Archive for category Amateur Radio

eBay Listings: Read Carefullly

What’s wrong with this picture? (clicky for more dots)

eBay - 40 pin IDC cable - header

eBay – 40 pin IDC cable – header

Not obvious?

Here’s the description, slightly reformatted for clarity:

New 5m IDC Standard 40 WAY 1.8” Multi-Color Flat Ribbon Cable Wire Connector


Type: IDC standard.

10 colors, 4 group, total 40 pcs cables per lot

5 meter per lot.

width: 4.7 cm / 1.8 inch

Package content: 5M Flat Color Ribbon Cable

If you divide the 1.8 inch cable width by its 40 conductors, you find the wires lie on a 45 mil pitch. If you were expecting this “IDC standard” cable to fit in standard insulation displacement cable connectors with a 50 mil pitch, you’d be sorely disappointed. You can get metric ribbon cable with a 1 mm = 39 mil pitch, but this ain’t that, either.

Here’s what an individual eBay wire (black jacket) looks like, compared to a wire from a standard ribbon cable (red jacket):

Ribbon cable - 26 AWG - eBay vs standard

Ribbon cable – 26 AWG – eBay vs standard

A closer look at the strands making up the wires:

Ribbon cable - 26 AWG - eBay vs standard - strands

Ribbon cable – 26 AWG – eBay vs standard – strands

As nearly as I can measure with my trusty caliper, the eBay ribbon cable has wire slightly smaller than 30 AWG, made up of seven 40 AWG strands, as opposed to standard 26 AWG wire made of seven 34 AWG strands. The good stuff might be 28 AWG / 7×36 AWG, but I was unwilling to break out the micrometer for more resolution.

I’d like to say I noticed that before buying the cable, but it came to light when I measured the total resistance of the whole cable: 80 Ω seemed rather high for 200 meters of 26 AWG wire. The wire tables say that’s about right for 31 AWG copper, though.

Changing the AWG number by three changes the conductor area by a factor of two, so you’re getting less than half the copper you expected. Bonus: it won’t fit any IDC connectors you have on the shelf, either.

Turns out a recent QEX article suggested building an LF loop antenna from a ribbon cable, so I was soldering all the conductors in series, rather than using connectors, and it should work reasonably well despite its higher DC resistance.



APRS/GPS + Voice Interface: Improved PTT Button Cap

Long ago, Mary picked out a PTT switch with a raised, square post that provided a distinct shape and positive tactile feedback:

PTT Button - bare post

PTT Button – bare post

Time passes, she dinged her thumb in the garden, and asked for a more rounded button. I have some switches with rounded caps, but replacing the existing switch looked a lot like work, sooooo:

PTT Button Cap - Slic3r preview

PTT Button Cap – Slic3r preview

As with all small objects, building them four at a time gives the plastic in each one time to cool before slapping the next layer on top:

PTT Button - on platform

PTT Button – on platform

The hole in the cap is 0.2 mm oversize, which results in a snug press fit on the small ridges barely visible around the post in the first image:

PTT Button - rounded cap

PTT Button – rounded cap

Rather than compute the chord covering the surface, I just resized a sphere to twice the desired dome height (picked as 6 threads, just for convenience) and plunked it atop a cylinder. Remember to expand the sphere diameter by 1/cos(180/sides) to make it match the cylinder and force both to have the same number of sides.

If it falls off, I have three backups.

The OpenSCAD source code as a GitHub Gist:



Audio Direction Finding

Given a point source of audio (or RF, for that matter) that’s far enough away to produce more-or-less plane wavefronts, the range difference between two microphones (or ears) is:

ΔR = (mic separation) x sin Θ

The angle lies between the perpendicular to the line from the midpoint between the mics counterclockwise to the source source: + for sounds to your left, – for sounds to your right. That’s the trig convention for angular measurement with 0° directly ahead, not the compass convention, but you can argue for either sign if you keep track of what’s going on.

The time delay between the mics, given c = speed of sound:

ΔT = ΔR / c

For microphones 300 mm apart and c = 344 m/s:

ΔT = 872 µs = 0.3 m / 344 m/s

If you delay the sound from the mic closest to the source by that amount, then add the mic signals, you get a monaural result that emphasizes, at least a little bit, sounds from that source in relation to all other sounds.

In principle, you could find the angle by listening for the loudest sound, but that’s a fool’s game.

There’s an obvious symmetry for a source on the same side, at the same angle, toward the rear.

A GNU Radio data flow diagram that lets you set the angle and listen to / watch the results:

Audio Direction Finding.grc

Audio Direction Finding.grc

The original doodles show it takes me a while to work around to the answer:

Audio direction finding doodles

Audio direction finding doodles


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Makerspace Starter Kit Available

For a variety of reasons that aren’t relevant here, I must dramatically reduce the amount of stuff in the Basement Laboratory / Machine Shop / Warehouse.

If you (or someone you know) has / is starting / will start a makerspace or similar organization, here’s an opportunity to go from zero to hero with a huge infusion of tools / instruments / make-froms / raw material / gadgets / surplus gear.

Think of it as a Makerspace Starter Kit: everything you need in one acquisition.

You’ve seen much of the stuff in these blog posts during the past five years, although I tightly crop the photos for reasons that should be obvious when you consider the backgrounds.

A few glimpses, carefully chosen to make the situation look much tidier than it really is:

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I’m not a hoarder, but I can look right over the fence into that territory…

I want to donate the whole collection to an organization that can figure out how to value it and let me write it off. Failing that, I’m willing to sell the whole collection to someone who will move it out and enjoy it / put it to good use / part it out / hoard it.

We can quibble over the value, which surely lies between scrap metal and filet mignon.

As nearly as I can estimate from our last two moves, I have 6±2 short tons of stuff:

  • Metal shop: old South Bend lathe / vertical mill-drill / bandsaw / hand tools / arbor press
  • Cabinets / shelves loaded with cutters / tools / micrometers / calipers / whatever
  • Gas & electric welding equipment, gas foundry furnace
  • Walls / bins / drawers of fasteners / wire nuts / plumbing fittings / pipe clamps / you-name-its
  • Bookshelves of references / magazines / databooks; I’ll keep at most one set of the magazines with my columns
  • Ham radio equipment / antennas / cables
  • Radial saw, blades, clamps, tooling, and a lumber / plywood stockpile
  • Labeled boxes of make-froms on steel shelving; you get the shelves, the boxes, and their contents.
  • Solvents, chemicals, metals, minerals, elements, etc.
  • Electronic / optical / mechanical surplus & doodads
  • Stockpiles of metal rods / pipes / beams / flanges / sheets / scrap parts
  • Tools & toys & treasures beyond your wildest imagination

When we left Raleigh, the moving company estimator observed “This will be like moving a Home Depot!”

You must take everything, which means you must have the ability & equipment to handle 6±2 tons of stuff in relatively small, rather heavy, not easily stackable lumps. You’ll need 1000+ square feet of space with at least a seven-foot ceiling on your end to unload the truck(s) and create a solid block of stuff with skinny aisles between the shelves. This is not a quick afternoon trip for you, your BFF, a pickup truck, and a storage unit.

I plan to keep the Sherline, the M2 3D printer, various small tools, some hardware / parts / stock, most of the electronic instruments (antique-ish, at best) and components, plus odds and ends. I’ll extract or clearly mark those items, leaving your team to move everything else without (too many) on-the-fly decisions.

I can provide photos and descriptions, but, realistically, you should evaluate the situation in person.

Although we’re not planning to move in the near future, if you’re thinking of moving into the Mid Hudson Valley and always wanted a house with a ready-to-run Basement Shop, we can probably work something out. Note: all of the online real estate descriptions, including Zillow, seem confused, as the two houses on our two-acre property contain the total square footage / rooms / baths / whatever. Contact us for the Ground Truth after you’ve contemplated the satellite view.

As the saying goes, “Serious inquiries only.”


Vacuum Tube LEDs: Platter Chassis

Given that Greenlee chassis punches fit the ersatz vacuum tube sockets, this makes a certain perverse sense:

Greenlee puch on hard drive platter

Greenlee puch on hard drive platter

This set of punches is probably worth its weight in, uh, tool steel, because Greenlee got out of the Radio Chassis Punch business quite a while ago:

Greenlee 730 Radio Chassis Punch Assortment

Greenlee 730 Radio Chassis Punch Assortment

As far as a Greenlee punch is concerned, a hard drive platter looks a lot like thin aluminum sheet:

Greenlee punched drive platter

Greenlee punched drive platter

I lathe-turned that white bushing to align the hard drive platter around the screw inside the punch. The right way to make that bushing in this day & age definitely involves 3D printing, but I was standing next to the lathe and spotted a nylon rod in the remnants bucket underneath.

The inner ring crumples around the bushing inside the die, while the platter outside remains flat & undamaged through the entire experience.

I match-marked the socket & “plate cap lead” holes on the punched platter and introduced it to Mr Drill Press, but the right way to do that for more than one socket / plate involves a Sherline mill fixture and some CNC.

And then It Just Worked:

Vacuum Tube LEDs - IBM 21HB5A drive platter socket

Vacuum Tube LEDs – IBM 21HB5A drive platter socket

That’s obviously a proof of concept; the socket rests on the desk with the rest of the tubes / sockets / Neopixels tailing off to the right. The plate cap lead should pass through a brass tube fitting on the platter, just for pretty.

The 7- and 9-pin sockets have a raised disk that’s slightly smaller than the 25 mm hard drive hole; the easiest fix involves slightly enlarging the disk to match the hole. Although CDs / DVDs have a 15 mm hole and Greenlee punches work surprisingly well on polycarbonate, if I’m going to CNC-drill the screw / wire holes anyway, CNC milling the middle hole should go quickly and eliminate a messy manual process.

Come to think of it, that big tube would look better in the middle of a DVD amid all those nice diffraction patterns from the RGB LEDs in the cap…



Bike Helmet Earbud Replacement

A bag arrived from halfway around the planet, bearing five sets of cheap earbuds. There was no way to tell from the eBay description, but they’re vented on the side:

Cheap earbud - side vent detail

Cheap earbud – side vent detail

And also to the rear, down inside those deep slots below the chromed plastic cover:

Cheap earbud - back openings

Cheap earbud – back openings

The raised lettering is a nice touch; the other earbud has a script L.

The PET braid over the fragile wire should withstand a bit more abuse than usual. The strain relief isn’t anything to cheer, though, consisting of that rectangular channel with the wire loose inside. I figured I’d start minimal and fix whatever crops up; I have nine more earbuds to go.

The motivation for all this was having the Gorilla Tape peel off the helmet, leaving a hardened mass of glue behind, then snagging the earbud wires. This is the new, somewhat better protected, wiring:

Bell Helmet - mic-earbud wire - hardened tape adhesive

Bell Helmet – mic-earbud wire – hardened tape adhesive

In a triumph of hope over experience, I applied more Gorilla Tape:

Bell Helmet - re-taped mic-earbud wiring

Bell Helmet – re-taped mic-earbud wiring

The helmet may need replacing after another iteration or two.

My solid modeling hand has become stronger these days, so I should gimmick up a flat-ish wart anchoring the mic boom and all the wiring to the helmet shell.


Family Christmas Ride

This doesn’t happen very often nowadays:

The Family That Rides Together - DCRT near Parker - 2015-12-25

The Family That Rides Together – DCRT near Parker – 2015-12-25

That’s in the rock cut just east of the tunnel under Parker Avenue. In a normal winter, that rock wall completely shadows the asphalt and preserves an icy layer through March.

We rode out-and-back over the Walkway, with a few digressions along the way:

Christmas Ride 2015 - APRS track

Christmas Ride 2015 – APRS track

A good ride was had by all; we could get used to those empty roads…