This presentation is from back in 2011, but it does an excellent job of explaining how technically complex it was to implement 800 MHz conventional repeaters in districts 1 and 2 given the complex terrain:
To make a long story short, they needed so many 800 MHz repeaters to replace low band that they had to implement vote-scan on the mobile radio receive end just to make it work. If I’m understanding it right, basically each “channel” is actually a series of channels in a scan list in the users mobile radio, all with a common Tx frequency. All the repeaters within the coverage area of that vote-scan channel listen to that common input frequency but then broadcast on their own individual repeater output frequencies. The mobile radio then scans through all those repeater output frequencies, “votes” (picks the strongest one) and then listens to that output frequency. This allows the mobile to roam around between a coverage area of different repeaters on different output frequencies without needing to change channels on the radio.
I often wondered how CALTRANS employees were expected to be such experts on local geography that they could constantly change channels to the closest mountain top every few minutes of driving, and it turns out this is how.
Since the drivers don’t have to change channels very often, the only maps that would exist for what you’re asking would be internal engineering documents for state radio techs, so your best bet is to figure it out through trial and error and determine which groups of repeaters constitute which “vote-scan channel.”
Thanks for the great information, both the link and how you explained how these roadside repeaters work. I think I read this document one other time, but an additional read and your explanation was needed to fully understand it well enough to program a scanner. I think trying to lock in tones would be counterproductive as there might be many repeaters on the same highway segment with the same output frequency. Counterproductive in that it would take several channels to cover each tone, using memory and slowing the scanner down.
Here in the eastern Sierra there are some problematic areas of coverage for 800 MHz. I'm real familiar with the Mono County and northern Inyo County areas that have lousy coverage. SR 158, the June Lake Loop and SR 120 west of U.S. 395 have never had 800 MHz coverage, since the 1982 conversion of CT D9. This is especially dangerous on SR 120 as we are talking about Tioga Pass, the eastern entrance to Yosemite. The June Lake Loop is also dangerous. There are some avalanche prone areas on it, plus a lot of traffic in the winter due to the June Mountain Ski Area. The northern portion of the loop is closed for much of the winter as a CT employee lost his life in an avalanche there. There is one resort and a horse packing business in that stretch, neither operate in the winter, so closing the stretch is not a huge issue. On the south side of June Lake (not the town of the same name) there is a large avalanche zone. A system of propane fueled explosive tubes have been installed, which can be fired remotely, has been installed to trigger avalanches when the road is closed. A county road was completed north of the town, which now provides bypass access during avalanche closures here. Snow removal in the spring on Tioga Pass is quite hazardous and about 5 miles west of 395, there is no coverage for anyone except CHP and the U.S. Forest Service, although iffy in spots.
Caltrans has been issued several 400 Mhz licenses in these two areas, however, I haven't seen any new 800 MHz licenses. They have had a couple of phases of licensing and not all the sites have been kept under license in the 2-3 phases of licensing they have undertaken. So I will continue to watch new licensing in an attempt to figure it out.
In northern Inyo County they have some problems along U.S. 6 due to shadowing from the Silver Peak site in the White Mountains. There are also some spotty areas along SR 168 east of U.S. 395. There are some grades and canyons along this route that need coverage improvement, although the traffic on this road is light and a lower priority. It is a highway section posted with "Snow Not Removed 4 p.m. to 8 a.m." or similar.
It will be interesting to see what CT does in these 3 areas. All are subject to snowfall and all need coverage.
In southern Inyo County I think some peaks and one elevated electronic site are providing coverage, however, my time down there is limited to 10-15, camping trips of 1-4 nights in Death Valley and one 4 night trip to the small towns of Shoshone and Tecopa. If you want more than 1 restaurant to choose from this is no place for you!
I wish we could get our hands on an "as built" list of channels for every district, similar to the one provided in this rural radio coverage presentation these two engineers did.
Thanks!
Regards,
-Frank C.
After reading inigo88's post and the document he provided a link to, I think you need to reconsider locking on to a single frequency while mobile. When I scan Caltrans I have all the CT district's frequencies in one bank, sometimes by itself and sometimes combined with CHP. What we need is a listing of all the 800 MHz frequencies used CT district by district, including all the voting or roadside repeaters. .