The Maximum Distance  >  10,000 miles!

If based on your experience you find this assertion unbelievable, please remember this disbelief as you look through the following link budget calculations.

Assumptions:

Frequency = 146 MHz
15 KHz bandwidth
System Noise Figure = 3 dB
10 dB C/N needed for threshold
transmitters are 5 watts
antennas are as good as dipoles
Path is totally freespace

Transmit ERP = +37 dBm + 2 dB = +39 dBm ERP
Receiver noise floor ~ KTB = -132 dBm in 15 KHz
Carrier power needed for threshold = N+10 dB = -122 dBm (.18 uV in 50 ohms)

Maximum allowable path loss = +39 dBm - (-122 dBm) = 161 dB
Path loss = 37 + 20*log(distance) + 20*log(Fmhz)
20*log(distance) = Path loss - 37 - 20*log(Fmhz) = 81
Maximum Distance = 10,900 miles

If your initial answer was more like 10 or 100 miles, consider the reasons you were surprised to find the above answer indicated an error of 100 to 1000 times, or 40 to 60 dB.

If you have used handheld radios your experience no doubt indicates that a pair of handheld radios on 2M may only be able to communicate a few miles at best. The large difference between experience and theory is due to the nature of typical amateur paths and the excess loss that often accompanies them.

If you have experience with amateur satellites, you may be able to reconcile this somewhat by asking yourself whether a handheld radio and small yagi might be able to hear a 5 watt geosynchronous satellite transponder if all the power were put into one nbfm channel.

It's really true, amateurs commonly use paths which provide 1/1,000,000 or less of the potential information capacity of their radios. This situation is intolerable as higher capacity, higher speed systems are required.

The Shannon limit for the above example is about 50 Kbps over the 10,000 mile path. If the path were shortened to 10 miles, the 60 dB increase in C/N could theoretically provide 350 Kbps in the same 15 KHz bandwidth or over 50 Gigabits/sec if wide enough bandwidth was used. This would require increasing the center frequency far above 146 MHz to accomodate the information rate and using antennas with directivity. If directional antennas as big as the 146 MHz dipole were used at both ends, even higher data rates are theoretically possible.

I find that most amateurs greatly underestimate the degree of waste involved in communications over typical amateur paths and are surprised by these numbers.

While there are many ways that amateur radio information systems can be improved, I believe that the first step toward high speed data is most often at the physical layer. It takes appreciation of the magnitude of the problem to even begin this step.

Cellular telephones systems work as well as they do largely because one end of their link is at a site which is well situated. Even so, much of the time they experience a moderately large amount of excess loss due to the locations of the users. These losses can be tolerated due to the relatively low datarates required to support a single audio channel.

If amateurs are going to have success with higher speed information systems, it will be necessary to improve paths beyond what is currently considered acceptable.

The problem of excess loss was very evident when we deployed our moderate speed (230kbps) radios in Northern California. Here are some practical details that may help put all this in context.