

Radio Line of Sight Calculator/VHF/UHF

Here is a simple line of sight calculator that will do the complicated math for you to determine just how far the horizon is from your HT or your base station antenna at any height above level and flat ground (or calm water) on the VHF/UHF ham bands.



This calculator assumes nothing is in the way of the radio signal between the antenna and the horizon at a chosen height above ground. It does not take into consideration the height of the "other" station" antenna you may be communicating with nor any attenuation caused by weather, band conditions, antenna gain, path loss, or other factors such as dB loss in coaxial cable.



To use the calculator, simply type in your antenna height above ground in the appropriate window below and click the "Compute" button. You will see the actual line of sight distance to the horizon in the "Distance" section before the radio waves are attenuated by the curvature of the Earth.



As an example using "Height in Feet" in the calculator below, and assuming we are using an HT, the antenna is about 5 and 1/2 feet above the ground, then just plug in 5.5 in the "Feet" window, click "Compute" and read the answer in the "Miles" window directly below. You should see 2.9 miles in the answer.



So with an antenna height of 5 1/2 feet above the ground and assuming there are no obstructions and the ground between you and the horizon is perfectly flat, then it is 2.9 miles before the curvature of the Earth starts to take effect on your signal strength at the level of the horizon! It does not account for any higher angle radiation coming from your antenna that may be "seen" by a much taller antenna such as on a tall tower standing beyond the horizon. It acts much like you were aiming a laser beam or spot light toward the horizon rather than a radio wave. The result is like a straight and level line from your antenna to the start of the curvature of the Earth.



At any distance beyond that 2.9 miles, your signal is attenuated rapidly due to the Earth being in the way of your signal. Again, this is assuming that the "other" station antenna is "below" the horizon from your.



Click the "Clear" button to start over if you make a mistake or if you want to do other height calculations...have fun!

Enter Values Some of you "newbies" to rf out there may ask:



"Wait a minute, I'm not sure I understand! I can hit a repeater 10 miles away with my HT..how can this be if I am holding my HT at the 5 1/2 foot level standing on the ground in my front yard?"



Just remember that if your antenna IS in fact at 5 1/2 feet, then a repeater antenna may be at 50 to 1000 feet or more above the flat ground beyond the horizon! The much higher repeater antenna increases the line of sight distance between your "station" and the repeater antenna tremendously! This is because it "sees" over your horizon in your direction from it.



Some examples using different antenna heights and the calculators on this page:



In the example below, both station antennas are at the same height above ground.

Station A Height Station B Height Total Line of Sight Distance to horizon between stations Each station distance to horizon in miles 5 feet 5 feet 6 miles 3 25 feet 25 feet 14 miles 7 50 feet 50 feet 20 miles 10 100 feet 100 feet 28 miles 14

Now let's change the antenna height variables on the stations as an experiment!



Station A Height Station B Height Total Line of Sight Distance to horizon between stations Each station distance to horizon in miles 5 feet 10 feet 7 miles A =3 B=4 25 feet 10 feet 11 miles A= 7 B=4 50 feet 30 feet 18 miles A=10 B =8 100 feet 50 feet 20 miles A=14 B=10

Here are a couple of more dramatic examples on how the height of the repeater antenna or your antenna plays a very important part in the line of sight calculation.



1. Assume the repeater antenna is on top of a mountain that is 5000 feet above the surrounding flat terrain and you had a pair of binoculars at the exact height of the antenna...you could see the horizon about 86 miles away !



2. A space traveler in the International Space Station looks out of his window from an altitude of 200 miles! He can see the horizon of the Earth at 1275 miles according to the line of sight calculator!



You should always remember that most repeaters have high gain antennas and much higher rf output than your HT does and this will help on the received signal strength of your signal at the repeater site and also will help you receive the repeater much better than if both "ends" of this communications "circuit" were at the 5 foot or so ground level and at only a few miles apart using "HT" power which is usually around 5 watts or less.



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Experiment with different antenna heights using another calculator!



Below is a handy line of sight calculator that you can experiment with by plugging in your own "station antenna height" and the height of a known repeater antenna or other station antenna height to get an idea of how this all works. It will show you how antenna height makes loads of difference between stations as far as the line of sight is concerned.

It is supplied by www.easycalculation.com and comes directly from their web site to this page.



Note that this calculator may take a bit of time to display! If it does not display, refresh your browser and try again.



Don't go away yet! Here is more good info you might enjoy!

This editor (N4UJW) personally can attest to the long range capability of a simple 2 meter transmission from a great height above ground from an HT over several hundred miles....TWICE......! Read on!



Many years ago, when I lived near Ft. Lauderdale, Florida, the space shuttle made an orbital pass directly overhead. One of the astronauts aboard was a ham and he used a simple 5 watt hand held radio on 2 meters from inside the shuttle holding it close to a window and was talking to his Dad who was on the Gulf coast of Alabama! His range to his Dad was just under 600 miles and according to the calculator, his max range would have been around 1100 miles plus or minus! The space shuttle ham station created such a pile up over several states on "2" that the astronaut finally had to ask every one to be silent until he finished his QSO with his Dad.



This orbital pass only lasted about 10 - 12 minutes, but the "space to ground" communications were still going hard and heavy when his signal finally went over the horizon from my station and he faded within seconds! I never did get to talk to him, needless to say! And on top of all of this, as I recall, he was on simplex all the time!



On another occasion, when I lived in middle Tennessee, I witnessed a ham operator/passenger aboard a corporate Lear Jet that took off from Nashville, Tennessee heading to Birmingham, Alabama. He was also using an HT by the window of the aircraft and was bringing up repeaters from several states. Another huge "pile up" was created by him until just before they landed in Alabama! I was hearing him from one of our local repeaters which was perched on top of a major hill about 2000 feet up!



In the aircraft case above, the extreme antenna height above ground plus any gain from the repeater antenna was the determining factor in these extreme long range communications using line of sight!

So what's the bottom line ?

Get your VHF/UHF antenna higher! You will be glad you did! 73



Additional good reading about VHF/UHF propagation: (off site)

The Basics of VHF and UHF Signal Propagation

Long Distance Communications beyond 50 miles



How Far Can I Talk on 2 Meters? This is an excellent article!

by Paul H. Bock, Jr. K4MSG, Hamilton, Virginia





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