Feb 8 2014

Something about communication range

Ever wondered about the claims some manufacturers put on the box? 10, 20, 30, 40 kilometers between two handhelds? Did you ever achieve it?

Probably not, and I’ll try to explain why by using a simple example.

Before we go any further we’ll going to assume a few things.

  1. Both persons holding the radios are 1.90 meters (about 6″2) tall
  2. Both radios are identical in every respect
  3. We use the 2 meter amateur band (VHF) and 5 Watts output
  4. There are no obstacles between the two radios
  5. Nothing above ground affects the signal in a negative nor positive way
  6. There are no special band conditions

Under these circumstances, what is the maximum range?
a) up to 6 Km
b) up to 10 Km
c) up to 20 Km

The answer is: up to 6 Km, give or take a few meters (the exact number is 5.7 Km). The reason why the signal can’t travel any further has to do with the radius of the earth. Beyond 5.7 Km your signal will literally hit the ground.

Line of sight

You can use this formula:

  1. Take the square root of the antenna height. In this case the antenna height is identical to the length of the person who is holding the radio, e.g. 1.90 meters. √1.90 = 1.378404875.
  2. Multiply this number by 4.12.
  3. Now you have your maximum range: 1.378404875 * 4.12 = 5.7 Kilometers.

Hopefully you now also understand why more output power can’t change the final outcome.

In real life chances are slim that you will actually be able to keep in touch over such a distance. There’s not much chance you run into the ideal circumstances and assumptions I described above. A few factors are:

  • Free space loss. The signal will diminish as it spreads out.
  • Absorption loss. Signals might pass through a medium which is not totally transparent to radio signals. Even a glass pane will do that.
  • Diffraction. This occurs when an object blocks the path. The signal can diffract around the object, but can’t do so without losses.
  • Terrain, composition of the ground. The terrain over which signals travel can have a negative effect on the signal, even without hills or trees.

In reality you should be happy when achieving a range of 1 or 2 Km.

Ways to improve range

  1. Antenna height. The higher, the better.
  2. Radiation pattern. Antennas should not waste energy by radiating it in unwanted / useless directions.
  3. Free space loss can be partly compensated by increasing power output. Doubling the output power will not double the range, the theoretical improvement is about 35-40%.
  4. The use of repeaters at high elevations, which will compensate for the radius of the earth.

repeaterThis article is intentionally kept as simple as possible and doesn’t cover all aspects. If you want to read more, go here.

13 comments on “Something about communication range”

  1. Well said. And the same thing applies mobile-to-mobile, too. On a vehicle you’ve got a little bit more height to the top of the aerial, more power, probably a better radiation pattern, but the range still isn’t all that wonderful until there’s a base station in the equation or the lie of the land helps out.

  2. Hello,
    Realy true. Try in a city with building and range is less than few hundred of meters…
    It is sometimes interesting to remember the bascis to compare with commercial. Thanks !
    Best 73s.

  4. Great explanation. I always enjoy how marketing s-t-r-e-t-c-h-e-s the truth just a bit. Next challenges – how doubling the output means 3 db, and all those magical antenna gain figures aren’t possible.

    Great blog. Keep up the good work!

    73,
    Steve
    KE8YN

  7. Pingback: ‘Een portofoon met 20km bereik’ » Hamnieuws

  9. True for only handheld radio’s. When using better antenna’s and more power much larger distances are possible. There are many possibilities to work large distances using weak signal communication and propagation like tropo scatter, aircraft scatter, or improved propagation.
    Hamradio on higher frequencies is much more than only 6km range.
    I can work always ranges up to 800 km on UHF frequencies. Even on microwave frequencies ranges up to 500 km are most of the time possible.
    If you want to work more than 6 km range just get yourself a real ham radio setup and not only a handheld radio.

    • True, but you’re ignoring the clearly stated points 5 and 6 at the start of the explanation :
      5. Nothing above ground affects the signal in a negative nor positive way
      6. There are no special band conditions

      The whole mini-tutorial was aimed at explaining simple, local, handheld radio range. Only a “real ham” cares about super long-distance from one highly tuned base station to another 🙂

  10. I am a new HAM and not a mathematician, but I think you should double that calculated distance to get the theoretical max. distance between transmitter and receiver because the receiver can be the same distance from the horizon without having the signal hitting the earth, making it about 11.4Km (which still is way less than the claimed 20Km).

    A second issue is that I don’t understand where the number 4.12 comes from. According http://en.wikipedia.org/wiki/Horizon#Approximate_geometrical_formulas, this number should be 3.57.
    Using 3.57 would give 2*3.57*SQR1.9 = 9.8Km.

    Please correct me if I make wrong logical steps here.

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