Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Two of the external Li-Ion battery packs I’m using with the bike radios seemed to fail quickly after being charged, so I sawed them open to check the state of the cells. This time I used the fine-tooth cutoff blades, rather than a coarse slitting saw:
Li-Ion pack – sawing case
As before, a 2 mm depth-of-cut, done 0.25 mm per pass after the first millimeter, seems about right. I didn’t saw the front of the case near the jack, which proved to be a mistake; the interlocked case halves need cutting.
No cell trouble found, which leads me to suspect an intermittent short in the battery-to-radio cable that trips the battery protection circuit. The spare cables went into hiding during the shop cleanout, so I can’t swap in a known-good cable just yet; of course, the existing cable behaves perfectly on the bench. The suspect cable is now on my bike and, if the problem follows the cable, further surgery will be in order.
The object of soldering all 40 wires in the 5 m hank of ribbon cable in series is to build a 40 turn loop antenna to receive LF radio signals like WWVB at 60 kHz. The antenna, being basically a big coil of wire, will have an inductance that depends on its layout, so putting a capacitor in parallel turns it into a resonant tank circuit. Given a particular layout (and, thus, an inductance), you can choose the capacitor to make the antenna resonant at whatever frequency you need (within reason).
With the joints soldered & reinforced with epoxy, the inductance across all 40 turns:
535 µH – rolled into a compact bundle
6.66 mH – vaguely circular loop on the concrete floor
5.50 mH – lumpy rectangle on the concrete floor
Back in a slightly different circular layout on the floor:
6.8 mH – across all 40 turns, as above
2.0 mH – across either set of 20 turns from the center tap
Given that inductance varies as the square of the number of turns, you’d expect a factor of four between those two inductances, but that’s not how it worked out.
Hanging the loop from a pair of screws in the floor joists to make a droopy rectangle-oid shape and driving it from a 600 Ω signal generator through a 10 kΩ resistor, it’s self-resonant at 213 kHz. Repeating that with a 470 kΩ resistor drops the resonance to 210 kHz, which isn’t different enough to notice and surely has more to do with my moving the loop while dinking with resistors.
Adding parallel capacitance (measured with an LCR meter, just to be sure) changes the resonance thusly:
9.9 nF → 20 kHz
900 pF → 64 kHz
400 pF → 87 kHz
250 pF → 108 kHz
none → 213 kHz
Because the resonant frequency varies inversely as the square root of the capacitance, halving the resonant frequency means you’ve increased the capacitance by a factor of four. Because 250 pF halves the frequency (mostly kinda sorta close enough), the loop’s stray capacitance must be about 1/3 of that: 83 pF.
Yeah, 1/3, not 1/4: the additional capacitance adds to the stray capacitance, so it goes from 83 pF to 250 + 83 pF = 333 pF, which is four times 83 pF.
The self-resonant frequency of 213 kHz and the 83 pF stray capacitance determines the loop inductance:
L = 1/((2π · 213 kHz)^2 · 83 pF) = 6.9 mH
Pretty close to the measured value from the floor, I’d say.
To resonate the antenna at 60 kHz, the total capacitance must be:
60 kHz = 1/(2π · sqrt(6.9 mH · C)) → C = 1050 pF
Which means an additional 1050 – 83 = 970-ish pF should do the trick, which is about what you’d expect from the 64 kHz resonance with the 900 pF cap above. I paralleled pairs of caps until it resonated at 59.9 kHz.
The -3 dB points (voltage = 1/sqrt(2) down from the peak) turned out to be 58.1 and 60.1 kHz, so my kludged caps are slightly too large or, once again, I nudged the loop.
Figuring Q = (center frequency) / bandwidth = 59.1 / 2 = 30, which works out close enough to Q = X / R = 2600 / 80 = 33 to be satisfying. Using standard 26-ish AWG ribbon cable, rather than crappy 31-ish AWG eBay junk, would double the conductor area, halve the series resistance, and double the Q. Faced with that much resistance, I’m not sure better caps would make any difference.
Attaching the spectrum analyzer through a 470 Ω resistor to reduce the load:
Loop – 40T 1nF – spectrum
I’d love to believe that big peak over on the left at 57.1 kHz is WWVB, but it’s not.
What’s more important: the broad hump between 56 and 62 kHz, where the increased amount of background hash suggests the antenna really is resonant, with a center frequency around 59 kHz. The -3 dB points might be 57 and 61 kHz, but at 10 dB/div with 5 dB of hash, I’d be kidding myself.
Dang, I love it when the numbers work out!
It’s faintly possible the spectrum analyzer calibration is off by 2.5 kHz at the low end of its range. The internal 300 MHz reference shows 299.999925 and it puts FM stations where they should be, but the former could be self-referential error and the latter lacks enough resolution to be comforting. I must fire up the GPS frequency reference, let it settle for a few days, see whether it produces 10.000000 MHz like it should, then try again.
Riding into the Village of Wappingers Falls, there’s a lumpy patched pothole just ahead of the fairing & front wheel:
Water Droplets – 2016-07-19 – 0196
You can watch (and I can hear) the fairing flex as the front end jounces over the patch:
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The hydration pack slung behind the seat also jounces and, when the reservoir bag bottoms out, the sudden pressure increase squirts water out of the bite valve, all over my face and goggles, and way out in front of the camera:
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The camera runs at 60 images/second: those 28 images span all of 450 ms.
Two seconds later, the droplet stabilized into a nice round lens:
Water Droplets – 2016-07-19 – 0360
The low humidity of a lovely day evaporated the drop after another three minutes…
Four miles later, a blowout through a tread gash previously covered by the tire liner
A puncture flat directly through the tread
Basically, erosion from the (last remaining, I think) liner in the rear tire of Mary’s bike caused the first flat; I patched the tube and didn’t notice the gash. After the blowout, I patched the tube again, booted the gash (with a snippet from a roll of PET bottle plastic I carry around for exactly that purpose), stuck an ordinary patch atop the boot to cover its edges, and the whole mess has held air just fine for the last week. I’m reluctant to mess with success.
Not having a tire liner caused the third flat, this time on my bike. The wound looked like a nail or glass shard punched directly through the Kevlar armor behind the tread. Fortunately, it happened (or, more exactly, I realized I had a flat) half a mile from home, so I fired a CO2 cartridge into the tube and pedaled like crazy, which got me halfway to the goal and I rolled the rest of the way on a dead-flat tire.
Ya can’t win.
So I picked up a pair of Michelin Protek Max tubes, the weirdest things I’ve ever stuffed into a bike tire:
Michelin Protek Max Tube – carton
The bumps along the tread surface are much larger and uglier than shown in that picture:
Michelin Protek Max tube
The rubber forming the protrusions has the same thickness as the rest of the tube, so you’re looking at soft, flexible shapes, rather than thick bumps.
The “liquid” inside must be a thin film over the inner surface. I’ve never been a big fan of tire sealants, mostly because they’re reputed to ooze to the bottom of the tire into off-balance puddles.
For future reference, the Official Quasi-Instruction Manual / Blurb (clicky for more dots):
Mary used a garbage can lid to shelter some plants, left it in the garden for a while, and a critter moved into the new shelter. She first noticed two well-prepared front entrances:
Garden shelter – front entrances
And a rear entrance or, perhaps, the emergency exit:
Garden shelter – rear entrance
Gingerly lifting the lid, she found a dismantled bird corpse:
Garden shelter – bird corpse
Along with a large stash of sour cherries from a nearby bush:
Garden shelter – sour cherry stash
A good-size toad kept an eye on the proceedings:
Garden shelter – toad in lair
We didn’t know toads ate sour cherries, but the evidence seems clear:
Garden shelter – toad on sour cherries
The image of a toad taking down a bird can’t be unseen, but, more likely, a recently fledged nestling took shelter and couldn’t figure out how to get out again.
Given five meters of 40 conductor ribbon cable, the object is to make a 40 turn five foot diameter loop antenna by soldering the ends together with a slight offset. After squaring off, marking, and taping the cable ends, I stripped the wires:
LF Loop Antenna – wire stripping
Twirling those little snippets before pulling them off produced nicely twisted wire ends with no few loose strands. Separate the individual wires, wrap with transformer tape to prevent further separation, run a flux pen along the wire ends, tin with solder, repeat on the far end of the cable.
Tape one end to the ceramic tile. Align the other end with a one-wire lateral offset and the stripped sections overlapping, then tape it down. Slide a paper strip between the ends, passing under every other wire, to separate the top pairs from the bottom pairs, then tape the strip in place:
LF Loop Antenna – wire prep
Grab each left wire with a needle point tweezer, forcibly align with the corresponding right wire, touch with the iron, iterate:
LF Loop Antenna – top solder joints
The red wire trailing off to the left will become the center tap.
Slide a strip of the obligatory Kapton tape underneath the finished joints, slobber on enough clear epoxy to bond the insulation on both sides of the joints into a solid mass, squish another strip atop the epoxy, smooth down, wait for curing.
Untape from the tile, flip, re-tape, solder the bottom joints similarly, add Kapton / epoxy / Kapton, and that’s that:
LF Loop Antenna – complete joint
Prudence dictates checking for end-to-end continuity after you finish soldering and before you do the Kapton + epoxy thing, which is where I discovered I had 80 Ω of distributed resistance along 200 meters of cable. A quick check showed 40 Ω at the center tap and 20 Ω at the quarters (the black wires on the left mark those points), so it wasn’t a really crappy joint somewhere in the middle.
The joint and its dangly wires cry out for a 3D printed stiffener which shall remain on the to-do list until I see how the loop tunes up.
What’s wrong with this picture? (clicky for more dots)
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
Description
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
A closer look at the strands making up the wires:
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.
The Smell of Molten Projects in the Morning 