We live in an age when thinking can be dangerous. But at whatever risk I began wondering if there's any comparison between the well known Part 15 rules for AM transmission, and the laws pertaining speed limits for driving.
There's an unofficial general belief that going 5-miles over the posted speed limit is still within a safe margin as presumably tolerated by traffic enforcement. In fact driving at the exact speed limit usually attracts tailgaters who hate you badly.
So imagine a transmitter putting out 150mW and an antenna 3.5 meters, cheating in very tiny increments. Obviously any improvement in signal coverage would be practically negligible, but I'm tailgating here.
Remember, it's only a thought. But it's also a question of law: is there a margin of tolerance?
First, to clear up a common misconception, it's not 100 mW of RF output, it's 100 mW of DC power input to the final section of the RF amplifier. Huge distinction. The efficiency of the transmitter in turning that 100 mW of power into useful RF differentiates one transmitter from another. It's likely that none of our transmitters get anywhere near 100 mW out, so 150 mW output is easily double (or more) what a person would expect.
The question might be better put as "how can we get more OUT of the 100 mW IN?" 5% components instead of 20%? adjustable components rather than fixed value?
Same thing with the antenna. Too long is too long, but how about trying a capacitive hat, or center or top loading, or one of many other configurations?
Good faith experimenting sounds more interesting to me than cheating, anyone can do that 🙂
Interesting beginning of what will probably be an interesting thread.
From a technical perspective a "little" cheating doesn't go a long way just as going 5 over on a freeway only gets you there 5 minutes earlier after an hour drive. The field strength is proportional to the square root of the power so one would predict that the linear range increase by doubling the power is 1.4 or 40%. But since the area covered is proportional to the square of the linear range, the area is proportional to the power. Given a uniform density of listeners per area then the number of potential listeners is proportional to power. Now, how much "cheating" on the power is one willing to risk based on the increase in possible listeners?
Another approach is that anything which increases the field strength while operating legally under 15.219 is a plus. Double the FS and your potential listeners are quadrupled. How is this done?
One way, as mentioned, is to increase the efficiency of the transmitter since the input power is limited to 100 mW, but the output power is input power X efficiency. The maximum efficiency I measured on my Ramsey AM-25 is 30% which is very low for a class C RF amplifier. Room for improvement there, and if I recall Ermi Roos posted quite a bit on this topic some time back. A search may be helpful.
The other way is to improve the power radiated by the antenna system by minimizing the ground RF resistance and/or raising the radiation resistance (which is a function of the antenna length). A ground mounted vertical antenna with a wired ground radial system is probably the best the average hobbyist can do, yet advanced theory and experimentation may yield gains over the conventional wisdom.
So, though I have avoided comment on the morality or legality of "cheating" I have tried to put it into perspective regarding the gains vs. the risks as perceived by the operator. I have no comment on how much over the limit one can operate.
Neil
I am digging around, but I seem to remember that licensed AM stations are permitted 5% over and 10% under what they are authorized to allow for line voltages variations. So if one were to apply the same logic, then the input power would be a max of 105mW. Big Deal! LOL!
John,
I think a key word in you analysis is "permitted". There is no such allowance in part 15.219 for us. Also, as I know you know, 5% over the 100 mW limit will have a barely measurable effect in a lab and will have no practical effect.
73
DE Neil
🙂