Most of the AM medium wave transmitters for Part 15 low power do not use transformer modulation, with one exception being the Ramsey AM25 which has a small hand-wound transformer. But the shortwave transmitters on the blogs titled Talking Pixie2 and Big Talker got their start with a schematic that shows a small Radio Shack transformer, 8-ohms to 1K, with no stated power handling capacity. Even though that transformer does function, I checked around and found a Triad audio transformer with a rating of 1-Watt, which seemed more in line with the circuits being used. It remains to be tested.
Right now I am only interested in your point of view about low power, Part 15, modulation transformers simply as a topic of conversation.
In the adult-sized world of power radio, making a comparison, a plate modulation transformer for a 50kW station might way a ton or more.
Down here in micro-radio, what kind of power rating should a modulation transformer have?
About One Half
About one half the carrier power produced by the final. This is because the sidebands contain 1/2 the power of the carrier at full modulation. A 100 mW transmitter (100% efficient) would require 50 mW of audio power.
The impedance matching is also a factor. Edit to Add: The impedance of the secondary should be approximately the final supply voltage divided by the current. For the high efficiency 100 mW transmitter I built this calculates to be 57 ohms. If this is typical of a 100 mW unit then an 8 ohm to 1k ohm transformer would not be a good choice since the 1k secondary Z is too high.
Neil
Switching to Big Talker
These transformer comments link to Big Talker over on another thread…
http://www.part15.us/node/2870#comment-13497