Curiosity and high power AM transmitters...

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Phil Hystad:
This forum seems to be a rich source of good information with a number of RF engineers participating in the mix.  Thus, my question:  What technology is used for a modern high-power 50,000 watt AM transmitter of the ilk used by your typical high-power AM radio station such as KIRO in Seattle or KGO in San Francisco.  I presume the older technology is all tubes and I can imagine the types used but do they have solid-state transmitters or amplifiers at this range.  If so, what devices are used and how many are typically put in parallel push-pull to raise the bar to 50,000 watts.

I did a little research and got a lot of hits to Harris out of Melbourne Florida -- I presume this is the same Harris that our company used to compete with back in the late 1970s and 1980s and the creators of the Harris/7 32 bit midi-computer as they were also located in Melbourne.  

However, this Harris Company or Corporation or whatever seems to be the manufacturer of choice for high-power solid-state transmitters and amplifiers but I only know what I have read on their web pages and that is not much about the technology of their RF technology.

So, if anyone can help satisfy my curiosity I would appreciate comments or even pointers to good spots on the web.

Phil, K7PEH

Michael S. Higgins:
Today’s high power (50,000 Watt) AM radio transmitters use phase modulation if they are analog transmitters. Two transmitters are modulated 90 degrees out of phase from each other and their signals are combined to produce 100% modulation. This is much cheaper that using a high power brute force analog type modulator.

The new transmitters such as the Harris DX Destiny units are set up for direct digital modulation and use multiple digital modulator and transmitter exciters that are combined for the final output.

Typical power MOSFET modules are about 3 KW each for a total power of 50,000 watts. Each module can be hot swapped while the transmitter is in operation. The modulators are digital ramp type and are controlled by MOSFET switch mode controllers.

Steve Katz:
Most large BC stations have engineering info on their websites, or on "somebody's" website.

KGO, for example, has this on line:

http://www.dxhound.com/KGO%20visit%202005/jj_test_KGO.html

Shows the 50kW AM (solid state) transmitter complete, and also some modules and combiners, the antenna matching unit, tower feedpoints, etc.  Even shows one of the transmitter panel meter sets indicating 70V at 500A.

WB2WIK/6

Clark McDonald:
High level AM modulation has indeed gone the way of the dodo bird, in lieu of the phase situation described above so well.  

The SOUND of the old high level modulated high power transmitter is a wonderful thing IMO, though.  Good ears can pick them out and also tell the phase mod within a few seconds of listening.  Familiarity with the differences does that, I think.  

Make new friends but cherish the old ones,


.
 

Alex:
The trade name for high level 50kw am transmission was Ampliphase.  2 RF signals 135 degree out of phase was FM or Phase modulated to create the equivelent of plate modulated am.  It sounded the same to me as any other high level system.


Alex....

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