Chapter III

RADIO COMMUNICATIONS

Through written evaluation, the applicant will demonstrate:

1.       A basic knowledge of radio wave performance and the effects that terrain, distance, and structures have on Search and Rescue radio communications.

Your local sheriff operating through the Oregon State Sheriff's Association, is empowered to use the frequency of 155.805 MHz for Search and Rescue operations. This frequency is just a tad above the Amateur Radio 2 meter band.  So it follows that discussions about the HAM 2 meter band's propagation characteristics also apply to the SAR frequency with only slight deviations.

    Two meter simplex operations are Line of Sight.

    Line of sight means absolutely no obstructions - if you were to have a high power telescope pointed down the antenna boom, can you see the other antenna?

    Radio Frequency  line of sight in miles over flat terrain is equal to the square root of 2 times the antenna height in feet. 
    For example, if one tower is 50 ft ( 2 x 50 = 100, sq root of 100 = 10 miles ) and the other end is 100 ft ( 2 x 100 = 200, sq root of 200 = 14.1 miles) then  the expected line of sight distance would be 10 + 14.1 = 24.1 miles.

    Okay, now we have the hard math on how far we can communicate over treeless flat terrain.

    But there are other factors in play here.  For one the above math does not take into account the wrapping effect of some  terrain.

    Take a ten story building.  If you are on the opposite side of the building from your control station, you would be in the radio shadow of the building and would not be able to communicate.  But if you were to move far enough away --- say, the equivalent of nine stories --- it is quite possible that you would be able to make communications.

   Also, if you were to move up in height -- to say the equivalent of the seventh or eighth floor --- you might be able to establish communications.

    This is caused by the radio wave bending or spilling over the building.

    How does all this fit into SAR communications?  Well, if you are on a search and lose communications behind a large hill, there are steps you can take to reestablish contact.   Of course, you can retrace your route and get up on top of that large hill.  Or you can try moving away from the hill and up another hill. 
None of this is an exact science, but being aware of this phenomena can help you out in a tight spot.

Joe Doman
Coos County Search and Rescue
N8SAR