been reading the nab engineers handbook 10th edition and it seems most AM Transmitters now a days use digital modulation and class D amps. it goes on to state that class D can also reach close to 100% efficiency similar to Class E.
I'm wondering if any of this along with high stability low noise PLL or DDS synthesizer could be employed for Part 15 AM with an optimized large air wound matching coil to a 1/2 inch 3 meter copper pipe.
just an idea that maybe one of the RF design engineers here could try and come up with a design for.
maybe even a large cage wound coil using 1/4 inch copper tubing suspended by 3 or 4 supports to minimize heat loss which is something the wilson 1000 cb antenna's started doing many years ago.
maybe a larger version of this type of coil designed to match the transmitter output to a 3 meter whip.
Really like the looks of that coil on the whip.
Digital modulation is new to my ears, does it end up giving an analog signal once it gets heard on the normal radio?
it's digital modulator that ends up still producing an analog compatible AM Signal on a standard AM Receiver
It's possible to come up with a class D/Digital Mod low power TX for Part 15. Nothing really stands in its way except someone taking the time to design one.
But your comparing mountains to mole hills. The power levels involved with those stations versus Part 15 stuff is a far cry in between. There is a reason why those stations want that 99.999 percent efficiency, and it is not because of maximizing the RF transfer coefficient. It is to minimize the amount of electricity those things use while producing the same TPO requirements for their license but keeping the costs as low as possible.
For Part 15, the goal would be to maximize the RF transfer efficiency onto an inherently inefficient antenna. Getting a .5 percent less electrical power consumption is pointless and waste of time trying to achieve that. But the other side of that coin, getting the most RF transfer efficiency to the inefficient antenna is certainly worth the time and effort.
Penny picking is all that it will amount to. Besides that, designing a digital class D transmitter would throw its price tag WAY beyond the mere 200 buck mark. More closer to the 700 range and I would not sell one for anything less than that considering the time involved in designing it.
It is not necessary to go to the nth degree with Part 15 simply because of the heavy limits placed onto it to begin with. There is only so much wiggle room we have and though it would be nice to have every square micrometer of efficiency and what not, the work involved to get that nth decimal point efficiency factor wont be worth it in the end. Even if it is reachable, the end cost to the consumer (kit builder) would be less than ideal.
Its a nice idea and one worth study, perhaps at some point when the cost vs price curve gets better (if it gets better), such a design would be worth doing and marketing.
RFB