The Forest Service Region 5 VHF data channel is 170.525. The 406.425 downlink you are hearing Dave, might be the remote base downlink for 170.525. The data channel has two purposes. First, most of the fire apparatus and law enforcement vehicles have something that resembles the ham radio APRS (Automatic Positioning Radio System is the translation I believe) system. It has its own VHF antenna on the roof of the vehicles for this use. Second, backcountry trail crews and wilderness rangers can carry a very small keyboard and screen, about the size of a small laptop, but thinner and not as heavy. They use the data channel to send text messages for health and welfare traffic as well as logistical traffic. In the past trail crews that stayed in the backcountry for a month at a time would have to tie up Forest Net for up to an hour with supply requests and other messages for crew members. The Forest Service was using text messaging long before it became available in commercial cell phones so that Forest Net did not have to be tied up for so long.
Jay, you are hearing both the up link, which is the higher of the two frequencies, and the down link, at 9 MHz lower in frequency. I would bet that both of the links you are picking up are linking Fox Field , where the dispatch office is located for he Angeles, to the Forest's radio system on the mountain. I would have thought it is linked via microwave as the old office location in Arcadia, at the Forest Supervisor's Office, was linked in that manner. Try listening to both the Forest and Admin nets on VHF and see if the 400 MHz pairs are carrying the same traffic.
When you figure out the linking system for a National Forest or National Park, should it be on 400 MHz, you may lock out the VHF frequencies, as you can often hear the whole Forest or Park in the same manner as the dispatcher does at their console. In my Mammoth Lakes location I used to listen to the down link coming off of Silver Peak to the dispatch office in Bishop. It was set up so that the down link repeated the up link so I just permanently locked out the VHF Forest Net frequency as I could hear the whole Forest by monitoring the down link from the hub remote base on Silver Peak. I could even hear car to car traffic from as far away as Lone Pine, 100 miles to my south, by listening in on the 400 MHz link.
Many people don't know to search 400 MHz and 72 MHz frequencies to find links that can open up a whole new world of monitoring they previously didn't hear. It is right there ready to grab if you know where to look. I listen to the down links used by Mono County to link the hub site on Conway Summit, located about 45-50 miles to my north. I can hear some more northern repeaters that even a amplified antenna can only pick up with a lot of noise on VHF, but are clear as they can be on the 453.000 down links. What is interesting is that the 453 MHz links are carried on vertically polarized antennas that are pointed nearly 180 degrees from my location. I'm actually picking up the small amount of signal that is coming off the back side of the antenna from a location nearly 50 miles away. To have that happen I have to use LMR-400 coax and replace it about every 3-4 years, as my roof can be covered in snow 7-8 months of the year, which is hard on coax.
I'm writing files for my PSR-600 mobile for all the northern Nevada counties and I do a licensee search for each county to see if they are linking their repeaters with 450 MHz frequencies. I then program in those frequencies in addition to the VHF frequencies that almost every Nevada county uses.
There are some remote base and repeater sites on State of California radio systems, especially the CHP and Caltrans, that are not linked via microwave. These two agencies often use 72 MHz links to those sites and if you know what to listen for you can often pick up traffic you would otherwise not be able to hear.
I hope that this thread will result in some folks searching the UHF band and the 72 MHz area to find those wonderful links that are just waiting to be monitored. In addition, I hope we might be able to figure out if a new USFS Region 5 Air Guard linking frequency pair has been allocated. I used to have 415.550 programmed into my mobile scanners and could often hear very interesting traffic from some very distant locations while driving around in the Central Valley. The fun of listening to linking frequencies is that you have all the advantages of being at a remote base location on some mountaintop at the end of a bad dirt road, while in your home or your car down in the flatlands.
