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Multiple repeater coverage areas on map


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Sadly, any overlaps or gaps that would result from overlaid circles would be fictitious at best. Coverages on maps today are shown as circles out of simplicity and convenience. It may or may not represent maximum tested range in one or two arbitrary directions. Coverages are actually jagged, random, irregular shaped blobs in the real world, they are only circular in outer space.

I experimented with one just this past week in the mobile. Someone had informed me coverage was about 13 miles. Unreliable coverage began at about 3 miles and I lost complete comms at about 5 mile in my direction of travel. You can see that the circles would quickly become misleading if 13 miles were the official distance listed on the map.

But I do get it, seeing the circles is nice visual aid.


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  • 2 weeks later...

Coverage maps are hard because GMRS is a line-of-sight communications service. There will be spots where you can be less than 5 miles away from a hilltop repeater and not be able to open its squelch, and there are spots where you're scratchy into a repeater that's 100 miles away. Coverage maps can be very reliable tools if both the mobile station and repeater's powers, losses, and thresholds are properly entered; the correct antenna patterns and heights are used; proper statistical losses are taken into account; and an effort is taken to ensure the topography in the mapping software matches real topography. In my experience, none or very few of these tasks are done when generating a coverage map. Real coverage tends to be much less than the modeled coverage, typically because statistical loss has a huge effect (mobile coverage requires staying above the signal threshold more than n% of the time (I usually model at 70% or 87% depending on band), while spot coverage (often used by default) assumes you're standing still in the peak of a fluttery signal), your antenna (especially if portable) probably has less net gain than whatever mobile station the map-maker simulated coverage with, site noise is a very real thing that will reduce repeater sensitivity, repeater owners may not realize that they are using a directional antenna or tower mounting position, and not many repeater owners are willing to admit that it's possible to have 4 dB of loss between the transmitter and the antenna. Repeater owners and users also like to see good coverage come out of the simulations, regardless of actual coverage, so there's even incentive for misleading coverage maps. So, there's a lot of ways to alter the coverage depicted on a coverage map, and there's so much room for variation (about 25 dB worth) that it's not possible to make repeater-to-repeater comparisons unless the same person made all the maps with correct information.

And unfortunately, it isn't possible for MyGMRS to generate the coverage maps accurately, as there's a lot of room for variation with repeater hardware (dinky little solar power repeater running 5W into a counterfeit Nagoya antenna duct-taped to a chain-link fence, vs. someone running 50 watts into a solid duplexer with preamplified receive and a 10 dBd gain antenna) and repeater locations on the map are not always accurate for various reasons. So, we just generalize coverage into a circle around the repeater's map position. It's easier for a repeater owner to estimate how far their repeater can cover towards population centers than for the owner to generate coverage maps that are correctly parameterized. It's easier for MyGMRS to handle circular coverage patterns than the rasterized geo-referenced images outputted by coverage mapping software.

Radio coverage in general should be taken with a heaping handful of salt due to the wide degrees of variability that are out there, and instead test your actual coverage with a second radio or a friend. And if you know or learn mapping software such as Radio Mobile, you can even make your own coverage maps for estimation if there's enough information about a repeater's location in its MyGMRS listing.

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