I've wondered the same myself. In California the federal agencies have been using Air Tactics 4, 170.000 MHz for more years than I can remember. Other geographic areas (in the sense of how NIFC divides up the country) different air to ground frequencies are used, and in Nevada they seem to switch them from time to time. I don't know the reasons for the changes but it might have to do with the fact very few federal frequencies have clearance for nationwide, regional, or wide area use. 170.000 is cleared for everything west of the Mississippi river. With the Sierra Nevada being so much higher than much of Nevada and with the high utilization of the NIFC Air Tactical frequencies on the public lands in California, Nevada was probably forced to find a unique frequency. They might not have found something that has worked over the entire state that does not interfere in Oregon, Utah, Arizona, Idaho, and California.
Air to ground frequencies tend to be problematic as an aircraft at relatively low altitudes such as 10 to 15 thousand feet can interfere with use of that frequency hundreds of miles away. Even though air to ground is usually used for comms when the aircraft is less than 2000 feet above an incident, many of the incidents occur in mountainous areas where the terrain is several thousand feet above the valleys and plains that surround the mountains. This is the situation in Nevada where there are a lot of mountain ranges rising up from large areas of plains and wide valleys. I've picked up 170.000 traffic from 300 miles away on the Midland mobile in my USFS truck while traveling to fires in the Central Valley of California. This doesn't happen often, but it isn't rare either. The Southwest Geographical Area has different air to grounds and different air tactics for each of their dispatch zones, and they don't use the same frequencies over again until they are 200 or more miles apart.
When Minden dispatches a fire where aircraft is used, they normally announce the command, tactical, and air to ground frequencies to be used in a particular part of the order of the dispatch. I try to keep my ears tuned for such an announcement, because after it is made, the air to ground is just referred to as "air to ground" and you miss a significant amount of information when you don't know that frequency. I try to do this each year when the fire season starts as I want to verify what is going to be used in Nevada for the year. Yes, even in southern Mono County we can hear a lot of Toiyabe NF and BLM Carson City Field Office traffic.
That is all I can provide as to the reasons they might be switching the air to ground frequencies from time to time in Nevada.
