back up on a future weekend.

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The second find was a small, slant-top box with a 1mA DC meter movement already built into the top. It was configured originally to be a shunt ammeter, but I really just wanted the nice box and meter. Instead of measuring AC current, it’s now set up to measure RF power.

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The original unmodified meter – something out of some other amateur’s homebrew shack.

The only external sign that the box has been altered is the presence of a small SMA jack, but the internals are entirely different. I populated and installed one of the AD8307-based power meter PCB’s that I cooked up back in March, fed directly from the external SMA connector. The output feeds the mA meter through an op-amp buffer – I started with a direct connection, but the AD8307, with its 10k-15K output impedance, wasn’t up to the task. The voltage output from the AD8307 is also directly fed to the red external binding post, for measurement with a DMM. I added a little green power LED to remind me to turn the darn thing off. I left the original switch in place, just because it feels right.

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The final meter, with an SMA/BNC adapter hanging off it.

The schematic for this project is essentially the same as W7ZOI’s Power Meter published in QST – see that article for details, as well as Wes’ further errata. The meter covers the range of the AD8307’s output voltage, from about -70dBm to +20dBm. As I’ve been working on putting together my BITX homebuild (more on that soon!), I’ve been thinking that having an RF power meter with an analog movement would be very helpful in peaking filters and looking at relative input and output power. And now, I have one!

See you at the ‘fest!

73

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Bench Report – HamRadio360 Podcast, Beach-40, SigGen, Power Meter Projects

A few weeks, back, I spent about half an hour chatting with Jeremy, KF7IJZ, for a listener-projects segment on the HamRadio360 Workbench podcast. That episode just went live this morning, if you want to take a listen online. Jeremy and I had a swell time chatting about the QST 40-9er Transceiver project I built awhile back, and ended up packaging with a laser-cut faceplate.

Seeing the podcast pop up got me thinking – from the last few months of posts, you’d think that I’d been spending all my time operating and that my bench was empty. Not so! There are three major projects milling about the bench these days. None are complete enough to merit a full post, but consolidating them all in to one post feels right.


First on the bench is the continuation of my Beach 40 Transceiver project, that I’ve been working on since the Fall. I’ve been making steady progress recently on the penultimate stage, the RF amplifier, which will take the ~10dBm modulated voice signal from the balanced modulator and turn it into ~33dBm (2W) of RF output power to send to the LPF and out into the world. The amplifier is a three-stage design, with a 2n3904 buffer feeding a BD139 driver and another BD139 final.

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Beach40 PA Schematic and Layout, in Graphite-O-Vision.

The module is assembled manhattan-style on single-sided copper glad, just like the rest of the project. The layout surrounds the +12V-on-transmit bus down the center of the PCB… more on that error later.

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The Beach40 PA mid-assembly – you can see the +12VT buss running down the middle, and the buffer and driver stages starting to take shape.

The module assembly and testing went fairly smoothly – using a variable-voltage power supply, it’s easy to vary the output power of the amplifier, and to observe the changes in PA heating that this causes. You can see the two large screw-on heat sinks on the BD139’s in the following pictures, as well as the the large ferrite bead which feeds the Final. There’s also a chunky 100uF cap at the DC input to the board for additional decoupling.

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The assembled Beach40 PA.

 

The problem I’m having now is the same one that plagued me way back in my assembly of the Virgin Receiver, my first first homebrew receiver: this amplifier would really love to be an oscillator. With the PA voltage set at 12V, any time the signal imput is sufficiently strong, the PA will fall hard into oscillation, and won’t stop until the PA voltage is brought way down or cut. I’ve been experimenting with emitter degeneration for the final BD139 (the original schematic has the emitter directly grounded). That’s helped, but not much. I’ve also changed the RF-carrying wires from hookup wire to coax, to mitigate feedback through other modules. Another help, but so far, the oscillation problem is still there. The next step will be to move the modules themselves around, to limit the amount that the high-power RF coming from the PA has to pass by earlier stages of the transmit chain (VFO, mic amp, balanced modulator) to try to eliminate feedback that way. Fingers crossed!


Next on the bench is a repackaging of an old project – my SI5351-based “VFO,” which for the better part of a year has been serving as my primary signal generator for experimental projects, spread out in all its Al-Fresco glory on a breadboard. With the amount of troubleshooting I’ve been going through on the Beach 40 project, I’ve decided to finally box up the SI5351, Arduino, and display into a proper project box, and make the thing a real SI5351-based Signal Generator. The schematic is essentially unchanged (minus the PA) since I used the project in my SI5351-based transmitter project, which is reproduced below:

CW Transmitter sCHEMATIC

To that end, I purchased an inexpensive nibbling tool and an expensive project box from the local Fry’s electronics, and have been working in the past couple weeks to marry the spread-out guts of the previous project with the clean lines of the enclosure. Biting out the large hole for the display 1/8″ at a time was time-consuming and strangely soothing.

At this point, the display and Arduino are connected, but I have yet to wire the two control buttons, rotary encoder, and Si5351 breakout board back into the Arduino. I also need a provision for getting the signal out the front of the darn thing, so I’m waiting on a shipment of SMA connectors and a jumper to go from the SI5351 breakout board to the front panel. So far, so good.

The code for this project is still on Github.


The last project on the bench is an RF power meter circuit, based on a circuit by W7ZOI from 2001. While the old scope-probe-across-a-50-ohm-load technique has proven very useful, I’ve found myself wanting a way to more reliably measure RF power at low levels, and to do it in a way that could be interfaced to a microcontroller or computer. To that end, I plan on using the W7ZOI circuit connected to an Arduino, much like Vu2ESE’s Sweeperino, to make digital power measurements.

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I rolled a little PCB for this project from OSHPark, and since that service provides PCBs in multiples of 3, I figure I’ll connect one board to an analog meter (for that old-school feel), one to an independent Arduino (for digital measurements), and embed one inside the SigGen project, for marrying signal generator and power measurement at specific frequencies. This last project, I figure, will be especially useful for examining filter behavior at HF and piping the information to a PC for display an analysis.


So that’s what’s on the bench at the end of March 2017. All of these will hopefully merit full posts in future as the projects come to fruition, but for now, the is the smorgasbord that is my bench.

Hear you on the air!

73