Total posts : 45366
What astonishes me the most about the first enforcement action discussed in this thread is the very low efficiency of the system that was cited. I already posted about that. I wonder how typical the efficiency of this installation is of Part 15 AM installations in general. If this poor efficiency is typical, Part 15 AM is next to useless as it presently exists. A fundamental technical reevaluation is needed.
If this installation is typical, tweaking the power input above 100 mW and elevating the antenna are definitely not enough to make Part 15 AM useful. In the cited system, the power input was 12 times higher than what is allowed, and elevating the antenna six meters caused the radiation resistance to be increased by a factor of about 20 compared to a 3 m antenna at ground level. The total radiated power increase caused by the increased input power and increased elevation is 12X20 = 240. Even with this advantage, the total radiated power is only 22.2 uW.This is a very insignificant radiated power.
Using Rich’s example of a 3 m antenna that produces about a half watt of radiated power for 80 watts input to the antenna, the efficiency is .5/80 = .625 %. Rich assumed 20 ohms loss resistance. Let’s arrume 25 % transmiter efficiency. In that case, the radiated power produced by Rich’s antenna if its input power is 25 mW is 25X.00625 = .156 mW =156 uW. This a lot higher than the 22.2 uW actually measured at the cited system. The loss resistance has to be increased to about 140 ohms to get only 22.2 uW from Rich’s antenna fed by a 100 mW transmitter that is only 25 % efficient. Only 22.2 uW was obtained with a system that gives a power increase of a factor of 240 over a system that is compliant with Section 15.219.
It’s too bad that almost nobody has an accurate field strength meter to evaluate Part 15 AM installations. A field strength meter would allow determining the efficiency of a system, and then troubleshooting an inefficient system. Two causes that I can think of for the inefficiency of the cited system are very high ground resistance and improper tuning.