How Far?
The formula for calculating the horizon is 1.17 times the square root of your height of eye = distance to the horizon in nautical miles. This would make it about 25 miles at the beach looking at the ocean, based on the average height male adult. It varies with terrain by a few miles, and would be increased if one point was on a mountain top (which, for example, would be about 70 miles on a 3,330 foot (from its base) mountain top to a visible point below). Line of site is only to the horizon of ones view, unless the object on the other side of the horizon is very tall and can be seen. See my signature below.
Line of site is a general rule. In an urban setting with trees and tall buildings, line of site radio transmission may only be a few miles or less (even a few hundred feet). Inside a building is even worse (less). In a rural setting, the distance may approach the horizon and even surpass it. This is why agencies (public and private) put the radio transmissions sites as high as they can get them (tall buildings, hill tops, mountain tops, towers, etc.).
However, propagation of RF signal doesn't work exactly like that - line of site. First, standard radio communications RF propagation is not normally a flat line or a beam, but rather an omni-directional wave (from a standard antenna). The wave goes out in all directions (left, right, forward, backward, up, down, etc.). Of course, there are RF transmissions that are directed in a beam or flat line, such as microwave and unidirectional propagation. Second, frequencies penetrate, bounce, and refract off of or through objects (structures, leaves on trees, moving vehicles, hills, mountains, etc.), bodies of water, atmosphere, and other elements, causing either a reduction in distance or an increase in distance. In fact, a RF signal can be bounced off the moon and received half way across the Earth.
The power of the transmission has much to do with how far the signal will travel. For example, my kids little toy 49 MHz walkie talkies would not propagate much further than about 1/16 of a mile. However, I have picked up transmissions from FRS radios of hikers in the hills below Muholland Drive in Encino, about three miles from my home. FRS radios aren’t very high powered either. My 5.8 GHz phones work about 1/4 of a mile from my home.
I have tested a 5-watt Motorola MTS2000 900 MHz trunked radio in simplex mode, and was able to transmit and receive more than three miles away in Downtown Los Angeles (plenty of tall buildings and other obstacles). The same day and the same location, we tested a 2-watt Motorola HT1000 on VHF-hi and it only got about 1.5 miles and an older Motorola (can’t remember the model) 5-watt HT in the 42 MHz band (around there) and got less than a mile. The 42 MHz HT did much better when we took it to a wider open area – I think we got about 5 miles. The other radios did the about the same in the wide open area as they did Downtown (the VHF-hi did slightly better).
Note: the higher the frequency, the better it penetrates objects; the lower the frequency, the better it travels far distances. This is why public safety uses VHF-hi, and in more recent times, UHF in urban settings and why many public safety agencies like highway patrol uses VHF-low for the state-wide or rural settings (apart from statewide linked systems, both TRSs and conventional RSs).
