Antenna range is a common question that I receive from ham radio operators and scanner buffs looking to purchase my antennas. I wish there was an easy answer to this question, but unfortunately there are plenty of variables to consider when analyzing what kind of distance you’ll receive from your KB9VBR J-Pole or Slim Jim Antenna.
True radio transmitting distance for VHF and UHF antennas is calculated by analyzing four factors:
- Antenna height
- Antenna gain and feedline loss
- Transmitter power
- Terrain and structures
VHF communications are primarily line of sight. Unlike HF and shortwave radio signals that can bounce off the layers of the ionosphere to enhance distance, VHF and UHF signals are point to point. There is a type of propagation, called Sporadic-E layer propagation that can enhance your range, but this phenomenon is mostly dependent on the weather. An upper atmosphere inversion layer, or trapped area of warm air can create a channel or duct for your VHF signal to travel. But since this is can be quite sporadic, you can’t rely on Sporadic-E to reliably increase your range.
All of these variables work in concert with each other. For example, if you live in a hilly or urban area, increasing your height will overcome the limitations of terrain. If you are using low loss coax cable, then antenna gain may not be as much of an issue. Increasing transmitter power can help fill in weak spots, but won’t help in receiving weak signals.
There are quite a few ways to estimate range, from keeping notes on antenna performance, to the more elaborate method of breaking your geographic area into a grid and driving from zone to zone to test the range. But for those that want to be a little more technological, you can model antenna range in the computer
When I’m evaluating the capabilities of an antenna system, I like to use a modeling tool such as Radio Mobile. Plug in your coordinates into Radio Mobile along with transmitter power, antenna height, gain, and feedline and it will spit out a map estimating your antenna’s range. The program isn’t for the faint of heart, it will take a little experimentation to get an accurate picture of your antenna system’s capabilities. But the results are better than guessing in the dark.
For a more rule of thumb estimation on antenna range, I like to use my own real world example. If you mount the 2 meter J-Pole antenna 20 feet in the air, it would not be uncommon to be able to hear portable radios in a 2-3 mile circle, mobiles 5-10 miles, and repeaters up to 50-60 miles. These values are based on my own experiences in a residential environment. If you live in a very hilly or urban area, these figures could be reduced.
Do you have any real world range examples with my antennas? I’d love to hear them. Drop me a comment or leave a note below.