You Can Learn a Lot from a Dummy (Load)

If you work on RF circuits–even if you aren’t a ham radio operator–you ought to have a dummy load. A dummy load is a non-radiative “antenna” with known impedance that you can use to test your RF circuit without radiating. For radio work, you usually just need a 50-ohm resistor that is non-inductive (at least at the frequencies you are interested in) and that can dissipate the amount of power you’ll expect it to handle (at least for a short time). [VO1PWF] wanted a dummy load and built his own.
CantennaThe Cantenna (not the Pringle’s kind; see left) was a famous dummy load design when Heathkit was in business. It was a single carbon rod immersed in a paint can full transformer oil (which we now know was full of dangerous PCBs; and we don’t mean printed circuit boards). [VO1PWF’s] design is a little more practical, using some resistors in parallel (20 1K resistors), a plastic pipe housing, and mineral oil to keep it all cool.

The reason for the parallel resistors is to maximize the power handling capability. The resistors are 3W units, so the dummy load–in theory–can handle 60 watts. Often, high power resistors are wire wound and thus have a good bit of parasitic inductance that makes the dummy load reactive (not a good thing since that makes the load impedance vary by frequency). They do make non-inductive wire wound resistors, but these aren’t truly non-inductive. The wire winds in two different directions, so the inductance tends to cancel out. We wouldn’t trust them to be a pure resistance in a high-power dummy load design.

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The Grid Dip Meter: Forgotten Instrument

It used to be a major rite of passage for a hardware hacker to acquire an oscilloscope. Until recently, new instruments were rarely in normal people’s budgets, so you probably made do with a used scope. Now, there are lots of inexpensive options, especially if you include low-end PC scopes and “scope meters.” Digital meters are also now inexpensive (often free at some major stores), along with signal generators, frequency counters, and even logic analyzers.

But there is one piece of test equipment you don’t see as often as you used to and its a shame, because it is a very versatile piece of kit. Admittedly, if you aren’t doing wireless work, it might not be high on your wish list, but if you do anything with RF, it is not only a versatile tool, but a good value, too. What’s it called? That depends. Historically, they went by the name “Grid Dip Oscillator” or GDO. Sometimes you’d hear it called a “Grid Dip Meter” instead. However, modern versions don’t have tubes (and, thus, no grid) so sometimes you hear them now called dip meters or maybe just dippers.

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The Junk Bins of Akihabara

Akihabara, Tokyo has transformed over the years. In its present form Akihabara emerged from the ruins of a devastated Tokyo after World War 2 when the entire district was burnt to the ground. The area was rebuilt in the shadow of the Akiba Jinja (dedicated to the god of fire prevention), and a new breed of street vendors began to appear. Huddling under the protection of railway bridges, and dealing mostly in Black market radio parts, these vendors set a new tone to what would become Japan’s “Electric Town”. And as Japanese manufacturing prowess grew so too did Akihabara.

Maids touting for business

Now of course Akihabara is also home to Otaku culture, and is perhaps best known in this regard for its maid cafes. Streets are littered with maids touting their cafes, somewhat incongruously among computer outlets and precision tooling stores.

My interests however lie squarely in Akihabara’s glorious junk bins. Of all places I think I’m happiest  digging through this mass of discarded technology from Japan’s manufacturing past.

A tour through the junks bins is like an archaeological dig. And in this article I will present some recent finds, and ponder on their relevance to Japanese manufacturing.

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VirtualBench Tear Down

What do you get when you cross a mixed-signal oscilloscope, a function generator, a multimeter, a power supply, and some programmable digital I/O in a box? Sounds like the set up to a very geeky joke, but it is actually National Instrument’s VirtualBench product. [Shahriar] has one and wanted to know what was inside, so he did a tear down.

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RTL SDR As A Spectrum Analyzer

RTL-SDR, the USB TV tuner turned software-defined radio is an amazing device, capable of listening to nearly anything from 25MHz to 1750MHz, fits in your pocket, and costs about $20. Even more astonishing is that it’s also a kinda-okay spectrum analyzer. [Kerry D. Wong] tested out one of these USB TV tuner, and the results are exactly what you would expect: it lacks a little precision, and sampling bandwidth is only a tiny bit terrible, but it does work.

A stock USB TV tuner doesn’t come with a connector that would normally be used for spectrum analysis. A BNC connector can be easily attached, as can a terminator to match the 75Ω impedance of the SDR. This isn’t really necessary; the frequencies being measured are low enough that you can get away without one.

As far as software goes, [Kerry] first pulled out the usual suspects of the SDR world; rtlsdr-scanner distorted the measured spectrum, as did a lot of other SDR receivers. Gqrx SDR was the first one that worked well, but the king of this repurposing of USB TV tuners was OSMOCOM. There’s a huge number of tools for spectrum analysis right out of the box with this package.

How did the RTL SDR fare as a spectrum analyzer? Feeding some stuff in from a signal generator, [Kerry] discovered the LO in the RTL SDR was off by a hair. Also, OSMOCOM only measures amplitude in dB, not the dBm found in every other spectrum analyzer ever made. By measuring a 0 dBm signal whatever value displayed can be shifted up or down.

So, does it work? Yes, it does. If, for some reason, you need a spectrum analyzer now, can you use this? Yep. Pretty cool.


Here’s something that’s just a design study, but [Ivan]’s Apple IIe phone is a work of art. You’re not fitting a CRT in there, but someone out there has a 3D printer, an old LCD, and a GSM module. Make it happen. See also: the Frog Design Apple phone.

A few days ago we posted something on an old ‘286 machine that was able to load up the Hackaday retro site. For a few people, this was the first they’ve heard about our CSS and Javascript-less edition designed specifically for old computers. They dragged out some hardware, and [WTH] pulled up the site on a Dell Axim.It’s actually somewhat impressive that these machines have SD cards…

[Arduino Enigma] created a touchscreen Enigma machine. Why haven’t we seen an Arduino Colossus yet?

The crew at Adafruit now have a Flying Toaster OLED, which means we now have flying toaster bitmaps for all your OLED/graphic display projects.

[Ian] had an old rackmount programmable voltage standard. This was the remote programmable voltage standard, without front panel controls. No problem, just get an Arduino, shift register, and a few buttons. Video right here.

A few months ago, [Jan] released a neat device that stuffs a modelling synth inside a MIDI plug. He’s selling them now, and we’d love to see a few videos of this.

Think Before You Measure – Old Test Gear and Why It Is Awesome

Good, workable test gear is key to enabling our hobby. In this post we will discuss where to procure it at rock-bottom prices, what to look for, how to fix it, and how to tailor your laboratory practices around gear that may not be reliable.

We are lucky to be living in an era with plentiful high-quality test gear. Since the Second World War, surplus test gear has been in abundance at low costs enabling hobbyists, innovators, and academics to experiment and build great things. If you are willing to think before you measure you can save serious amounts of money and have a professional laboratory in your home.

Where to buy
The obvious answer is eBay, but the deals on test equipment are at the hamfests. Don’t be fooled by the name. Hamfests sell much more than amateur radio equipment. Hamfests are swap meets where hobbyists trade electronics of all kinds. Check out the ARRL hamfest calendar to find the next local one near you! I suggest you arrive early, however. The culture of hamfests tends to favor showing up as soon as the doors open and leaving about two hours before the official end. The early bird gets the worm!

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