Friday, July 10, 2015

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.

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