Total posts : 45366
The “nanowatts” you mention is the amount of RF energy radiated by the antenna conductor. The power level I mention is the transmitter final PA input power. They are different; quite different. The use of a lossy antenna would allow 10 to 25 milliwatts of transmitter input power to produce the appropriate power for legal field strength.
Understood. But even if the PA output power of a Part 15 FM tx is only 10% of the PA input power, that 10% (1-2.5 mW in your example) is still far more power than can be radiated even by a low-gain antenna without exceeding the Part 15 FM limit at 3 meters.
Even a very short “whip” antenna is a very efficient FM radiator. In fact the peak gain of a 1/10-wave dipole is about 91.5% that of a 1/2-wave dipole — and well above that of an isotropic radiator.
But suppose the whip antenna radiated only 10% of the 1 mW output of a Part 15 FM tx. The peak field at 3 meters then would be 23.4 mV/m, or about 94X higher than the Part 15 limit.
FCC certification does NOT ensure legal operation either. Just ask the folks at Ramsey Electronics.
It will if the certification was properly done, and the tx+antenna system is operated in the same configuration as when they were certified together in a single test. But of course it will NOT meet Part 15 FM rules when in operation if the user changes the system configuration (uses a different antenna design or adjustment, adds an r-f amplifier, etc). In such cases the original certification is invalid, anyway, and the responsibility for compliant operation must be assumed by the operator.
If you talk with Keith Hamilton (one of the good guys) about violating the letter of the rules with a Rangemaster transmitter, he will tell you they can be operated outside the rules and regs.
Certainly, but not without risk. I wonder if this statement you attribute to him really is his official position before the FCC. I doubt it. More likely it is just “wishful thinking,” useful to those wanting to operate that way.
Regarding the isotropic antenna. ..But theoretical gain, at least in popular publications on the subject, puts the gain figure of a dipole at 2.26 db gain over that of an isotropic antenna. That would make the gain of a 1/4 wave and the theoretical isotropic very close to one another, with the 1/4 wave system winning out due to non-linearity in the radiation pattern. AM broadcast rules have used this thumbnail comparison for over 50 years. Your text books may not use the same figures or definitions, but broadcast engineering practice for the past half century certainly has.
The 1/4-wave vertical antenna and radial ground system used in AM broadcasting have very nearly the gain of a 1/2-wave dipole (which is 2.15 dBi, not 2.26 dBi), not that of an isotropic radiator. Please refer to virtually any antenna engineering textbook to confirm (Kraus, Balanis, Terman etc).
The fundamental question remains how do uneducated everyday hobbyists stay comfortably inside the boundaries of the current Part 15 rules and regulations? I have asked this question on several boards and groups, and as yet no one has been able to offer a satisfactory fool proof solution or answer.
Actually, I did answer that question earlier in this thread, and again in this post. The answer is “Buy a Part 15 FM tx and antenna that have been certified together as a compliant system, and use it in that configuration.”
These posts are getting quite technical, so maybe we should take it off board to avoid the clutter? You have my email address.