A Stellar Time

I was wandering to the lab just now and ran into a group camped out near the physics buildings with a pair of  reflector telescopes.  One of them recognized me from the ham radio club, so I stopped by.

They were observing the moon with one — I saw magnificent detail in the craters and highlands along the light-dark line, and a red-blue binary I believe was Beta Lyrae  (the scope was angled very high and pointed just southwest, at 9:50 CDT in Grand Forks).

Link budget for Part 15 iBiquity

Here’s some theoretical calculations of FM iBiquity performance under Part 15 conditions.
=========
Reception
=========

2006 NPR testing indicated real-world reception certainties:

95% reception at 67 dBuV
85% reception at 60 dBuV

Concurrent operating rules provided for digital broadcasting at 1% ERP.  The reception thresholds at elevated power levels are thus expected to be

10% ERP:

95% at 57 dBuV
85% at 50 dBuV

100% ERP:

95% at 47 dBuV
85% at 40 dBuV

dBm = dBuV – 90 – 10 * log (Z), [Z ~ 376.73031 Ω]
dBm ~ dBuV – 115.76

dBm table

( 1%    10%    100% ERP)
95%:    -49    -59    -69
85%:    -56    -66    -76

============
Transmission
============

Part 15 rules allow for transmission on the FM band at field strength levels up to 250 uV / m at a distance of 3 meters.

(250 uV / m) ^2
—————    = 166 * 10 ^-12 W/m^2
376.73031 Ω

166 * 10^-12 W/m^2 from spherical radiator   * 4*pi*(3m)^2 = 18.8 * 10^-9 W

Assuming an antenna aperture of roughly 1 m^2 (an approximation on the FM band, where Aeff for a short dipole ranges between 1.39 and 0.92 m^2), and a characteristic impedance of free space ~ 376.73031 ohms,

This gives a signal level of -67.8 dBm at 3 meters.

===========
Data theory
===========

Hypothesis:  Original study recieved 100 kb/s channels.

Expected process gain:  ~3 dB per halving.  Signal modes available in iBiquity determined from Wikipedia
100  kb/s    ( 0  dB)
50   kb/s    (+3  dB)
25   kb/s    (+6  dB)    -standard all-digital fallback mode
12.5 kb/s    (+9  dB)
5    kb/s    (+13 dB)

=================
Ideal link budget
=================

Maximum transmission power at lowest frequency (88 MHz), power received by short dipole with 1.39 m^2 effective area.

(3m    10m    30m    100m)
dBm        -66    -77    -86    -97

Maximum Process Gain +13 dB

(3m    10m    30m    100m)
dBm(eff)    -53    -64    -73    -84

Compare with signal thresholds -69, -76 dBm:  Process gain makes it possible to get an 85% receivable signal at 40-50 meters.

==========
Conclusion
==========

Without any additional process gain (increased redundancy, etc) performance of an iBiquity station is not expected to be substantially “better” than with analog FM.  With the use of low-bitrate channels over all-digital modulation, it may be possible for a Part 15 station to have a reception radius of something like 40 meters.

Bear in mind that this is a theoretical inquiry based upon publicly available data.   I should point out at least two potential sources of error.  First, the signal thresholds are based on the single NPR study, which itself showed somewhat significant variation in signal detection depending on which station they tried to receive.  A wider basis of signal reception data is probably available and I’d appreciate links to take a peek.  Secondly, it’s been a while since I’ve done RF work so my math may be a bit off, I’d appreciate any corrections.

Finally, though it is used as a public broadcast standard in the United States, the HD Radio standards are not public information.  This makes amateur experimentation difficult.

KD0GWG Contacts

I did my first-ever HF calls with the help of K9DIG today!

Andy, RM5D at 2007Z

David, GO0AQD at 2032Z

Vojko, S51ZZ at 2043Z

Cheers and 73 from Johnathan, KD0GWG!

 

The quest for a cheap, portable HDMI monitor

Something that I haven’t quite solved in a day of searching:  I’ve been looking for a good, cheap, portable HDMI screen.

I’m looking for such a thing for two, maybe three reasons.  The first is that I’m called to the occasional computer repair task, and it’s always useful to have a functional monitor at hand for diagnostics.

The second thing is I’ve been looking into doing more portable computing, maybe off my car power supply, maybe off a luggable 12V rig.  A portable monitor designed for 12V power would really be ideal.

And third is:  My Raspberry Pi needs a screen to call its own!  Sure, it can have its way with any random 53″ television I happen to walk past, but having a dedicated screen would definitely put it farther ahead on my time-pile.

Rumours of a $100 Chinese wonder all lead back to a not-in-stock.  But hey, for $100 I could really just buy a full desktop display and lug it around.

Thus far, I’m only coming back with partials.  The Pi can output in to RCA composite video, making your typical $40-ish car video screen an option.  Newegg has a third-party seller listing one display for just over that, but the similarly priced Digital Prism ATSC710 looks like it has composite or component in, plus an HDTV tuner.  That’s everything *but* HDMI for my checklist.

Small displays *with* HDMI are another story still.  Something I didn’t know before is that there’s a market for $1000 portable displays as a DSLR accessory.  I guess for the money you get something you can drop on the ground once or twice — handy, but not what I need right now.

There’s another trend I seem to have missed: a plethora of “black box” video recorders, ranging up from $50 or so, designed to look out the windshield and record a 5 or 15 minute loop, typically to an SD card.  Many have HDMI out, so they’ve been popping up in my search results.  It’s interesting to look at, but I don’t think I’m sold on the concept unless there was at least a rear-facing camera too.

 

A Flag for Mars

I’ve read a little about Vexillology, and I’ve designed a couple of spec flags in my time;  today, I’m sharing a flag for the planet Mars.  Not that there’s a lot wrong with the popular Lee flag — I agree with its design theme — but to me a simple tricolour feels kind of wrong.  You don’t get any “planetary sense” out of it.  I look at it and think France and Italy tied the knot.

So here’s my proposal, the the Martian “Marble” Flag.  It keeps the basic Red, Green, Blue theme, but it uses recognizable discs that sort of lean into each other, the next stage in planetary development right on the horizon.

This flag has the bonus of being correct under a commonly used rule in heraldry – that is, colours shouldn’t run right together, but are always separated by white or gold.  It keeps the design elements distinct and recognizable.

I also seem to like 2:1 flags over other ratios, though Phi or 21/2 would be more natural, maybe. If you play around with it, I’d sure love to see what you come up with!

OMG Hax

MaximumWeeaboo.com‘s eye-pleasing Arras Theme is actually a security minefield, as it turns out.  Twice over the last month the site’s been down, causing a bit of a headache for my friends at phpwebhosting.

So for all those interested: the older versions of the Arras Theme are totally bugged out!  It mainly has to do with some scripts they use for image processing, so far as I can tell.  If you use it, keep it updated, unlike a lot of things in WordPress, it may not correctly auto-update.

There’s also some evidence of foul play with my other sites; It’s going to take a few days to clean out. *sigh*

The Hack Factory

I visited The Hack Factory in Minneapolis on Wednesday night.  It was awesome!  They’ve got a full wood shop, welding tools, loads of free stuff sitting on shelves, a super-cool sound system.  Everything you’d need to express your creativity, really!

The space used to be a lot more cramped, but they’ve recently expanded to take over the entire building they rent for what has to be an ungodly sum.  Still, the community seems to turn out in droves on Wednesday nights for Liquid-Nitrogen ice cream and podcasting;  They chug craft beer and homebrew by the gallon!

Even with the extra space, they’re still having junk-management issues.  They’re starting to use a “parking ticket” system to manage space and keep abandoned projects from piling up.  And there’s more than a few projects to improve the infrastructure itself;  they’re working on building a better antenna rig for their ham radio station, and a few folks are building better computer-controlled machine tools for the community to use!  Woot, programmable matter!

instructables

Ah instructables.  Right now, I’m working on a project from there – my friend wants a bit of help building him a crazy cool clock.

This one helped inspire my friends to start making home-cooked ramen.  But beware the other ones based on ramen!

My only complaint about the site is that they push users to register for no good reason.  Maybe they’ll get worse, but for now still a very positive makerspace.

Hello world!

I’m Johnathan McClure, an Electrical Engineering student at the University of North Dakota.  This site, design.j5mc.org will be the nexus for projects I’m working on, and other technical stuff I’ve been working with.

I anticipate that at least some of this will be worth releasing with OHANDA or a similar open hardware licence.  I look forward to doing some good, hard work!