I'm trying to help a few friends set up a radio communication system between the two because they are interested in the science and they want to be preppers. Right now they are only getting a GMRS license from the FCC which limits the antenna to be 25 meters from the ground, have a frequency range in the 462 MHz band, and limits transmission amplitude to 5 watts.

On average with an omnidirectional antenna, you can get a 25-mile clear signal with that but I want to know for sure if I can reach 40 with a dish or not. Also, I want to keep an equation like this for later use for my own projects and put it in my wizard book, if such an equation exists.

 

Hello,

Using a dish for the metioned frequency will be huge.
You might want to have a look at the collinear antenna:
http://www.rason.org/Projects/collant/collant.htm

Bertus

 

I think you would be better using a yagi antenna as these are directional and more suited to point to point communications. If you Google "70cm yagi antenna" you will find many designs. These will be for use on the 432 Mhz amateur radio band so you would need to scale the dimensions to use it for 462 Mhz.

Les.

 

There is no equation you can use to determine a reliable propagation distance between an RF source and an RF receiver. The propagation environment is continuously changing and there are multiple physical phenomena that effect propagation. For example the moisture content of the atmosphere will affect the propagation of VHF signals. The best you can hope for is to gather data on what works and what doesn't work over the distance involved. A serious prepper would not bother with GMRS, they would get an Amateur Extra license which takes only three multiple choice exams and has way more flexibility for their purposes. The requirement to learn Morse Code to obtain a license has been eliminated, but if they were smart they would do that too.

 

I am not an expert in GMRS....but have installed many 460 repeaters and radio links in my younger days.

A couple of things..........GMRS would be the perfect setup for emergency short range communications.

I believe that the antenna has to be omnidirectional. And limited to 20 FEET above existing structure.

The range is dependent on path. It will depend on local terrain. I have installed repeaters that cover hundreds of square miles..................on five watts. You need elevation.

The most important thing.........is to shorten or eliminate the feed line. One can do this by mounting radio at the top of antenna structure.

You supply power, a key switch and audio thru cable wiring to the radio. In other words...eliminate the coax.....by remotely controlling radio thru control wires. 20 ft of coax can easily swallow half of 5 watts for most 460 MHz, un-professional installs. You can use a length of coax for a dummy load on 460. If this is not feasible for you...then get as much elevation as possible. 2 watts at high point..........works better than 5 watts at low point.

With a small solar panel......and a micro controller......one could install and hide...several tone encoded GMRS repeaters on the hills and ridges around your location. Not sure of all the legalities with tone encoding......but preppers don't worry about that. If you are a flat lander.....don't expect very long distances, without very tall structures.

And as mentioned before.............any serious prepper.........will have ham gear and several antennas...........already up and running.

To this very day............nothing beats a brass key.

 

You can get a rough idea using the free-space path loss equation.
https://www.allaboutcircuits.com/tools/free-space-path-loss-calculator/

http://www.antenna-theory.com/basics/friis.php