At first let me talk about combiner , no it doesn't mean modulator , consider that you have some transmitters and you want to increase the out put power . can you use some transmitter in parallel ? of course yes , but not easily . you have many ways , such as a master/slave feed back , but at very high powers ( around 500KW ) it isn't a good choice . so you have to use a combiner , a combiner , means a magnetic section which will sum , the out put of your transmitters , magnetically . i hope you got my mean .
And about medium and low and high , i didn't outbreak myself with these definitions , because it isn't important . for example suppose that a company give you an offer for a transmitter , they won't tell you , to make a madium power or low power transmitter . they'll tell you , for example we want a 1 MW or 800 KW or less or higher , transmitter in the range of ( e.g 30MHZ ) and impedance of antenna is around 50 ohms .
Best Wishes
Goldsmith