The VHF/UHF Line of Sight Horizon

How High Determines How Far You, and Your Radio, Can See

VHF and UHF radio propagation is often referred to as line of sight. There are many propagation modes that will vastly increase your range past line of sight distances, such as tropo ducting, tropo scatter and sporatic E, but clear line of sight is what you can always count on.

To give you an idea of what that is, here are a few examples. The actual radio horizon is usually a tiny bit further than the visual horizon, as much as 15 percent at some frequencies. Height is everything. Not only the height of one antenna, but the height of the other. Each has its own horizon, and the line of sight between the two is the "cohorizon" or what distance the two horizons just touch.

So what is the mutual co-horizon of two people on level ground using HT's? Assuming an antenna height of six feet on each end and no intervening obstructions:

Effective co-horizon is 6.9 miles

What happens if you mount an outside antenna on a single story house at the level of the eves, say about 14 feet up and talk to someone using an HT on the same level ground:

Effective co-horizon is 8.7 miles

What happens if one person is on the 10th floor lanai of a condo or has a fancy 100 foot tower:

Effective co-horizon is 17.6 miles

How about someone on the top of Mauna Kea on the Big Island talking to an HT at sea level, such as a standing in a yacht offshore:

Effective co-horizon is 169.7 miles

And finally, how about someone on Mauna Kea talking to someone on Tantalus on Oahu?:

Effective co-horizon is 226.1 miles!

As you can see, when altitudes start being measured in thousands of feet, the horizon starts to really stretch outwards. This is the basis of "mountain topping", the practice of taking VHF rigs to very high locations to see how distant a station you can work.

On the other hand, an outside antenna mounted on the eve of your single story house, is not going to improve your horizon that much over a handheld in your living room. BUT it is going to make a big difference in the efficiency of the radiation, being outside and mounted high and clear in an optimum place, above at least some of the surrounding vegetation etc.

Often condo dwellers can get remarkable VHF ranges by hanging up a roll up J-pole in an optimally exposed spot on their lanai. When their height above ground, which can be significant if they are on the 20th or 30th floor, is combined with height above sea level, which even in Honolulu can be several hundred feet, they end up with a rather impressive radio horizon. This can make such a location a surprisingly good VHF Simplex Net Control Station.

Take the case of a 20th floor lanai on a condo that is up by H-1 on Oahu to our same person with a hand held at sea level:

Effective co-horizon is 40.8 miles

In a recent VHF contest a station with a 25 watt mobile rig and a roll up ribbon J-Pole on a sixth floor lanai, worked both North and South sides of Kauai, Molokai, the Big Island and stations all over Oahu on both sides of the mountains. Maui stations were heard, but could not quite be worked. This was FM simplex operation with no repeaters. Most of the contacts were solid copy on only five watts.

VHF contacts from the same location in Moiliili were regularly made during a recent yacht race with boats in the Molokai Channel fully 15 miles from Diamondhead using 25 watts and a simple 5/8ths wave ground plane antenna. Contact was maintained all the way into the AlaWai Yacht Harbor except for a black out area when the boats were close under Diamondhead.

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