Marine VHF Radio Coverage

After some offshore fishing with family last week (thanks Bill!), I was idly wondering about the Coast Guard's monitoring of emergency radio calls.  As it turns out, they've got a great system of towers along all of the U.S. coastline called Rescue 21.  Overall it gives reliable coverage out to 20 – 40 miles from the coast.  It looks like they get these ranges, which are similar to broadcast band FM ranges, by sticking antennas on the same commercial towers.  Makes sense.

So, how does this impact offshore fishing in the Apalachicola area?  Here's the USCG-supplied Longley-Rice propagation model coverage prediction map, with a 40 mile reference bar extending southwest from the tip of Cape San Blas:

(click to embiggen)

Hmm.  We're most of the way there.  Now, those coverage calculations were based on a 1 watt transmitter 6' over the water, but the boat's radio belts out 25w with the antenna more like 12' over the water.  According to the Egli model, that factor of 25 in transmission power roughly doubles the coverage range, which is plenty for any sane offshore fishing possibilities.  However, this is more of a line-of-sight bounded problem.  Adjusting for the change in antenna height only adds a couple of miles to the total range in this case.  What's more in bad weather with rough seas, which is really when you'd want things to work, LoS propagation can be significantly reduced.

Is this extra kick in power and increase in height enough?  If I had to draw a red-yellow-green map (sort of what I do for a living), from what we have here the area of interest would solidly be in the yellow area.  This doesn't mean it won't reach – I'm expecting that it will – but it does mean it's worth a test.  It is a marginal case.

Smackdown.

In a stunning victory for common sense, Denton Cove development in Apalachicola is a no-go.  Read about it in The Times.

Dirt Rag Interviews Bob Weir of the Grateful Dead

...in 1992.  Part of their Blast From The Past series.  <link>  OK, it's a tad back there in time, but still worth a read.

While we're on the subject of mountain bikes, the butcher's bill stacks up a few dollars more today with the ongoing dental repairs from a 2002 race at Chickasawbogue.  Oral surgery this time, to remove shards of a root canal gone bad.  Looks like I'm implant bound.  Hey, it's an excuse to buy some more titanium – except this time it bolts onto me, not the bike.

J-Pole: The Magic Dimensions

Last winter we had a post rounding up a bunch of J-pole links.  Now Paul Thirst over at Engineering Radio has posted about emergency broadcast FM transmission antennas.  Most importantly, he has included a diagram marked out not in inches, cm, etc., but in wavelengths.  Even more importantly, the diagram has the elusive "how far from the bottom of the J to tap" dimension.

Do you know how many ham "how to's" do not list this dimension at all but leave finding the correct feed point as an exercise for the student?  "Adjust for minimum SWR."  GRRRRR.  (calm down, calm down, calm down)

Anyway, many thanks to PT and his blog for putting the info out there.  (SWR of 1.4 and he's not happy with that?  What a perfectionist!)  Even though he's more concerned with broadcast band FM frequencies (88-108 MHz), this design can easily be adapted for the 2m ham, marine VHF, MURS, etc. bands.  You just have to know the center frequency to plug in.  Oh, and the speed of light in a vacuum, but that doesn't seem to change very much these days.

Just in case his blog ever goes away, here's the critical figure:

Astronomy Picture of the Day, #150708

Looks almost CGI in its abstract perfection, doesn't it?  But no, it is as IRL as it gets.  Folks, I've said it several times already: if you're missing out on NASA's APOD site, you're really missing out.

One for the Job Hunters

No, really, don't do it this way:

Unless you are totally, completely sure that the interviewer is a giant nerd.  Then go for it, it'll probably get you the job.  Even then, only if you can get all the quotes exactly right.

From xkcd.

iPhone Screenshot

I was needing to take a screenshot of something that showed up on my iPhone, so to youtube I went: here.

OK, that was easy enough if not completely obvious, but got what I needed.  Afterward, the inevitable "you also might be interested in these videos" showed up.  Think on it for a minute... yes, "iPhone" and "shot"....  OK, now go look at this video.

Heh!